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Title: The Science of Brickmaking
Author: Harris, Georg F.
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

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Transcriber’s Note:

In this Plain Text version of the eBook, italic text is indicated by
_underscores_ and chemical compounds on pages 76–77 are shown without
subscripts, e.g., H2O. Other Notes will be found at the end of this
eBook.



ADVERTISEMENTS.


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  CHARLES ERITH & CO.,

  70 GRACECHURCH STREET, LONDON, E.C.



[Illustration]

ADVERTISEMENTS.


  THE CARDIFF KILNS (LEE’S PATENT)

  HIGH-CLASS

  BUILDING BRICK, BLUE & VITRIFIED BRICK,
  FRONT AND ORNAMENTAL BRICK,
  FIRE BRICK, SANITARY WARE,
  SALT GLAZING, &c., &c.

CAN be burned BETTER and CHEAPER in these IMPROVED REGENERATIVE
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They are CHEAP to erect, DURABLE, SIMPLE in construction, and
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EASY TO BURN. REQUIRE ONLY SMALL COAL, AND ALLOW NO WASTE HEAT. GIVE
PERFECT RESULTS,

  AND ARE
  THE KILNS
  OF THE
  DAY.

It has long been admitted that the “DOWN-DRAUGHT” is the only reliable
Kiln for burning the highest class goods, and the best for ordinary
purposes, the only drawback to its general adoption for lower grade
wares and Building Bricks being the heavy consumption of fuel. THIS,
the ONLY disadvantage, is ENTIRELY overcome by the patent CARDIFF KILN,
and the improvements recently devised in their construction, whereby
all the advantages of the HOFFMANN system are combined with those of
DOWN-DRAUGHT system, while avoiding the defects of the Hoffmann, thus
rendering these kilns the BEST, CHEAPEST, and MOST ECONOMICAL of all
modern kilns, producing the BEST GOODS at the LOWEST COST. The system
may be applied to existing Down-draught Kilns at very moderate cost.

  FOR PARTICULARS APPLY TO
  The Patentee, E. P. LEE,
  29, The Parade, CARDIFF.



THE SCIENCE OF BRICKMAKING



  THE SCIENCE
  OF
  BRICKMAKING:

  WITH SOME ACCOUNT OF
  THE STRUCTURE AND PHYSICAL
  PROPERTIES OF BRICKS.


  BY
  GEORGE F. HARRIS, F.G.S.,

  Membre de la Société Belge de Géologie, Paléontologie et
  Hydrologie; Lecturer on Geology, The Practical Applications of
  Geology, and Mineralogy, in the Birkbeck Institution, London;
  etc., etc., etc.


  LONDON:
  H. GREVILLE MONTGOMERY,
  43, ESSEX STREET, STRAND.
  1897.

  [ALL RIGHTS RESERVED.]



  PRINTED AT THE VICTORIA PRINTING WORKS,
  STANSTEAD ROAD FOREST HILL S.E.



PREFACE.


The substance of this little work was first published as a series of
articles in the _British Clayworker_, in 1895–96, and I am indebted
to the courtesy of the Proprietor of that Journal for permission to
reproduce them.

An attempt is here made to furnish some information of an elementary
character on a special branch of technical education which has been
seriously neglected in this country. But the reader will understand
that it is only an elementary treatise. Its publication in serial form,
where each article must, more or less, be complete in itself, has to a
large extent determined the method of handling the subject, and I am
fully cognisant of the drawbacks of the work in that respect.

At the same time, it is hoped that the book will be useful to the more
advanced class of brickmakers and clayworkers generally, many of whom
have expressed a desire to see the articles in this form.

                                    GEO. F. HARRIS.

  Birkbeck Institution,
    Bream’s Buildings, Chancery Lane, E.C.
  1st February, 1897.



CONTENTS.


  CHAPTER.                                                         PAGE.

     I. FLUVIATILE BRICK-EARTHS                                        1

    II. LACUSTRINE AND FLUVIATILE BRICK-EARTHS                        17

   III. MARINE BRICK-EARTHS                                           22

    IV. THE MINERAL CONSTITUTION OF BRICK-EARTHS                      28

     V. MINERALS: THEIR BEHAVIOUR IN THE KILN                         39

    VI. MINERALS: THEIR BEHAVIOUR IN THE KILN (_Contd_)               49

   VII. THE CHEMISTRY OF BRICK-EARTHS                                 58

  VIII. THE CHEMISTRY OF BRICK-EARTHS (_Continued_)                   75

    IX. DRYING AND BURNING                                            94

     X. THE DURABILITY OF BRICKS                                     103

    XI. THE MICRO-STRUCTURE OF BRICKS                                118

   XII. THE MICRO-STRUCTURE OF BRICKS (_Continued_)                  128

  XIII. ABSORPTION                                                   132

   XIV. STRENGTH OF BRICKS                                           136

    XV. ABRASION, SPECIFIC GRAVITY                                   146



THE SCIENCE

OF

BRICKMAKING.



CHAPTER I.

FLUVIATILE BRICK-EARTHS.


Let us go to Crayford, near Erith, or to Ilford, in Essex, and take a
superficial glance at some of the brickyards found at those places; in
particular, let us ascertain a little concerning the earths employed.
We find in one brickyard a series of stiff brown or bluish clays,
interstratified between sandy clays or “loams,” with thin brownish
partings. In another, the loam will become very sandy, and the earth
light, with a slight greenish tinge. A third has thin pebble or gravel
beds developed, or small stones sparingly scattered in the clays and
loams on certain horizons. A fourth contains, in addition to some of
the beds above described, a lime-clay or marl[1] with small pellets
of chalk. It will be noticed on entering the yards that these various
horizons, or beds, as they are conveniently termed, are disposed in
regular lines or layers, more or less horizontal; in other words, the
beds are “stratified.” On the face of the working being dug into, it
will often be found that these thin beds, a few inches or feet each
in thickness, vary in depth, and frequently disappear altogether, or
“thin out,” whilst, on the other hand, a bed only a few inches thick
may become as many feet, and new beds are found to be developed. A
pure sand may in like manner become loamy on being dug into, and, on
being further developed, pass insensibly into a stiff clay. And many
other changes take place into which we will not enquire at the moment.
Suffice it to say, that in such brickyards the strata are very locally
developed, though it follows from the circumstance of their existence
for so many years, that what changes have taken place, to some extent
compensate each other, so that the material is still an earth suitable
for making bricks. Again, certain beds of much economic value may be
more persistent than others, both in character and development. Having
noticed all these things, we perceive a couple of men digging with care
into the brick-earth, and presently they bring some objects to us which
we have no difficulty in recognizing as the remains of the lower jaw
of an elephant’s skull. Returning to the spot where they were exhumed,
the upper jaw and tusks also are uncovered. To the clay workers these
things are well known; in their time they have found many similar
skulls of animals in the brick-earth; but they know next to nothing
concerning them, or how they got there. Another expedition to the same
localities may yield the remains of rhinoceros, the musk sheep, grizzly
bear, hippopotamus, reindeer, and many other animals. A fine series of
the remains of these, obtained from the brick-earths of the valley of
the Thames at several points, is exhibited in the geological department
of the British Museum (Natural History), South Kensington, and more
or less complete skeletons obtained from the same source may be found
in other, and local museums. One of the most interesting points
concerning these remains is that so many of the animals represented
in the brick-earths are of extinct species--there are no species
included in this latter category of precisely similar kinds to animals
now living, Thus the elephant was different to modern elephants; we
know, from remains found elsewhere, that it was clothed with wool. The
same also with the rhinoceros. The reindeer no longer lives in this
country, being confined to northerly latitudes; whilst the musk sheep
is a denizen of the Arctic regions, and the hippopotamus is restricted
to the tropical or sub-tropical climes. But we might continue for a
long time expatiating on the character of the very numerous mammalian
remains found in our common brick-earths. What a curious assemblage of
animals! It is wonderful to contemplate the time when the reindeer and
musk sheep lived side by side with the elephant and rhinoceros on the
site whereon London now stands.

That is not all, however. In the same brick-earths and gravels, tools
(flint implements), fashioned by the hands of man, are also frequently
discovered, and in one place at Crayford, the spot whereon flint
implements were manufactured has been ’lighted upon. Each flake chipped
off has been collected and pieced together, and the shape of the
original flint has thus been determined. Clearly, from this evidence,
the earth from which millions of bricks have been made has formed
since primæval man (and with him the animals alluded to) inhabited
the valleys of the Thames and its tributaries. It is interesting, too,
to reflect on the circumstance that the materials upon which many of
these facts of great philosophical significance are based, have been
collected through the instrumentality of the workmen. Palæontologists
are proud to acknowledge that; their debt of gratitude to the
intelligent and persevering men can never be fully repaid.

Pursuing the matter still further, we discover a quantity of shells,
blanched and very frail--they seem to be deprived of much of their
original substance, so to speak; their entombment in the brick-earth
has taken all the natural colour out of them. Studying these, we soon
ascertain that they belong to land snails and mollusca which inhabit
fresh water. Living representatives of the same species are, with few
exceptions, found in Kent and Essex.

Putting all this evidence together, we come to the conclusion that the
brick-earths alluded to accumulated in the channel of a river; they are
found above the present level of the Thames, for the simple reason that
they have been elevated into that position partly by earth movements
and partly by the channel of the river being cut deeper by natural
causes, of which abundant proof will be adduced. The snails were washed
down from the land by freshets, or caught by the river in flood; the
elephant, rhinoceros, hippopotamus, and musk sheep were overcome,
perhaps, by floods, drowned, and subsequently covered up by the mud
of the swollen current. We can imagine that the savage hunter, in his
canoe, attacking the animals swimming in the river, loses his tomahawk,
or his frail bark may be upset, and he is striving to gain the shore
for dear life. Or, it may be winter time; the river is frozen over,
and he is cutting a hole in the ice with his flint chisel wherein to
fish; his hands are benumbed, and he loses his grasp of the tool; it
falls into the water, to be discovered in the brick-earth by one of our
intelligent friends. Truly, the revelations of the brickyard enable us
to construct a picture of one of the most interesting phases of the
past history of the Earth.

We have given an outline of the evidence upon which certain
brick-earths in the Thames valley are proved to be of fresh-water
origin--to have accumulated in quiet reaches of the river, and at
other convenient spots along its course--but we have used that as
an illustration only; phenomena of precisely the same character are
manifested in nearly all river valleys in this country, especially
those in which the bottom of the valley has only a slight gradient down
to the sea.

The brickmaker may ask: What is the practical bearing of these
observations? What difference does it make to us whether the earths
we use are of fresh-water, lacustrine, or marine origin? All the
difference in the world, from the points of view of structure,
composition and suitability of the earths, and especially of their
distribution over the face of the country. How much easier it is to
value an extensive brickmaking property when you feel perfectly certain
as to whether the face of earth as shown in the pit will die out on
being worked into for a few yards, or whether it will be persistent
throughout the whole of the property to be valued. Better still, when
your knowledge enables you to state definitely whether the quality of
earth now being worked in a pit is likely to continue the same, or
whether it will get better, or worse. The disposition of the earths, in
some instances, is so clear that no brickmaker with an eye to business
could fail to trace their extent over his property. But this is not
often the case, for the earths being used are for the most part covered
by a superficial mantle, or overburden, which masks the true character
of the beds beneath. A very slight acquaintance with the principles of
geology overcomes these difficulties as a rule; and we are about to
lay down the elements of these principles, so far as they apply to the
immediate subject in hand. By seeing why it is the beds of brick-earth
vary in structure and composition we shall be in a better position to
make forecasts of their general behaviour.

In regard to fluviatile deposits, it goes without saying that every
river flows along a general depression more or less pronounced, called
a valley, and that this valley is bounded physiographically by a ridge,
except in the region of its entrance to the sea or lake, or, if a
tributary, of its joining a main stream. The watershed of a river and
its tributaries includes and comprises what is technically termed the
“river basin.” All valleys are, in the end, the result of denudation
taking place in them. In other words, on the birth of a valley a very
slight depression or other physical feature determined its general
direction for the time being, but the little rivulet once being
formed proceeded, through the medium of the “agents of denudation,”
to carve out its channel more clearly, and eventually to eat into the
rocks over which it flowed, until a large valley had been formed. The
“agents of denudation” in river valleys may be summarised as rain,
snow, ice, heat, and wind, and their general effect on rocks is called
“weathering.” We need not stop to enquire into the precise methods
adopted by these agents in accomplishing their work; it suffices at
present to say that the rock destroyed or broken up is removed by the
running water constituting the rivulet, stream, or river, as the case
may be. Some of the material is chemically dissolved in the water,
whilst another and larger proportion is taken away in suspension, or
is said to be dealt with mechanically by the river. The agents of
denudation do their work very slowly, as a rule, and yet no one who
stands on London Bridge and contemplates the swollen stream laden with
muddy sediment passing under it after a few days’ rain, could say that
they are not doing their duty effectually. To give some idea of the
quantity of sand, gravel, and mud removed from the land through the
medium of rivers, we may remark that the Mississippi discharges into
the Gulf of Mexico annually a mass of earthy matter equal to a prism
268 feet in height with a base area of one square mile. In regard to
denudation by chemical means we may say that the Thames carries past
Kingston 19 grains of mineral salts in every gallon of water, or a
total of 1,502 tons every 24 hours, or 548,230 tons every year; this is
not taking into account the muddy sediment, gravel, &c., annually sent
down to the Nore, which must be infinitely greater in quantity.

Enough has now been said to show that stupendous quantities of mineral
matter derived from the destruction of the land are sent down to the
sea by natural agencies, and we may at once state that a very large
proportion of this, which finds a resting-place in and about the
mouths of the rivers and their backwaters, is material suitable for
brickmaking at places where it is obtainable. Enormous quantities of
muddy sediment, sand and gravel, however, never reach as far as the
sea with great rivers. This material is arrested at sundry convenient
spots, and, as a rule, forms excellent brick-earth.

[Illustration: Fig. 1.--Formation of Brick-earth in a river valley.]

See Fig. 1, which represents part of a river of slow current with
three bends, A, B, C. The water is flowing in the direction indicated
by the arrows; and it is part of the mechanics of such a river that in
rounding a bend its velocity is greatest (and its eroding power also)
at the outer portions of the curves approximately indicated by the
arrow points. The water “wheels round” such portions of the curves,
and “marks time” at the points _x_ _x_ _x_, and, indeed, its progress
may be altogether arrested for a time at the latter places. Now the
transporting power of a river is its velocity, and, naturally, the
greater the velocity, the coarser will be the fragments or particles
of rock carried along. It is interesting in this connection to quote
the figures calculated by Mr. David Stephenson, giving the power of
transport of different velocities of river currents:--

  Ins. per  Mile per
  second.    hour.
     3   =  0.170   will just begin to work on fine clay.
     6   =  0.340   will lift fine sand.
     8   =  0.4545  sand as coarse as linseed.
    12   =  0.6819  will sweep along fine gravel.
    24   =  1.3638  will roll along rounded pebbles 1 inch in diameter.
    36   =  2.045   will sweep along slippery angular stones of the
                      size of an egg.

These figures[2] have greater interest for us than in the connection
at present used, as will be noticed hereafter. We have seen that in
rounding the bends (Fig. 1) A, B, C, different portions of the stream
possess different velocities. We know it is charged with sediment and
stones all the time. The tendency, therefore, will be for the large
stones and coarse detritus to go round the outer side of the bend, to
bombard the banks near the points shown by the arrows, and to erode the
channel deepest in those situations; whilst a goodly proportion of the
fine muddy sediment will find its way to the quiet and shallow parts
near _x x x_, and in course of time become deposited there, whilst the
main course of the stream is eating its way and shifting its course
as indicated by the dotted lines _a a_. This action proceeds, it may
be, until the course of the river becomes straighter, as shown by the
dotted lines _b b_, when the whole of the loop B D is abandoned, its
former course there being evidenced by pools of water and irregular
heaps of gravel, sand and mud. The reader will now see that the whole
of the land marked _x x x_ has been formed of sediment brought down
by the river, and in the majority of cases such fine silt and sandy
mud or clay is specially suitable for brickmaking--many of our largest
brickmakers obtain their material from such a source. It should be
observed that the valley, as shown between the lines _v v_, may be two
or three miles in width, and it is often much more, so that the actual
amount of land made by the river at _x x x_ may be several thousands of
acres in extent.

Now as to the practical application of the foregoing observations. In
the first place, it will be seen that such deposits of brick-earth as
are made in this manner cannot be very thick, their total thickness
perhaps, resting on the bottom of the valley, not being more than 20
feet, and it is frequently much less. The next thing to be noticed is
that they must be very variable in character, a bed changing perhaps
every 100 feet or so horizontally, and more often every few feet.
Individual beds must of necessity be very irregularly developed under
the circumstances. The velocity of the stream being greater at certain
seasons of the year than at others, we frequently find some such
section as the following developed:--

[Illustration: Fig. 2.--Section of Fluviatile Brick-earth.]

  _a_ = Mould and soil, of no use to the brickmaker.

  _b_ = Sandy clay, with a large proportion of sand; useful for
          moulding or incorporating with the “fat” clays below for
          brickmaking.

  _c_ = Gravel bed, lenticularly developed; suitable for mending
          roads, paths, &c.

  _d_ = Sandy clay; similar to _b_.

  _e_ = Thin bed of marl, with a fair proportion of lime.

  _f_ = Sands and small pebbles, irregularly stratified
          (false-bedded).

  _g_ = Laminated sandy clay.

  _h_ = Stiff clay; can be mixed with _f_ and passed through the
          pug mill.

  _i_ = Sand; an irregular bed of very local occurrence.

  _j_ = Gravel bed, with much sand.

The above is typical of deposits accumulated in river valleys; it
is different in character to deposits laid down in the sea (as will
presently be described); the section exhibits very different classes
of brick-earth also, and yields a totally different kind of brick to
that obtainable from brick-earths of marine origin. The importance of
the question of origin of a brick-earth, therefore, is just beginning
to dawn upon us. Many rivers are noted as having throughout a long
period of time wandered from one side of the valley to the other (by
the process depicted in Fig. 1) several times, in which cases the
brick-earth sections relating to them are liable to still greater
variation. The reader would perhaps be very much astonished to find how
much is known concerning peregrinations of that description in regard
to particular localities, by competent authorities--usually field
geologists.

We come to another important point in regard to river deposits. The
ceaseless flow of the river, and the abrading action of the large
stones rolled along at the bottom of its channel, tend to cut the
latter deeper and deeper, and we have excellent evidence that most
English rivers once flowed at a greater elevation in their valleys
than they now do. In consequence of this, the brickmaker may find his
pit somewhat higher than the neighbouring river, which at an earlier
stage of its existence made his brick-earths. To a certain extent,
small earth movements, as previously explained, are also undoubtedly
responsible for many of these brick-earths now being at a considerable
elevation above the surface of the river. This phenomenon is
illustrated in Fig. 3.

[Illustration: Fig. 3.--Section across a river valley, showing
formation of terraces of gravel and brick-earth.]

This type of disposition of fluviatile deposits is of common
occurrence. We will assume that the valley is carved out of clay (shown
by horizontal lines and dots). On both sides of it, and at the same
relative heights, are two masses (marked 1 and 2) of brick-earths and
gravels running along so as to form two distinct broad terraces. These
beds were laid down when the river, in flood, though occupying only
a small portion of the valley, was approximately of the height shown
by the dotted lines _a b_. Denudation has been hard at work, however,
since then, and only vestiges of these beds clinging to the sides of
the valley, as shown, remain. At a later period, and coming on towards
modern times, the broad expanse of beds (comparable in disposition
with those depicted in Fig. 2) some miles in width, marked 3, were laid
down, and we notice the river channel, as it now is, cutting its way
through them. Thus it comes to pass that brickyards may be situated
in terraces one above the other; and what is much more important, the
brick-earths may vary very widely in quality along these horizons,
those in 1 differing from 2 and both from 3. The brickyards may be
quite close to each other, and to the unscientific eye the earths
are of similar appearance, but they do not yield the same class of
brick, and no one seems to trouble to enquire the reason why. These
differences have resulted primarily from the materials having been
derived from other collecting grounds, other watersheds, than those
comprised within the basin of the river as at present constituted. They
are the inevitable accompaniment of the evolution of the river system,
and throw light on successive phases of the past history of the stream
and its tributaries. For us, as we have seen, they possess considerable
practical value of the first importance in selecting the site for a
brickyard.

Apart from differences of the character just described, serious
alterations sometimes take place on these brick-earths being traced
higher up the valley, and indeed an excellent brickmaking material may
become absolutely worthless in that respect, for the reasons about to
be explained. The reader will agree that neither stones nor sediment
can travel up a valley, and he will understand that no sediment can be
found in the valley earths other than that derived from the destruction
of rocks within the watershed of the river system, to which the valleys
belong, or did belong, at the time the earths were formed. We desire to
put the case in a very simple light, so as to be clearly comprehended.
Let us contemplate Fig. 4.

[Illustration: Fig. 4.--Map shewing river basin, with geological
formations depicted.]

Here we have represented a river basin, the limits (watershed) of which
are indicated by a sinuous dotted line. Three geological formations are
found therein; in the upper reaches of the main river is a series of
clays marked A; a large tract in the middle, B, is sandstone; and the
lower part, C, is occupied by limestone. Seeing that nothing but clay
crops out in the part A, it follows that the deposits of the river in
that region must be principally of an argillaceous character, to the
point _a_. On flowing over the sandstone B, the main stream, already
charged with clay particles, will be mixed with sand; the proportion
of sand increases as the first large tributary (_b_) to the east is
encountered, and is considerably augmented as the still more important
tributary (_b_) to the west enters it. The superficial deposits in
the valleys of the area B will likewise be very sandy and perhaps
gravelly at _b b_, but at _c c¹_ the sands and gravels will be mixed
with much clay. On passing over into the area C, much carbonate of
lime is added, though the larger proportion denuded from the rocks is
taken away, chemically, in solution. Nevertheless, nodules of “race”
(lime concretions), limestone pebbles, and perhaps chert and flint
gravel will come upon the scene at about the point marked _e_. At _d_
the deposits would principally consist of gravel and impure marls. To
sum up, the clays at _a_ would no doubt be too stiff of themselves to
make good bricks; similarly the beds at _b b_ would be nothing but
sand, though these might be made, with a little judicious treatment,
into a species of fire-brick; at _c_ we should find alternating loams
and clays suitable for turning out fair bricks; at _c¹_ the beds would
be more variable in character and more locally developed; they would
consist of thin beds of sand, clays, loams and gravels (principally
sandstone fragments), which as a whole might be made serviceable,
though difficult to deal with; nothing of much use to us would come
from point _d_, nor bordering the tributary running over C; there
would be too much lime present, though a trade might be started in
basic bricks should there be any demand for them in the neighbourhood;
this, however, would only pay under extremely favourable conditions.
At _e_ there may be a mixture of all the foregoing deposits, and
providing the beds above were easily weathered and thick beds of loam
were thus fairly well developed, good sites for brick-earth might be
found. The point _e_ might possess this advantage over the other sites
mentioned, viz., that marls would no doubt be present, and thus no
necessity should arise for grinding lime to be incorporated with the
brick-earth; the only danger would be that lumps of limestone might be
too numerous--especially if c were a hard limestone.

The general character of the deposits might be slightly modified by
mineral matter brought up in springs and thrown down at convenient
spots.



CHAPTER II.

LACUSTRINE AND FLUVIATILE BRICK-EARTHS.


The great variability of brick-earths deposited in river valleys is
reflected to some extent in those laid down in lakes, though the size
of the latter is frequently a controlling factor. The chief difference
consists in the broader expanse of the sediment laid down--especially
in large lakes--and variation in structure is not so noticeable
horizontally. Let us consider a simple case in which a lake is fed by a
large river bringing down abundant sediment. The lake acts as a species
of settling tank, and the method of deposition of the sediment by the
river is mainly guided by the velocity of the stream. The tendency
under normal conditions is for the river to commence parting with its
sediment immediately on entering the lake. The detritus alluded to is
only held in suspension by the velocity of the water; when the latter
is checked, as on entering the lake, the grosser pieces subside, and
as its rapidity becomes progressively curbed, medium-sized fragments
are compelled to give way, until at last only very minute particles are
left in the water. In due time most of these also are deposited. Thus
gravel is laid down before grit, grit before sand, and sand before clay.

If the velocity of the river always remained the same, we should be
presented with thick accumulations of the same character in sharply
defined areas. But it is always changing. With every storm and every
steady rain the motion of the river becomes greatly accelerated, with
the result that the deposits for the time being are deposited farther
out in the lake than in more quiescent periods. In this way we may have
a gravel thrown down on sand, sand on clay, and so on.

From the foregoing observations it will be gleaned that, in general,
deposits in large lakes are more persistent in character than are river
deposits; indeed, in very large sheets of water, as Lake Superior, Lake
Erie, &c., they are in this sense more comparable with sediment of
marine origin.

The practical value of this knowledge hinges on the correct
determination of the origin of the deposits, and it is not always easy
to identify a brick-earth of lacustrine origin. In all probability
the tyro, on meeting one, would be disposed to regard it as a river
deposit pure and simple. The valuation of a brick-earth property under
such circumstances would thus be greatly in favour of the prospective
purchaser; but it would be disastrous for the seller. A random section,
except in the case of a very large lake, would show gravels, sands
and clays in much the same manner as the river deposits described in
the last article of this series. But, as previously remarked, on the
whole they would be more continuous and persistent, and what is quite
as important, the mineral composition of each stratum would be equally
homogeneous when traced over wide areas. The geologist distinguishes a
lacustrine deposit from one of fluviatile origin more from its mineral
constitution and the general disposition of the beds, as ascertained by
mapping, than from evidence afforded by fossils--these latter for the
most part being similar to those found in the deposits left by rivers.

The well-known brick-earth called “Reading mottled clay,” so
extensively developed on the outskirts of the London basin, and in the
Isle of Wight and Hampshire generally, furnishes a good example of a
lacustrine deposit. Many millions of bricks are made from this bed
every year, and in some parts of the districts mentioned the stratum
is thick and extensively developed. It is pure enough to be suitable
for terra-cotta manufacture here and there. No one who had seen this
remarkable deposit could possibly fail to recognise it again. The
natural colour of the clay when damp is brilliant red, scarlet or
crimson, in large blotches and patches mottled tea-green and yellow,
and locally white.

We have been intensely amused to note the efforts in recent years
to obtain possession of a few acres of this coveted deposit for
brickmaking in divers localities. Not long since we visited a large
brickmaking establishment where these Reading plastic clays are
actively raised and used, the works being situated four miles from the
nearest railway. There were no other brickworks between it and the
railway line, and there was no water accommodation. Enquiry revealed
the fact that the greater part of the intervening land belonged to the
same landowner as the ground where the brickyard stands, and that no
difficulty was apprehended of the owner letting out such intervening
land for the same uses and on the same terms if other brickyards were
contemplated. The proprietor of the brickyard in question volunteered
the information that the reason he started so far from the railway was
because the earth at the point selected was the only kind suitable for
brickmaking in the neighbourhood. We then questioned him as to his
knowledge of the brick-earths in the district, and eventually elicited
the fact that he chanced upon the spot selected, without any reasoning
therefor, and commenced operations. As a matter of fact, precisely the
same clay extended from his works all the way to the railway line, and
had he known anything whatever of the geology of the district (even
the merest boy’s knowledge of the subject), he would have seen how to
save that four miles of road carriage. What prevented him from knowing
the fact was a thin mantle of gravel and soil about four feet in
thickness, which covered the plastic clay in the area generally, except
in the immediate vicinity of his brickyard. That was in reference to a
lacustrine deposit--the Reading plastic clay--and shows the value of
knowing something of its persistent character; if it had been a river
deposit there would not have been so much room for wonderment.

To give some idea of the extent of that particular horizon, we may
say that not only is the plastic clay alluded to found so extensively
in the London and Hampshire basins, it is even more expanded in the
north-eastern parts of France, and is there as much utilised as on this
side of the Channel for brickmaking.

Lacustrine deposits are sometimes of enormous value to the clayworker,
on account of the general purity of the clays. This is more
particularly the case when the material deposited is in part or wholly
derived from chemical disintegration of granitic rocks, as in the
celebrated Bovey Heathfield clays near Newton Abbot, so well described
in a small pamphlet by Mr. S. Smith Harvey. Here an experimental boring
proved the clays to a depth of 130 feet with no signs of exhaustion.
In the divers clay-pits but a small proportion of waste is found,
the different levels vary in composition, and, like almost all thick
clays, improve in quality as the depth increases. The strata are very
irregular towards the surface, due perhaps to the action of local
freshets in the final periods of the history of the lake. These clays
are extensively employed for the manufacture of stoneware pipes,
facing and other bricks, fire-bricks, etc. They constitute a somewhat
remarkable exception to the class of clays laid down in lakes, as a
rule, and, as will have been observed, are of enormous thickness.

We have very little to say in regard to estuarine brick-earths; as
might readily be anticipated, they are intermediate in character
between fluviatile and marine deposits, and approach the one or
the other according to position in the estuary. On the whole, they
are variable in character, individual beds being thin. The strata
frequently contain abundant plant remains (pieces of wood, etc.), and,
except in the case of large rivers, are not noted for yielding very
good brick-earths. Sometimes, however, the quality of the clays is not
bad, as instance the bricks made in Lincolnshire and Northamptonshire
from Jurassic Estuarine clays.



CHAPTER III.

MARINE BRICK-EARTHS.


Turning to brick-earths of marine origin, we may say that these
constitute by far the largest class of deposits from which bricks are
made in this country, and it will be useful to deal with their origin
in some detail. If we attentively watch the action of the weather on
a friable sea-cliff we notice that large pieces tumble at intervals
on to the beach, and in due time these are washed away by the waves,
thus encouraging more to fall when the time is ripe. This process of
denudation each year takes tens of thousands of tons of sandy clays
and the like from the beaches around our islands. Large pieces of
rock, too, are detached by the weather, and eventually succumb to wave
action. During storms large stones are hurled against the cliffs,
and the general effect of this bombardment is to wear them away, and
reduce them to powder and sand grains with all possible expedition.
No one who has not seen the waves at work at such times can have any
idea of their tremendous power of moving blocks of stone many tons in
weight. During calm weather the slight movement of the waves on the
beach is manufacturing tons and tons of sand. A mass of gravel falls
from the cliff; the finer particles are floated away at the earliest
opportunity; the angular stones have their rough projections knocked
off by striking against each other; and the incessant movement up and
down the beach slope reduces the rough stone to a pebble, all the
time the particles thus shaved off are taken out to sea for greater
or less distances. If the cliffs are of limestone, or similar rock,
both chemical and mechanical methods of denudation come into play, and
considerable quantities of lime, &c., are taken away by the sea water
in suspension and solution. Large quantities of lime are daily added to
the sea through the agency of rivers also.

Now, what becomes of these vast quantities of detritus furnished to the
sea? That depends on the shore currents at the particular locality. If
there is not much of a current, the larger grains of grit and sand are
soon separated from the rest, and fall to the bottom, whilst the clays
are taken farther out to sea before being laid down. But, in any case,
the reader will readily perceive that marine deposits must of necessity
be on a grander scale, and of a much more substantial character, as a
rule, than river, lacustrine, or estuarine deposits. By their mode of
origin, too, they must be more homogeneous, whilst they are frequently
several hundreds of feet in thickness. In their process of deposition
they were not influenced by every storm and freshet; nothing short
of great earth-movements in process of time, or some other equally
grand phenomena, could disturb the even tenour of their existence. How
different to the comparatively insignificant strata formed by the other
methods alluded to!

Take samples of brick-earth of fluviatile origin at intervals and
analyse them; no two analyses will be alike, except by a most
remarkable coincidence--more by accident than otherwise. On the other
hand, take a thick marine clay, and compare its chemical composition
as ascertained at the present time with that of it made, say, 20 years
ago in the same brickyard, and the analyses will, in most instances, be
practically identical--at any rate, so far as they may be of use to the
brickmaker.

A brickmaker using a marine clay possesses innumerable advantages over
another employing brick-earths due to river action. It is no uncommon
thing for a marine clay--say, 300 feet in thickness--to continue
across country for hundreds of miles, stretching from the North of
England to the South, and over into the Continent, save for the slight
break occasioned by the scooping out of the English Channel. The
composition of the Oxford Clay, from which the well-known bricks at
Peterborough are made, does not differ in the slightest degree, so far
as suitability for brickmaking is concerned, from the Oxford Clay of
Bourges or Chateauroux, in the centre of France, or indeed at almost
any other point _en route_. With marine beds it is possible to deal
with the matter on broad lines, but it is not so with any other class
of deposits.

If a marine clay in a specified locality is found to be unsuitable for
bricks at one point, by reason of the presence of too much lime, it
would be a phenomenon if clay along the same geological horizon did
not present the same unfavourable features at every other point within
the district. The homogeneous composition, both from mineralogical and
chemical standpoints, of thick marine clays renders them of special use
to the brickmaker. Having by sundry processes, after infinite labour,
produced a certain class of brick from such an earth, he does not as a
rule have to materially modify those processes as the earth is dug into
to continue manufacturing the same brick. He is dealing with an earth
which, comparatively speaking, is a constant quantity--when the clays
are thick, and no lines of bedding are distinctly visible.

We find that a rooted conviction exists in many brickyards that clays
of marine origin are no good for brickmaking, because (so the opinion
runs) they always contain so much salt. It is wonderful that such
ignorance prevails, when the slightest acquaintance with the subject
would teach otherwise. It is perfectly true that such deposits might
have contained salt during and for some time after deposition, but
it is absurd to suppose that their marine origin has anything to do
with the presence of common salt in the clay at the present time.
Salt is soluble in water, and has been removed from such clays by the
percolation of underground water in 99 cases out of a hundred. Indeed,
as a matter of experience, we find that salt is most commonly found in
beds of lacustrine origin, or those laid down in enclosed portions of
the sea, for reasons we need not enter into at the present moment. Of
course, when material is taken from the sea-shore to make into bricks,
a considerable quantity of salt is manifest, but that is a totally
different thing to the clays deposited--we should not like to say
how many thousands of years ago. Clays of all kinds, however, may be
impregnated with salt (as in parts of Cheshire), owing to the proximity
of other beds containing that mineral; also by the percolation of
underground water with much salt in solution.

To give some idea of the antiquity of the Oxford Clay alluded to--and
that is quite a “young clay” geologically speaking--we may remark
that at the time it was laid down not a single species of animal
existed like those now living. The only mammals found, very small and
very lowly organised, were like kangaroo rats; the birds were more
like flying reptiles than anything else; it was the age of reptiles,
and enormous, unwieldy brutes swam in the water or floundered about
on land; huge sharks abounded, and armour-clad fish of kinds very
different to those now existing roamed the sea; even the “shell-fish”
were not altogether like modern ones; whilst the plants find their
nearest modern analogues in the wilds of Australasia. No elephants,
tigers, lions, bears, or dogs lived then, and the face of Nature wore
a totally different aspect to what obtains at the present time in any
part of the globe.

And this seems a fitting opportunity to the writer to put on record
the fact that many of the most wonderful remains found in the Oxford
Clay and the neighbouring Kimeridge Clay are due to the discoveries
of brickmakers. Without their valuable aid scientists would be quite
unable to clearly depict the life of those remote epochs. We have
mentioned Peterborough; some most interesting remains have been found
in the clays near that town during the past few years. To appreciate
this let the reader visit the fossil reptile gallery of the British
Museum (Natural History), at South Kensington. One of the most recent
acquisitions, set up a year or two ago, is the skeleton of a young
_Plesiosaurus_--without doubt the most perfect specimen in the world of
its kind--from Peterborough. The _Plesiosaurus_ was a large swimming
reptile, with paddles, and a long neck.

We mention these things not only to instil philosophical interest in
such brick-earths, which may be reflected upon after business hours,
but to impart some idea of the extreme remoteness of the epoch from the
human point of view, and to insist on the immensity of the intervening
time throughout which circulating underground waters--even in such an
impervious material as stiff clay--may have exerted chemical action.
The “mineralisation” of the fossils is an eloquent witness of the
effect of such changes. The reader will perceive from this that there
is scant possibility of soluble salts being present in such marine
clays; and the geological circumstances are fully borne out by the
results of hundreds of chemical analyses of thick marine clays.

The invertebrate fossils more particularly testify to the marine
origin of the clays, and are thus invested with considerable practical
interest. The man whose duty it is to determine the persistence, or
otherwise, of valuable marine brick-earths has thus a much easier task
than when called upon to decide the value of a large tract of land for
brickmaking purposes, of fluviatile origin. Finally, brick-earths do
not, except in extremely rare instances, vary materially in character
when dug into horizontally, thus every opportunity is afforded to
the manufacturer for making an unvariable quality brick, tile, or
drain pipe. It should be borne in mind, however, that these clays
often weather a brown colour, which on being dug into changes to a
bluish-black tint, the latter being the unaltered and best portion as
a rule. The only practical advantage the worker of a superficial river
deposit possesses over his neighbour using thick marine clay is in the
great range of variation in materials disclosed in the former kind of
pit. By judiciously mixing the different beds he may be able to live
well where the worker of marine clays, especially where the clay is
too stiff, or contains too much lime, “comes to grief.” A good marine
clay is a great boon, a bad one cannot be remedied other than by the
sacrifice of much money.



CHAPTER IV.


THE MINERAL CONSTITUTION OF BRICK-EARTHS.

There cannot be any question that the applicability or otherwise, of
an earth for making good bricks, to a large extent depends on the
mineral constitution of that earth. A chemical analysis of a sample
of such earth will tell us how much silica, alumina, lime, iron,
etc., is present therein, and this information is frequently of great
value when given by a scientific chemist; but it does not tell us the
state in which those constituents exist in the earth--an essential
_desideratum_, if we are to understand the scientific aspects of the
question of burning in the kiln. Further, the size of the granules
and particles composing the earth is well worth knowing, as we shall
presently see. It is a great mistake to imagine that all clays are
essentially chemical deposits. The majority of them have been in part
derived from chemical disintegration, it is true; but the resulting
deposits contain so much also that is purely of mechanical origin, that
the behaviour of the whole is materially modified, from a metallurgical
point of view. Take one ingredient, for example--say, silica. That
may exist in a brick-earth in a variety of ways, both in a free and
combined state; but its behaviour in the kiln is largely dependent on
the particular form assumed, not only whether it is free or combined,
but as to how it is combined. In a certain sense, it is very doubtful
whether even in the best-burnt brick much of the raw material becomes
chemically combined; a sort of agglutination takes place locally, as is
clearly shown by the microscope; at such points true fusion undoubtedly
takes place, and there may be actual chemical combination. In the vast
majority of cases, however, such fusion or possible combination is of
an extremely partial and elementary character, whilst it hardly exists
in the average “rubber.” The microscope shows that even in the hardest
burnt brick there still remain enormous quantities of what may be
termed mineral grains, that have by no means succumbed to the burning
process. The edges of the grains may occasionally be seen merging into
the more or less vitreous ground mass in which they are embedded, but
beyond that they appear tolerably fresh, and their action on polarised
light remains unimpaired.

We did not intend to say anything yet concerning the microscopic
structure of bricks--that will be gone into in a subsequent chapter;
but we thought it useful to state the foregoing elementary facts in
order to endeavour to uproot a conviction that seems to be very firmly
grounded--viz., that the chemical composition of a brick-earth imparts
an accurate idea of the possible active agents, on the earth being
subjected to the kiln. As a matter of fact, some of these would-be
agents are imprisoned in the mineral grains and particles that have not
become involved in the partial melting or agglutination of the mass,
and might as well not be present in the earth for any work they may
accomplish either for good or for evil. There is greater probability
of the bulk of these grains and particles being of active service
when they are ground up exceedingly fine; but the clayworker’s idea
of “fineness,” as demonstrated by what passes through an ordinary
clayworking mill, and “fineness” in the sense here intended, are two
totally different things. We mean something that shall render the
particles so small as that they shall only be observable on being
magnified, say, 50 diameters. Hardly any clays used in brickmaking
are in bulk made of such small particles as this; there are a few, of
which the best terra-cotta and porcelain are manufactured, however,
but even these have to be very carefully prepared to exclude grosser
foreign particles. From what we have said, it will be gathered that
the terra-cotta and porcelain manufacturer is at the present time in a
better position to judge of the work done in the kiln or oven than is
the brickmaker. But that is simply a matter of education; the problems
presented to the average brickmaker are rather more complicated than to
the terra-cotta manufacturer, but they may be unravelled on sufficient
application, as we hope to point out.

Even under the most favourable conditions, however--when the particles
composing the mass require a ¼-inch objective for their elucidation--we
find that the best burnt brick is largely made up of them in an
unmelted condition. And we should be very sorry to get rid of them; for
if they disappeared, the stony attributes of the brick would disappear
also, and the general value of the substance would be deteriorated to
such an extent that it would be unsaleable as a building material.
The brick would nearest resemble a form of slag. All we now insist
upon is that in brickmaking a chemical analysis is only useful up to
a certain point, beyond which we must appeal to the microscope to aid
us, and this in conjunction with as perfect a knowledge as possible as
to the behaviour of earths of certain mineral composition when under
the influence of high temperatures. In many instances, the value of the
brick depends almost entirely on incapacity for fusion on the part of a
large proportion of the minerals of which the brick is made. Possibly,
a good all-round brick would be where the bulk of its mineral particles
were infusible at the temperature employed, and when the remainder
were fusible enough to partially run, so as to cement or agglutinate
the infusible particles firmly together. In order to bring about such
conditions artificially, or to arrive at them even approximately, we
must know at least three things, viz.--(1) the nature of the mineral
particles involved in the whole operation; (2) their behaviour
under high temperatures; and (3) a knowledge of certain branches of
metallurgical chemistry. Now, obviously, we cannot undertake to teach
even the spirit of what is involved in these three _desiderata_ in a
small book like this; but we can, and shall, attempt to do something
in that direction, and we must ask the reader’s indulgence to take
for granted observations to be occasionally made, in the inevitable
prospect of our not being able to explain them at sufficient length.

The following are the principal minerals found in clays used in
brickmaking, together with their more important attributes from our
point of view.


KAOLIN.

Pure clay is, theoretically, composed of this mineral alone, but pure
clay does not exist in Nature, except as a mineralogical curiosity.
What is generally called pure clay is a white, or light-grey plastic
material, composed of kaolin with many other substances to a small
degree, from which it frequently has to, as far as possible,
be separated before being put to its highest uses in porcelain
manufacture. Chemically, pure kaolin may be regarded as a hydrous
silicate of alumina, viz.--silica = 46.3, alumina = 39.8, and water
= 13.9. Under the microscope, in reflected light, it is seen to be
made up of extremely minute, thin, six-sided plates, which are said
(doubtfully) to crystallize in the rhombic system; though, when
regarded with the naked eye, one would not suppose that it possessed
a crystalline structure, as it appears to be an earthy, unctuous
substance. It is commonly mixed with grains and small crystals and
fragments of quartz, which mineral will presently be described. Being
derived from the decomposition of felspars, the microscope reveals
the fact that in addition to the six-sided plates alluded to, a great
deal of opaque matter, as particles of mud, occurs in the substance
universally known as kaolin. It is very difficult to satisfactorily
state what this mud is; micro-chemically, its general character may
be brought out. There is no doubt, however, that in converting the
kaolin into china-ware, these particles are more active than the
minute kaolin crystals in uniting with other substances to form a
species of flux. The subject has been investigated to a very limited
extent, but from the foregoing observations it will be seen that the
proportion of amorphous mud particles to the minute crystals must be
an important factor in determining the nature of the fluxing material,
and of the quantity of this latter to be used. Correlatively, the
fusing point can be determined in the same manner. For, in itself,
kaolin is an infusible mineral, and before it can be made use of for
brickmaking, terra-cotta, or any kindred purpose, it must be rendered
artificially fusible by the addition of a fluxing substance. When,
therefore, we learn that kaolin is being used for these purposes, we
know, if used direct as it comes from the pit, that it must be impure
from a mineralogical standpoint, or that it is being mixed with other
substances. We say that kaolin is infusible (refractory); we mean at
any temperature used in the industrial arts, including brickmaking.
With the recent improvements in the electric furnace, the temperature
generated is so high that practically any mineral substance may be
melted; it is hard to speak of anything being infusible.

But the mineral matter called kaolin in ordinary clays, such as the
brown and blue London Clay, the Oxford Clay, “brick-earths,” etc.,
has very little in common with the more or less pure china-clay. The
microscope shows that in the vast majority of such clays scales of true
kaolin are few and far between, that opaque mud particles are more
frequent, and, above all, that pieces of highly decomposed felspar
(called “kaolinised” matter) are present. Eliminating all other and
foreign substances from the clay, the whole of what would commonly be
called kaolin and kaolinised matter, taken together, is of very varied
chemical composition, and might, indeed, be fusible in the ordinary
sense of that term. From this, the reader will perceive that the term
kaolin is very ambiguous and altogether too wide in its meaning. We
think it highly desirable, therefore, to describe kaolin as a true
mineral and not as a rock, reserving the term for the crystalline
plates. The mud particles referred to we may call “kaolinised
particles;” and the highly decomposed felspar “kaolinised matter.” To
sum up the relative fusibility of these substances, _per se_, we should
say that (1) kaolin crystals are practically infusible; (2) kaolinised
particles are either fusible, partly fusible, or infusible, depending
on the actual nature of the particles; and (3) that kaolinised matter
may be difficultly fusible or infusible. A mixture of (1) and (2) may
not be fusible, and could not be unless a great proportion of (2) of a
fusible character, so as to form a flux, were present. The reasons for
this will appear in considering the different kinds of felspar, next to
be described.


FELSPAR.

This mineral, a very common constituent of nearly all clays and
brick-earths is very variable in character, but may be separated into a
number of mineral species, each of which possesses a definite structure
and a more or less constant chemical composition. To show the range
of variation, the following kinds of felspar, with their chemical
composition, may be quoted:--[3]


_Chemical Composition of Felspars._

  -------------+---------+----------+---------+-------+-------
               | Silica. | Alumina. | Potash. | Soda. | Lime.
               +---------+----------+---------+-------+-------
  Orthoclase   |  64.6   |  18.5    |  16.9   |       |
  Albite       |  68.6   |  19.5    |         | 11.8  |
  Oligoclase   |  63.7   |  23.9    |   1.20  |  8.1  |  2.0
  Labradorite  |  52.9   |  30.3    |         |  4.5  | 12.3
  Anorthite    |  43.0   |  36.8    |         |       | 20.1
  -------------+---------+----------+---------+-------+-------

Orthoclase felspar, in addition to the above, frequently has small
proportions of lime, iron, magnesia and soda. Amongst other things it
is an essential constituent of granite, and on the decomposition of
that rock is the first mineral to become affected. When attacked in
the open air by rain and the ordinary agents of denudation, granite
ultimately gives way by the dissolution of the felspar, and on being
removed, the felspathic matter may accumulate in convenient situations
to form kaolin. If we now compare the chemical constitution of
orthoclase felspar with that of kaolin as previously given, we notice
that the potash has disappeared in the decomposing process; it has
been dissolved and taken away by rivulets, and the like, or washed
by rain direct into the sea. We also observe that there has been a
re-distribution, so to speak, of the relative proportions of silica and
alumina--following well-known laws.

Of the remaining felspars the commonest for our purposes is oligoclase,
a mineral found in nearly all British “granites” in a greater or less
degree. That contains a higher percentage of alumina than orthoclase,
and there is a fair proportion of soda and little lime, but much
less potash. The lime-soda felspar, labradorite, and its near ally,
anorthite, are not often met with in a recognisable form in clays.
If present, they are generally as “kaolinised matter,” too highly
decomposed to exhibit their characteristic optical properties.

It is pretty generally stated, and too often assumed by some, that pure
china-clay is derived from the direct decomposition of rocks containing
“orthoclase” felspar. Yet, this cannot really be so, if we reflect on
the mineral composition of many of the rocks, which, obviously, have
yielded the china-clays in question. Take the china-clays of Devon and
Cornwall; they have undoubtedly been derived from the “granites” of
those counties. To some extent, as previously remarked, the orthoclase
is attacked, and provides the material of which china-clay is made. But
in the “West of England,” we have yet to learn that some of the other
felspars are not also involved in the process. If we examine a fresh
piece of granite from the flanks of Dartmoor, or from the neighbourhood
of Liskeard, or St. Austell, we find no difficulty in recognising a
fair proportion of triclinic felspar (one or more of those mentioned
in the table except orthoclase) in it. There is a difference in
the composition (and therefore the commercial applicability) of a
china-clay derived from a rock containing orthoclase alone, and one
from a rock having orthoclase and one or more triclinic felspars
in addition. The latter minerals are more easily decomposed than
orthoclase, especially the lime and lime-soda varieties. We should
not have raised this point only that, by reason of the granites
being to some extent mechanically as well as chemically decomposed,
a large proportion of “kaolinised particles” and “kaolinised matter”
is introduced into certain china-clays, which render them different
in their behaviour under intense heat from those china-clays in which
orthoclase alone has been principally concerned. In other words,
great practical advantages accrue from an accurate knowledge of the
constitution and origin of the china-clays in question. Two clays of
the same chemical composition often behave in a different manner in the
kiln; the cause of this is frequently to be found in the prevalence of
“mechanical fragments” of felspar in one of the clays; and the absence
of these, but the presence of “kaolinised particles” of the same
chemical composition, in the other.

Another point to which we may draw attention is the erroneous
supposition that granites which have yielded china-clay have in all
instances been reduced to the condition in which we now find them by
the action of atmospheric agents of denudation alone. Granites, as a
matter of fact, yield very slowly to the action of the atmosphere,
and taken as a whole no building stone is as durable as they. How
comes it, then, that they have decomposed to such an extent as to
have formed extensive deposits of china-clay in a very short space of
time, geologically? We think the answer is to be sought, at any rate
in some instances, in the alteration the rock as a whole has undergone
in certain situations, whereby it became more easily decomposable.
Take the rotten china-stone of the neighbourhood of St. Austell, for
example. In that material we clearly see a stone from which the “life”
has been sapped, and instead of a bright, sparkling, porphyritic
granite, as it once was, we now notice only a ghost of its former self.
The large orthoclase felspars may be seen in it as skeletons, the
mica is reduced to mere iron-stains (when present at all), whilst the
quartz is also slightly affected. This altered and comparatively rotten
material (although sometimes hard enough to be used as building stone)
extends to an enormous depth from the surface; it has not been bottomed
in some parts of the district. Such an extensive transformation could
not possibly be due to ordinary agents of denudation which do their
work at and near the surface of the rock only. It seems to arise from
an enormous regional alteration, acting underground to an unfathomable
depth, and which may not be unconnected with the mineral veins so
common in, and in the immediate vicinity of the workings.[4]

Yet another thing to be remembered is that, under certain conditions,
as near St. Austell, china-clay has been formed _in situ_, and has
therefore not been deposited by the action of running water, as have
the majority of china and other clays. Mr. Collins remarks that this
china-clay is very irregular in its occurrence. It seems to be formed
of various granite masses decomposed in place; it often occupies
considerable surface areas, and extends to a depth unknown. He remarks
that at Beam mine, and also at Rocks mine, both near St. Austell,
china-clay was found to a depth considerably exceeding 60 fathoms from
the surface. This china-clay, in its natural condition, is very much
the same as china-stone; but the decomposition has proceeded further,
the felspar being completely changed into clay; and nothing more is
necessary for extracting the clay than the disintegration of the whole
mass by a stream of water directed upon it, when the clay is carried
away in suspension and collected at convenient spots. Thus there is
every gradation between the true crystalline orthoclase and triclinic
felspars, through china-stone into china-clay formed _in situ_, so
into china-clay deposited from water by natural or artificial means,
and into a pure clay containing a large proportion of kaolin crystals,
“kaolinised particles” and “kaolinised matter.” But although we can
state that much, a great deal yet remains to be done in connecting
mineral structure with chemical composition of the purer clays, and
in defining the various grades scientifically, in order that full
advantage may be derived from them in a commercial sense.



CHAPTER V.

MINERALS: THEIR BEHAVIOUR IN THE KILN.


THE SILICA GROUP.

Silica, the oxide of silicon, is found in brickmaking clays principally
in two conditions when not combined with other substances: in one of
these the free silica may be crystalline, when it is known as _quartz_;
in the other it may be hard, but not crystalline, as _flint_. We may
consider these in order.

QUARTZ.--When pure this mineral is perfectly white and transparent,
like ordinary window glass. It is exceedingly hard, and this property
is of much service as enabling us by the most elementary examination
to distinguish it from certain other minerals, which it is not unlike
at first sight. One of the latter is calcite, a crystalline form of
carbonate of lime, also white and transparent. Quartz and calcite
behave in a very different manner in the kiln, and as we shall see,
they are both rather common constituents of brick-earth. The difference
in hardness may easily be ascertained by the point of a good steel
knife; the steel will not scratch the quartz, but it will, easily, the
calcite.

When it has plenty of room wherein to crystallise, and is not hemmed
in, as it were, by other hard crystalline matter, quartz often forms
beautiful six-sided prisms surmounted by a six-sided pyramid, and,
rarely, pyramids are found at both ends of a prism. There are no
lines, or “planes of cleavage,” to interfere with the transparency,
either in the extremely minute forms of the mineral as investigated
by the microscope, or in the gigantic crystals occasionally found.
Regular crystals of quartz, although by no means rare in Nature, are
seldom met with entire in brick-earths. The most common form of the
mineral is in irregular aggregates with other minerals, as in the rock
granite, which is composed essentially, as previously mentioned, of
quartz, felspar, and mica. We have traced the history of the felspar
on the decomposition of that rock, and it may now be said that on
complete disintegration of the granite a great part of the quartz
present is simply resolved into fragments and dealt with by rain and
other transporting agents. For quartz is practically imperishable; it
is almost proof against the deleterious acids in the atmosphere, which
so readily attack many other common minerals. In dealing with it, all
Nature can do (at least at the surface of the earth) is to carry the
small quartz grains and pieces about from place to place; She can,
and does, in this process, reduce the quartzose fragments by causing
them to continually knock against each other and against other mineral
fragments and masses until the grains and pieces find a resting place;
She may put them in a mill and grind them to powder, but the quartz is
still there.

Another manner in which quartz occurs in Nature is as filling cracks
in rocks, but this is comparatively unimportant for our present
purposes. The purest quartz is known as rock crystal; but by far the
commonest kinds of the mineral are impure; they may contain iron,
schorl (a black needle-like crystal), and many other minerals. One of
the most interesting points about it, and which undoubtedly in certain
cases is of importance to the brick manufacturer as modifying its
melting properties, is the presence of myriads of extremely minute
so-called cavities, generally filled (or nearly filled) by liquids
of different kinds, the precise nature of which is not as well-known
as it might be, though in some instances it has been determined with
tolerable certainty. In some cases these inclusions are so numerous as
to obliterate the transparency of the quartz crystal, causing it to
present a frosted appearance. The fluids in these cavities may have
beautiful little crystals of other minerals, such as salt, floating
about--but it must be remembered that we are referring to something
infinitely little. These slight differences in the constitution of
minerals, however, have their influence in the kiln. For instance,
although the fluid present is usually water, that often contains carbon
dioxide, which acts as a species of flux to the quartz when present in
sufficient quantity.

In reference to the second form of silica present in brick-earths,
flint, that is of precisely the same chemical composition as quartz,
only that it is not crystalline, nor transparent, though thin pieces
of flint are translucent. Flint is by no means as common in Nature
as quartz; it is very hard, but brittle, and breaks with what is
termed a conchoidal fracture, from the fact that the fractured surface
frequently resembles the external appearance of the shell of a bivalve
mollusc. It occurs in a variety of ways; (1) often as hard lumps or
nodules running along in fairly regular layers in limestone rocks such
as chalk, and (2) occasionally filling up cracks or joints in such
rocks. It is hard to describe its origin in a few words, and we shall
not attempt it; all that need be noted is that it is frequently full of
the remains of extinct organisms of small size, which may, or may not,
constitute an impurity depending on the particular organism and its
present condition. When flint contains a fair proportion of iron it is
called chert--an extremely common constituent of brick-earths in some
localities--though that term refers to other rocks, such for instance,
as those made up almost exclusively of the siliceous spicules (hard
parts made of silica) of fossil sponges.

A more or less crystalline kind of silica is found, forming the
skeletons of minute aquatic plants, and these accumulating to some
depth, constitute the basis of such materials as Kieselguhr and the
diatom earth of the Isle of Skye, both of which, especially the former,
are used for making firebricks.

There is very little to be said concerning the behaviour of free
silica--quartz and flint--in the kiln. It is infusible except at
higher temperatures than are employed by the brickmaker. But, as we
have already remarked, the impurities often present in the minerals
form a species of flux which naturally brings them into the range
of fusible substances, though even then the temperature required is
far beyond what is usually attained in the majority of brickyards,
though it might be frequently arrived at in the manufacture of certain
fire-bricks. For all ordinary purposes, therefore, quartz and flint may
be regarded as infusible. In presence of much lime, iron, or similar
substances, however, both of them are readily melted, and it is part of
the science of brickmaking to know exactly how much lime, &c., to add
to yield the best results. Many brick-earths contain large quantities
of the calcareous and ferruginous substances alluded to, and are then
capable of being made into bricks direct, without any addition. But
although such natural brickmaking earths are frequently employed by the
manufacturer, nearly all of them could be made to yield a better brick
by a little artificial mixing. We must keep urging this point; there is
room for great improvement all round.

As with the majority of comparatively refractory substances, the
size of the grains and pieces of quartz and flint makes a difference
in their readiness to become fusible. The larger the grain the more
difficult it is to break down; fusion commences at the outside of a
quartz grain, the centre of which may at the same time be comparatively
unaffected. By arresting the fusing process, the microscope shows the
outside of the grain to have become softened (so much so as to affect
its doubly refracting properties), whilst the innermost parts still
retain their usual optical characters.


MICA.

The different varieties of mica are important as rock-forming minerals,
but they are not as often met with in brick-earths as is generally
supposed, except in insignificant quantity. Some of the purest clays,
however, contain a great deal of mica, derived almost directly from the
destruction of granite. The two commonest varieties of the mineral are
_biotite_ and _muscovite_.

BIOTITE MICA.--This mineral, usually known as ferro-magnesian mica,
is composed of silicates of magnesia, alumina, iron, and alkalies
in variable proportions. It occurs as six-sided plates or irregular
scales, usually of a bronze-black colour. Biotite weathers with
comparative facility, hence the reason why it is not more commonly met
with in brown and other impure clays.

MUSCOVITE MICA.--This is sometimes called potash- or alumino-alkaline
mica, composed of the silicates of alumina, alkalies, iron, and
magnesia; the proportion of silica ranges from 45 to 50 per cent. It
may usually be distinguished at sight from biotite by its silvery white
or light brown colour. When large enough, both the micas mentioned
may be split up into thin plates, muscovite yielding large transparent
sheets. Compared with all other constituents of brick-earth, the micas
are bright and of semi-metallic lustre. Muscovite is more durable
than biotite, and is much more frequently met with in brick-earths,
especially in the sandy varieties.

The influence of mica in the kiln is not of much importance in ordinary
brickmaking; in general its alkaline character renders it fusible,
though a high temperature is necessary at all times to effect that.
In china-clay mica is regarded as a nuisance, and in breaking down
the material it is separated in the washing process by running water,
the mineral collecting in depressions or basins, called “micas.” When
muscovite contains much fluorine, as it frequently does, it is very
undesirable in clays for high-class purposes. At the best of times the
proportion of iron in mica is sufficient to mar the quality of the
otherwise most excellent clays. In the kiln, or porcelain furnace, the
presence of mica (more particularly biotite) is apt to create yellow
and brown specks, or a species of mottling. It is highly satisfactory,
therefore, to note that these little shiny flakes may be easily floated
off by a moderate amount of care in washing, and thus separated from
the other constituents of the clay.


IRON.

Except in regard to white kaolin clays, nearly all earths used in
brickmaking contain more or less iron, which is usually present as
protoxide in many mineral constituents. The colouring matter of clays
is generally iron in some form, and blue clays weather into brown by
the alteration of that mineral. It is unnecessary for us to consider
the various minerals of the iron group; all we need do is to state the
mode of occurrence of iron oxides in clays and earths, to consider a
variety known as iron-pyrite, and the general effects of ferruginous
minerals in the kiln.

Iron may occur in clays simply as a stain, when it is usually not in
large quantity, or it may occur combined with some mineral or minerals
present--as for instance certain felspars and micas. The brown, yellow,
or blue appearance of the clay is due to it. In loam it may be found
also as a species of ochreous earth, and in thin bedded loams (as the
upper part of the Woolwich and Reading series of the London basin)
each layer frequently varies in the proportion of iron present. In the
more arenaceous parts of these loamy deposits, little grains of iron
sometimes make their appearance, as also in certain sands employed in
brickmaking; on careful examination, however, many of these grains are
found to be other mineral substances coated with iron. Certain horizons
in what are known as the Jurassic rocks contain great quantities of
ferruginous matter in little pellets.

Iron, in large proportion, acts as a flux to other constituents when
the brick-earth is subjected to great heat in the kiln, and on that
account must be carefully watched. But, to the average brickmaker,
the ferruginous constituent is far more interesting as a colouring
medium. At a later stage we shall have something to say concerning the
colouring of bricks, &c., but it may now be remarked that red bricks,
in practically all cases, owe their colour to the effects of firing
on iron. It is a great mistake to imagine, however, that a large
percentage of iron in a clay will necessarily produce a good red tint.
In the first place, a great deal depends on the way the clay has been
mixed or prepared; and in the second, the method of burning and the
temperature employed, taken in conjunction with the general composition
of the earth, are all important. This much may be said, however,
that without the iron (or some mineral colouring matter possessing
similar properties in the kiln) a red brick would not result. An even
colour is the effect of thorough and homogeneous incorporation of
the iron with the brick-earth; that may have been brought about by
natural processes, but it is most frequently obtained in the careful
preparation and mixing of the clays. A very essential point is that
the earths must be of such a character as to withstand the requisite
heat in the kiln without becoming vitreous, or twisting or warping.
It must not be forgotten that a certain proportion of the iron, under
great temperatures, may be carried away out of the kiln in union with
other things, in the form of vapour. To successfully treat a raw earth,
so that all these points may be taken into account, and to produce a
thoroughly uniform red brick, that shall not vary in tint from kiln
to kiln, is a matter requiring considerable skill and attention,
though fairly good bricks of that character have been produced by
sheer accident in burning natural earths fairly rich in thoroughly
disseminated iron oxides.

Two minerals commonly met with in earths used for brickmaking
are pyrite and marcasite, both of which are of the same chemical
composition, namely, iron disulphide. We may first consider them
separately, for they are of great importance to the brickmaker.

Iron pyrite occurs as regular cubic crystals, or irregular streaks,
or as nodules or lumps; in clay, the last-mentioned is its commonest
form. It is a good petrifying medium, so that it is frequently
associated with organic remains, as is exemplified in almost any yard
where stiff clay is being worked. The nodules, on being broken open,
ordinarily exhibit a radiating structure of brassy lustre and extremely
beautiful appearance, though often marred by brown iron stains due to
decomposition of the mineral. In the refuse of slates, now so largely
used in several parts of the world for brickmaking, pyrite is most
frequently found as fine cubic crystals of a durable nature.

Marcasite, on the other hand, crystallizes in a different manner (in
the rhombic system of mineralogists), but is chiefly found in fibrous
masses or dirty-brown nodules, the last-mentioned being common in
clays. When bright it is paler in tint than pyrite, though this is not
a constant character. It occurs abundantly in almost all sedimentary
rocks diffused as minute particles, but sometimes in irregular layers.
Sir Archibald Geikie states[5] that this form of the sulphide is
especially characteristic of stratified rocks, and more particularly
of those of Secondary and Tertiary age. That it is not abundant in
Primary rocks is not to be wondered at when we consider its liability
to rapid decomposition; indeed, for it to be preserved at all it must
be well shielded from atmospheric agents by Nature. Exposure even for
a short time to the air causes it to become brown, free sulphuric acid
is produced, which may attack surrounding minerals, sometimes at once
forming sulphates, at other times decomposing aluminous silicates and
dissolving them in considerable quantity. It plays even a larger part
than pyrite as a petrifying medium, at any rate in the younger rocks.
Both pyrite and marcasite are abundant in many other rocks than those
of special interest to the brickmaker; the former, in fact, is almost
universal in its occurrence.

It will be convenient to consider the behaviour of these two minerals
in the kiln together, as the difference between them from that point
of view is practically _nil_. Under the action of the intense heat
met with there, they become partially decomposed; oxide of iron and
basic sulphides of iron remain. When, at a subsequent period, bricks
containing these substances are exposed to the action of the weather,
oxidation takes place, sulphate of iron and sometimes of lime are
formed, which on crystallizing expand with considerable force and split
or crack the brick. From this it is evident that sulphide of iron
in any form is not to be tolerated in brick manufacture, and if the
earth used in the first place contains much, it must be removed in the
preparing process. If permitted to remain, it is impossible to obtain
either a durable, or a good coloured brick.



CHAPTER VI.

MINERALS: THEIR BEHAVIOUR IN THE KILN (_continued_).


CALCITE, ARAGONITE, &c.

Carbonate of lime may occur in a crystalline form, or as earthy
substances, and many varieties of it are found in clays used by the
brickmaker. The commonest are calcite, aragonite, and a white earth.

Calcite, known also as calc-spar, crystallises in the hexagonal system,
though true hexagons are not very common. It occurs principally as
rhombohedra and scalenohedra, with variations therefrom; also fibrous,
lamellar, granular, compact, nodular, and stalactitic. When pure,
calcite is colourless and usually transparent, but when mixed with iron
or other mineral colouring matter it commonly assumes yellow and brown
tints.

Aragonite is also a crystalline form of carbonate of lime, but is by no
means as common in Nature as calcite. It crystallises in the rhombic
system, which assists the mineralogist to distinguish it from the
last-mentioned mineral, from which it differs also in being harder and
of higher specific gravity. Aragonite may occur as globular masses,
or as incrusting other substances, or in the stalactitic form. It is
sometimes white, but more often yellowish, or grey, and it is not,
commonly, as transparent as calcite, whilst it often possesses one to
two per cent. of carbonate of strontia, or other impurity.

It is generally stated that carbonate of lime, when deposited from cold
solutions, crystallizes in hexagonal (calcite), and when from warm
solutions, in rhombic (aragonite) forms. No doubt, on the whole, that
is the case; but we ought not to forget that many marine organisms
make their hard parts of aragonite, which, under the circumstances, is
certainly not obtained from warm solutions. These crystalline forms of
carbonate of lime are both of them found in fossil shells and the like
in clays, and in not a few instances the calcareous constituent found
in the brick-earth is present almost exclusively in the fossils, which
are ground up with the rest in preparing the material for the moulding
machine.

When present as hard crystalline lumps or pebbles, they have been
derived from the destruction of limestones, and are then the greatest
nuisance imaginable to the brickmaker and the most dangerous
constituent at the same time. With proper machinery these hard lumps
may be ground down to fine particles, but they are even then only to be
admitted into the earth on sufferance. The best plan, without doubt,
is to remove them altogether from the raw earth. They are commonly met
with in what the geologist calls “boulder clay”--a deposit owing its
origin to glaciers and icebergs. Very often the pebbles alluded to
are not crystalline, but of an earthy character, as is the case when
made of chalk. In the semi-dry process of manufacture, it is next to
impossible to incorporate the ground-up particles of carbonate lime
sufficiently well to result in the production of such a homogeneous
earth as is desirable for making a first-class brick.

In sandy clays or loams, and in a few stiff clays used for brickmaking,
certain remarkable concretions called “race” are found, the deleterious
properties whereof are so well known to the average brickmaker that
he carefully avoids the particular strata in which they occur. It is
fortunate that these concretions have a habit of being confined to
narrow limits along definite horizons in the brickyard section, so that
they may be readily discarded in working. But that is not always the
case, and little nodules of “race” are usually more or less frequent
also in the beds above and below the horizons referred to. They are
composed wholly of carbonate of lime, and their general effect in the
kiln, and afterwards, will presently be explained. Other forms of
concretions are known as “septaria,”--tabular or rounded masses of
argillaceous limestone found in practically all stiff clays. These are
often of enormous size, and are disposed in regular lines which the
field geologist takes to indicate bedding planes in the clay--otherwise
often very difficult to make out. In certain stiff clays little pellets
of the same substance are found. The larger septaria have commonly been
cracked in various directions, the fissures being subsequently filled
with calcite.

Coprolites are impure varieties of phosphate of lime, and the term
should, properly speaking, be restricted to a substance of organic
origin,--the fossilised excrement of animals. But the name is now
loosely employed to designate phosphatic concretions in general, such
as are commonly found in stiff clays, in certain “greensands,” and in
other sedimentary deposits. The dark brown phosphate of lime has formed
on and often completely envelopes many fossils; in certain cases it
has in fact been utilised as a petrifying medium, in which form it
ordinarily occurs in the thick black clays of Peterborough, Cambridge,
the gault of Kent, Surrey, etc.

Summing up the effects of carbonates and other kinds of lime in the
kiln, it may be at once said that when present in any other form than
as extremely minute particles, they are distinctly to be avoided. The
small pellets and large pebbles especially are to be avoided, for the
following reasons. Carbonate of lime is made up of lime and carbonic
acid; if a lump of this be subjected to great heat and thus calcined,
the carbonic acid is driven off, escaping by means of flues, the
open chimney, or kiln. The product is lime pure and simple--ordinary
builders’ lime. Everyone knows that on the addition of water builders’
lime becomes “slacked,” and eventually, after a fashion, “sets.”
Precisely the same thing occurs in the brick-kiln. The raw brick is
often composed of pieces of chalk or other limestone, in limestone
districts and in areas where boulder clays are largely employed for
brickmaking. On being subjected to the heat of the kiln these pieces
are promptly reduced to the condition of lime. During the process
of conversion considerable expansion takes place, and subsequently
contraction, leading to the formation of cracks radiating from the
fragments of limestone, the homogeneity of the bricks being at once
destroyed. Apart from this, when placed in the open air the lime
becomes slacked, and the quality of the brick is seriously impaired.

Lime is a highly refractory substance, strongly basic in character,
and forms fusible compounds with silica and other acid bodies. It is,
therefore, useful as a flux in many earths used in brickmaking, being
added to them expressly for that purpose, to the general improvement
of the brick. The celebrated Dinas bricks, for instance, are composed
of a highly refractory earth containing about 97 per cent. silica, the
remainder being lime, oxide of iron, alumina, alkali and water. To
render this material fusible and so as to make refractory bricks, from
1 to 3 per cent. of lime is added.

But what we more particularly desire to draw the reader’s attention
to at the present stage, is not the employment of lime in making
fire-bricks so much as its mixture with ordinary brick-earth, as in the
manufacture of malm bricks. Sometimes the mixture has been effected
by Nature, as is the case with true marls; but the brickmaker does
not care so much for these, as without considerable and expensive
artificial assistance they do not often make readily saleable bricks.
The common practice is, briefly, to grind chalk or similar earthy
limestone in the wet state, and then to introduce it to the brick-earth
with which it is thoroughly incorporated; and there are many ways of
doing this, which we shall not attempt to describe now. The object
of adding chalk to the brick-earth is twofold; in the first place
it assists in diminishing the contraction of the brick on drying,
_i.e._, before burning; and secondly, it acts as a flux in the kiln
by combining with the free silica, or the silicates, in the earth.
Undoubtedly the second is, theoretically, its chief function; but its
beneficial effects in that direction are largely marred by insufficient
burning, whereby a large proportion of the chalk is not actively
engaged, as may be seen on examining the majority of malm bricks with
the microscope. Indeed, the eagerness to save fuel, and to turn out the
bricks as rapidly as possible, often leads to the chalk particles being
utterly useless. And, if we may judge from conversations with several
brickmakers, it would seem that the real reason why the limestone is
used at all is unknown to them, except that it produces bricks of a
saleable colour. This question of colour is the all-predominating one
with most malm brickmakers.

We said just now that the fragments of limestone in the raw brick are
reduced to lime on being burnt; some of the latter, however, as may be
anticipated from our subsequent remarks, is engaged in forming a flux
wherever possible in the immediate neighbourhood of such fragments:
it is the “kernel” that is left which becomes “slacked,” and weakens
the brick. The object of utilising the smallest particles only of
the carbonate of lime is thus obvious; and if it were possible to
use ordinary builders’ lime instead of carbonate of lime, the result
would be better still. The difficulty in utilising builders’ lime is,
of course, its certainty of slacking during the preparation of the
brick-earth with which it would have to be thoroughly incorporated.


SELENITE.

The “petrified water” of the brickmaker. It is a crystalline form of
gypsum--a hydrous sulphate of lime, occurring in large quantities in
the commonest clays used in brickmaking. Large and beautiful crystals,
some of them radiating from a central point, are found in the London
Clay, Kimeridge Clay, Oxford Clay, &c. By expelling the water from
selenite, or gypsum, plaster of Paris may be prepared. In the kiln,
therefore, it is important that this constituent be as finely ground as
possible, so as to localise the effects of the anhydrous sulphate on
being moistened subsequently. In hard burnt bricks, no doubt, a great
deal of it is effectively used as a flux to other constituents of the
clay; but in by far the larger quantity of bricks this sulphate is
reduced to fine powdery particles easily picked out as being softer and
lighter in tint than the remaining constituents. The weather-resisting
qualities of the brick are naturally, not improved when much baked
selenite is present; and the colour of the whole is apt to become
variegated--that is, in a fairly soft brick.


DOLOMITE.

Dolomite is, chemically, composed of the carbonates of lime and
magnesia in about equal proportions. It is found as rhombohedral
crystals, the faces of which are often curved; also in granular and
massive conditions. Its prevailing colour is light yellow both in
crystals and rock masses, but, as with most other minerals, impurities
occasionally make it assume other tints, principally red and green.
Carbonate of iron is frequently present, sometimes to such an extent as
to entirely alter the character of the substance. As separate crystals
dolomite has very little interest for us, though rarely it may take
the place of calcite or aragonite in the fossils of brick-earths and
clays. But in its massive condition, as magnesian limestone, it is of
increasing importance to the brickmaker. For many years it has been
utilised in the manufacture of basic bricks, though at the present
moment the market in these materials is attentively looking at the
possibilities of the next mineral to be described.


MAGNESITE.

Magnesite is pure carbonate of magnesia--that is, magnesia = 47.6, and
carbonic acid 52.4 per cent. It usually occurs massive or fibrous,
but sometimes granular, and its fine rhombohedral crystals are well
known. Like dolomite, its prevailing tint is yellow or light brown,
but, when very pure, is as white as snow. It is usually associated
with serpentine rocks. In the kiln it is highly refractory, and
behaves very much in the same way as lime--forming fusible compounds
with silica and silicates. For the higher grades of basic bricks it
is at this moment largely exploited in the few localities where it
occurs in paying quantities. A few years since, investigation to
determine the best basic refractory material was actively prosecuted
in Germany, and magnesia, preheated at the highest white heat, was
awarded the palm. Magnesite, when calcined, yields magnesia, which,
however, still contains the impurities that might have been present in
the raw material. An average percentage composition of the magnesite
of commerce shows it to contain magnesia 45, carbonic acid 50, lime
1.5, protoxide of iron 1.6, the remainder being silica, alumina,
and protoxide of manganese. The presence of silica in magnesite is
an objection, because it is liable to have a fluxing effect at high
temperatures.

Magnesite has been found in paying quantities in California, Styria,
and recently in Greece. In Eubœa, in the last-mentioned country,
the mineral occurs in lodes which, near Krimasi, are worked on two
levels 30 to 40 feet from the top, and dipping at an angle of about
70 degrees. The general average of the lode gives 88 per cent. of
carbonate of magnesia, and the substance is peculiarly suitable for
the manufacture of basic bricks. A novelty with the raw material is
that the proprietors sell either by guaranteed degree, or degree of
analysis, the former being 95 per cent. of pure magnesia, whilst the
latter often gives as much as 97.8 per cent. In inferior grades the
principal increase is in the proportion of silica.


SALT.

Chloride of sodium, or common salt, is present in many natural clays,
especially (in England) in that formation known to geologists as the
Trias, developed largely in Cheshire. The influence of a salt-bearing
bed is, naturally, not confined to the immediate vicinity of the
formation; salt being so readily soluble in water, it comes forth from
the rocks in springs, which, flowing over loams and other similar
absorbent earths, impart a saline character to them. In this manner
otherwise useful earths for brickmaking are rendered absolutely unfit
for the purpose. Salt is one of the most powerful fluxes known; when
mixed even in very small quantities with clay it becomes impossible
to make a good brick of the substance. But we must recur to this
matter at a later period in another connection. The fluxing property
is sometimes taken advantage of by mixing salt with sand in moulding,
or in employing a sand already saline, as when dredged from the sea,
or obtained between tide-marks. A species of glaze is produced on the
brick by the action of such moulding sand.

We may ignore the presence of a number of minerals such as rutile,
augite, and hornblende in brick-earths, as they only exist therein in
such small proportion, and have no appreciable effect in the kiln.



CHAPTER VII.

THE CHEMISTRY OF BRICK-EARTHS.


Introduction: THE BLOWPIPE.

It is not our intention to write an elementary treatise on chemistry;
but we know it is the custom for brickmakers to have chemical analyses
of their raw earths made, and we are aware also that the precise
meaning to be attached to these analyses is very little understood. Our
principal aim in introducing this subject, then, is to interpret, in an
elementary manner, certain typical analyses of earths and substances
used in brickmaking; but before doing so we shall explain some easy
methods of examining earths by means of the blowpipe, which will not
merely give some insight into their chemical constitution, but will
afford the intelligent brickmaker a means of investigation which he can
himself put into practice.

The results of a chemical analysis of a compound earth, as ordinarily
used by the brickmaker, widely differ from those obtained by a
mineralogical or petrological examination. The petrologist views
the earth as a mineral aggregate, the constituents of which may be
ascertained on appeal to a properly-constructed microscope--that is, in
the majority of instances. By noting the relative proportions of the
different minerals, he is enabled to state, with approximate accuracy,
what is the ultimate chemical composition of the whole. From this it
would appear that a rough chemical analysis could be drawn up by the
petrologist without having recourse to the ordinary methods of chemical
investigation. And in a limited sense that is true. But we should not
lose sight of the fact that there is, in too many cases, an amorphous
residuum in earths, the nature whereof the microscope is powerless to
reveal. It is upon this remnant that the chemist should direct his most
careful attention.

The mineralogist also can give a shrewd idea of the chemical
composition of a brick-earth by using a blowpipe and accessories. This,
in fact, may be regarded as a chemical means of investigation; but
it possesses this serious drawback, viz., the blowpipe only yields a
qualitative, and not a quantitative analysis. In other words, it can
tell us something concerning chemical compounds present in an earth,
but rarely informs us as to the relative proportions of them. Even
this, however, is of great service in many instances, though it does
not possess the value of a quantitative analysis. For example, we
have stated previously that certain ingredients are very undesirable
in a brick-earth, even in minute quantities; and that fact becomes of
increased value if we extend the field to earths used in terra-cotta,
and china and porcelain manufacture. Now, the blowpipe is a handy
instrument; it may be carried about by the prospector with its usual
accessories, and occupies but little space. Suppose he discovers a bed
of white earth which he believes to be good china-clay; he can prove
that fact, or at least obtain a great deal of information to that
end, by the mere use of that useful little instrument. Knowing, for
example, that fluorine is an undesirable constituent in such a clay for
many high-class purposes, he might test first of all for that; iron,
perhaps, may come next, and so in a few minutes he is enabled to arrive
at some valuable particulars that would take much longer to obtain by
chemistry in the wet way.

It will be profitable, therefore, for us to briefly describe the
blowpipe and the most common of its accessories, stating results
obtained in dealing with substances frequently met with in
brick-earths. With but little practice anyone can use the instrument,
though, as with most other methods of scientific investigation, it
requires expert knowledge to yield really excellent results. The simple
minerals and compounds to which we shall direct attention may be
detected with the greatest ease.

The essential constituents of a blowpipe outfit are as follow:--

  1. Blowpipe.
  2. Lamp.
  3. Platinum-pointed forceps.
  4. Platinum wire.
  5. Charcoal.
  6. Glass tubes.
  7. Chemical reagents.
  8. Miscellaneous articles.

[Illustration: Fig. 5.--Blowpipes.]

_1. The Blowpipe._--Common forms of blowpipe are shown in fig. 5. A
may be described as follows. It consists of three separate parts: a
tube _a b_ having a mouthpiece; an air chamber _c_ to retain moisture
caused by the breath of the person blowing; and a side tube _d_ ending
in a platinum-tipped jet. Another form of blowpipe, which, however,
does not differ essentially from that just alluded to, is shown in
fig. 5, B. It is not absolutely necessary to have the jet made of or
tipped with platinum, though certain examinations with the instrument
are facilitated by the use of such a tip. An essential point is, that
the hole in the jet should be of proper size, usually about 0.4 mm. The
trumpet-shaped mouthpiece shown in the diagram may be dispensed with.

[Illustration: Fig. 6.--Blowpipe Lamp, &c.]

_2. The Lamp, or Candle._--A convenient form of lamp is a Bunsen
gas-burner furnished with a special jet (fig. 6, A). For certain
purposes, however, this flame cannot be employed, as when testing a
substance for sulphur, as coal-gas frequently contains sufficient
sulphur to vitiate results. Moreover, in country districts and in
the field coal-gas is not always procurable. A convenient form of
lamp, though rather too large for transporting purposes, is known as
Berzelius’ blowpipe lamp. This, as improved by Plattner, is shown in
fig. 6 B. This consists of an oil vessel on a stand provided with
two openings closed with screw-caps, the one opening being used for
charging the lamp with oil, the other being fitted with a burner
bearing a flat wick. The lamp may be adjusted to any required height
on the stand by means of a screw. Olive oil, or refined rape oil,
is usually burnt. A spirit lamp with a flat wick is sometimes used.
In countries where neither coal-gas, alcohol, nor oil are readily
available, the prospector may use a small grease lamp. This consists
of a cylindrical box of thin metal having a wick-holder soldered on
one side, through which a flattened wick is drawn. The box may then be
filled with grease, solid paraffin, old candle-ends, or fat of similar
description. Professor Cole describes[6] it as follows:--When brought
into use the wick is lighted, and the flame directed with the blowpipe
upon the surface of the solid tallow or fat, until this is melted to
a depth of about a quarter of an inch. The lamp will then become hot
enough during use for a continuous supply to be maintained; but it is
still better to hold the lamp with the pliers over a spirit lamp until
all the contents become fluid. When about half or three-quarters empty,
it is well to drop in extra lumps of fuel--a single candle-end or
so--during use, and this additional material becomes melted up slowly
with the rest. The wick must be freely supplied with fluid fuel, or it
will char and waste away. If the lamp is kept sufficiently hot, the
wick will not require raising during a day’s work; but it can be easily
thrust up with a knife point after the flame has been at work a few
minutes. A cylindrical cap fits down upon the lamp when put aside. For
many ordinary purposes a good carriage-candle may be employed to give a
blowpipe flame, but candles have the disadvantage of not remaining at a
constant level--an important point when one is comfortably at work.

_3. Platinum-pointed Forceps._--At least one pair of forceps is needed,
and it should preferably be made of steel, nickel-plated to prevent
rusting. One end has platinum points self-closing by means of a spring,
so that the piece of mineral to be heated, placed between them, may
be firmly supported. At the other end are other forceps of ordinary
pattern for picking up small fragments; this end, however, should never
be placed in the flame. A pair of common self-closing forceps might
also be at hand for holding test-tubes, etc., in the flame.

_4. Platinum Wire._--A few inches of thin platinum wire are
indispensable, and lengths of an inch or so may be fixed into suitable
handles. A convenient method is to have a small glass rod for a handle,
and by fusing the tip of one end of the rod the glass may readily be
made to hold the piece of wire. Pieces of platinum foil are useful,
also, as will presently be seen.

_5. Charcoal._--The outfit should comprise several pieces of charcoal,
and a convenient form for each piece is a circular disc about an inch
in diameter, flat at the top and convex beneath. Long prisms of the
same material, square in section, are occasionally required; these may
be up to 6 inches, or so, in length.

_6. Glass Tubes._--These should be of hard glass, small, of several
diameters, the bore being large enough to place fragments of minerals
or earthy substances within. Closed tubes, such as test-tubes, are
always requisite.

_7. Chemical Reagents._--These are, for the most part, used as fluxes,
and those most commonly employed are borax (sodium tetraborate),
soda (sodium carbonate), and salt of phosphorus or microcosmic salt
(phosphate of soda and ammonia). Small quantities of potassium
bisulphate (in a glass bottle), as also small bottles of hydrochloric,
nitric, and sulphuric acids, and a solution of cobalt nitrate, are also
useful in certain cases. It is hardly necessary to remark that the
chemicals employed must be of the highest degree of purity.

_8. Miscellaneous Articles._--Strips of test paper, both turmeric and
blue litmus, a small hammer, a steel anvil about an inch cube, a bar
magnet, a pair of cutting pliers, a three-cornered file, and a few
small watch-glasses are very desirable, though not absolutely essential.

The reader, on glancing at the foregoing formidable list of articles,
may possibly imagine that some considerable outlay is requisite, and
that they must occupy much space. But that is not the case. An ordinary
blowpipe, a grease lamp, a small spirit lamp, and all the articles
mentioned in paragraphs 3 to 8, both inclusive, occupy but a small
space. They may be packed in a box specially fitted, and one in the
writer’s possession, containing all of them, measures only 10 inches by
5 inches by 3¼ inches, and is less than 3 lbs. in weight.

Now, as to the use of these various things. First of all, let us
examine the flame, as produced by a candle, which is typical of flames
obtained by other means described, except the Bunsen lamp. A candle
flame (see fig. 7) consists of the following parts:--

1. A dark core (_a_), which contains the gaseous products of
decomposition given off by the melted tallow drawn up by the wick.

2. A highly luminous zone (_b_), in which only partial burning of the
combustible gases takes place. In this, oxygen from the air combines
chiefly with the combustible hydrogen, whilst the carbon is separated
in a highly heated state, which causes the luminosity.

3. An outer mantle of blue tint (_c_), where the oxygen of the air is
always present in excess, so that the separated carbon is here burnt.
The highest temperature is found in this part of the flame.

[Illustration: Fig. 7.--Candle and Gas Flames.]

Technically, the outermost zone (_c_) is known as the _oxidising_
flame, and the inner luminous zone (_b_) the _reducing_ flame. The
two portions of the candle flame act in different manners on specific
mineral substances, and the blowpipe operator may use either of them
at will. The method of doing this is illustrated in the same figure.
To obtain the reducing flame, the blowpipe jet is brought to the edge
of the flame a little distance above the burner, or wick. The operator
then produces a gentle blast, which deflects the latter (upper figure)
without altogether passing into it, so that the flame is still charged
with glowing carbon. A yellowish luminous flame is the result, the most
active part of which lies at a short distance from the end.

On the other hand, the oxidising flame is utilised by passing the
blowpipe jet a little farther into the flame (lower figure) and blowing
more strongly. A pointed non-luminous flame is the result. This will
be seen to possess an inner blue cone, before the point of which the
hottest part is situated. Substances to be fused are placed in this
part of the flame, whilst those to be oxidised are placed a little
farther away, in order that they may be exposed to the air at the time
they are being highly heated.

The “platinum wire” is an absolutely indispensable adjunct to a
blowpipe outfit, and is employed as follows:--A short piece of
the wire, an inch or so in length, being attached to a handle, as
previously described, the free end of it is bent into a loop about the
size of this O. This may be heated in the flame employed, or, better
still, in the flame of a spirit lamp, and, when hot enough, it may be
dipped into a small quantity of the powdered borax or microcosmic salt,
some of which will be found to adhere to the wire. On further heating
the borax it will swell out and form a number of irregular bubbles,
which (heat still being applied) will subsequently settle down into a
clear, colourless bead in the loop of the platinum wire. A satisfactory
bead having now been made, a portion of the mineral substance to be
analysed (in the shape of small grains) is taken up by dipping the
heated borax bead therein.

The actual operation of determining the nature of the substance then
commences. Using the blowpipe, and directing either the _reducing_
flame (R.F.), or the _oxidising_ flame (O.F.), on to the substance on
the borax, according to circumstances presently to be detailed, the
operator notes the change in colour (if any) of the flame yielded by
the process. At this point a very annoying thing sometimes happens;
for, in liquefying the borax bead, it is apt to fall off the wire, and
another bead has then to be made. To avoid this, great care should
be taken not to blow too vigorously at first. With the microcosmic
salt especial care and dexterity must be exercised in this connection.
If all goes well, the powdered mineral substance (if fusible in the
borax) readily melts down, and becomes incorporated with the borax. On
permitting the latter to cool, which it very rapidly does, the bead
should now be carefully examined, and any change in tint noted. Most
beautiful transparent colours, pregnant with meaning, are often seen to
have formed with the borax as flux.

The operator may test his skill by making the following brilliant
experiments. Take up a few small fragments of the mineral malachite (a
carbonate of copper) by means of the clear, colourless, heated borax
bead, and then introduce them to the _oxidising_ flame. They slowly
dissolve in the borax, and, whilst doing so, the tip of the blowpipe
flame becomes emerald-green in colour. After applying this flame for a
minute or two, the whole of the mineral will have become incorporated
with the borax, and, when the bead is still hot, note that it is also
of a rich green tint, but that, on cooling, it turns blue. If too much
malachite has been taken up in the first instance, a very dark green
tint is imparted, which still remains when the bead is cold, and it
appears to be quite black. Its true colour, however, may be ascertained
by flattening the bead out before it is quite cold. It is always well
to begin by using a small quantity of the mineral substance at first,
and adding to this as may be required.

Assuming that a fine rich green bead has been produced, and that it
contains a relatively large amount of copper, the operator may now
hold it in the _reducing_ flame and re-melt the bead; if the operation
has been conducted carefully, the bead will then show red, and be
practically opaque when cold. The red bead may now be re-heated in the
_oxidising_ flame, when it will be found once more to return to a green
colour. The student will find this easy operation excellent practice,
as proving to him, in the absence of a demonstrator, that he is really
able to recognise and use the oxidising and reducing flames at will.
Many mineral substances yield a distinctive colour in this way--a
useful factor in a qualitative analysis.

Before using the platinum wire, be careful to ascertain that it is
quite clean; a borax bead made thereon should be perfectly white and
transparent.

The “platinum foil” is employed as a support during fusions; pieces
about an inch and a half long, by half an inch in width, are generally
used. A small platinum spoon is sometimes adopted when fusing
substances with acid, potassium, sulphate, or nitre.

Minerals may be tested to see whether, in the ordinary blowpipe flame,
they are fusible or not. To do this, a fragment of the substance to be
tested is held in the flame by means of the “platinum-pointed forceps.”
If the mineral is found to be fusible, then its “degree of fusibility”
may be ascertained according to the following table. The “degrees of
fusibility” are six in number:--

  1. Fusible in ordinary gasflame, even in large fragments. Example:
     _Stibnite_, or grey antimony.

  2. Fusible in fine, thin pieces, in the ordinary gasflame, and in
     larger fragments in the blowpipe-flame. Example: _Natrolite_, a
     hydrous silicate of alumina and soda.

  3. If very thin splinters be used, fusible without difficulty with
     the blowpipe-flame. Example: _Almandite_, or iron-alumina-garnet.


  4. In thin splinters fusible to a globule. Example: _Actinolite_, a
     non-aluminous variety of hornblende.

  5. Thin edges may be fused and rounded without great difficulty.
     Example: _Orthoclase_ felspar--already described.

  6. Fusible with great difficulty on the finest edges. Example:
     _Bronzite_, one of the augite group of minerals.

Now, it is highly probable that many of our readers will not
understand, or be able to recognise the six minerals above enumerated;
and we recommend those who may be sufficiently interested, to purchase
them from a mineral dealer--such as Damon, of Weymouth, or Russell, or
Gregory, or Henson, or Butler, in London. A set, comprising the six,
should cost from two to three shillings. With these, as a standard
for comparison, the operator readily grasps the method of assigning a
fusible mineral to its proper degree in the scale.

Another object of examination in the forceps is to see what colour
(if any) is imparted to the flame by the divers minerals experimented
upon. It is a good rule not to permit the specimen, when being fused,
to touch the forceps in the neighbourhood of the actual part fused. For
a mineral containing antimony or arsenic would tend to form a fusible
alloy with the platinum points, and so ruin the forceps.

The pieces of “charcoal” alluded to in our inventory, are used for
placing the mineral substance upon in certain parts of the blowpipe
operation, which may be briefly described. Essentially the charcoal
forms a support to the substance during fusion; but the glowing carbon
has also a kind of reducing effect. Taking a long prism of charcoal,
such as that described, page 63 _ante_, the mineral to be dealt
with should be placed near one end of a flat surface and the prism
so held that the flame from the blowpipe, will sweep down its full
length. The object of so doing is to give a chance to any volatile
substance (derived by the operation from the mineral) to deposit on
the comparatively cool surface, which deposit is often indicative of
the chemical nature of the mineral. To carry this point home, the
following experiments may be conducted by the student. Taking a piece
of _stibnite_ (sulphide of antimony), which, as we have just learnt,
is a most fusible mineral, we place it on the charcoal in the manner
indicated. Whilst melting, and the blowpipe flame be continued to be
directed upon it after it has become fused, it will be noticed that a
yellowish-white deposit is taking place on the length of charcoal; this
is called a _sublimate_.

Mineral substances may also be assisted in fusing on the charcoal by
using the reagents described in our list of chemicals, &c., included in
a blowpipe set.

In regard to the use of the “glass tubes,” it may be remarked that they
are used principally for the examination of minerals which yield a
volatile substance on being heated therein, and to detect the presence
of water and the like. It is important to make a distinction between
the closed and the open tubes. When a mineral fragment is placed in a
tube, closed at one end, whatever takes place will be in presence of
very little air, or oxygen; on the other hand, when the tube is open at
both ends, and is inclined during the experiment, a constant stream of
oxygen passes through the tube, and the mineral is being dealt with in
presence of that. The employment of this oxygen makes a great deal of
difference in the results obtained, as a few elementary experiments
will show. If we place a piece of sulphur in a tube, closed at one
end, and heat it gently, we notice that a yellow coating takes place
inside the tube; but if we now employ a tube open at both ends and
heat it very slowly indeed, we notice that the sulphur goes off as an
invisible gas, and if the experiment has been properly conducted, there
should hardly be a trace of the sulphur left on the glass. A number of
experiments of a similar nature might be quoted, but enough has been
said for the present to show the utility of the tubes.

The “chemical reagents” alluded to have already been sufficiently
described to render any further discussion on them unnecessary for our
immediate purpose.

In regard to the “miscellaneous articles” mentioned, it may be remarked
that the test papers are employed in the detection of certain acids
and bases; whilst a strip of brazil-wood paper is for the detection
of fluorine. The hammer and anvil are for breaking the substance to
be tested into small fragments; the magnet for withdrawing particles
of iron from the pulverised material; the three-cornered file for
assisting in determining the relative hardness of minerals, &c., &c.

In examining substances before the blowpipe, it is highly desirable
that the various operations should be carried out in some definite
order. The following has been found convenient:--

  _a._ In a glass tube closed at one end.
  _b._ In an open tube.
  _c._ On charcoal.
  _d._ With borax and microcosmic salt.
  _e._ As to flame colouration.
  _f._ With other reagents.

The size of the fragment to be dealt with in an examination, depends
on circumstances, but for ordinary purposes a piece of the size of a
small rabbit-shot will be found sufficient.

It is convenient in this place to describe a few chemical reactions
without the use of the blowpipe; that will render the effects on
certain minerals, presently to be mentioned, clearer to the reader.

In the first place it may be ascertained whether the mineral is soluble
in water, and if so, to what extent. Then as to whether it becomes
soluble in certain acids, such as hydrochloric or nitric acid. The
former acid is generally used, except for metallic sulphides, and
those minerals containing heavy metals, such as lead, silver, &c.; the
latter is employed for the exceptions named. Several minerals, even
when in a powdered state, are hardly, if at all, affected by acids. The
results to be noted during the test with acids, commonly fall into the
following three groups.[7]

A. The mineral may dissolve quietly with or without colouring the
solution; this holds good, for example, with hematite (a variety of
iron), also of many of the sulphates and phosphates.

B. There may be a bubbling off or effervescence of a gas, which gas is
usually carbon dioxide; but may be hydrogen sulphide. Chlorine may be
liberated, or reddish fumes of nitrogen.

C. There may be separation of some insoluble substance as sulphur,
silica, &c.

We will close this chapter by stating the behaviour under blowpipe
examination of various minerals, given in preceding pages, as being
common in clays and earths used in brickmaking:--

_Quartz._--This is infusible, and remains undissolved, even in a
microcosmic salt bead; but it fuses readily with soda, on charcoal.
In the flame it splinters into fragments, which fly off with great
rapidity. It is soluble in hydrofluoric acid. _Flint_, when pure,
behaves in a similar manner.

_Orthoclase Felspar._--Fusibility, 5; flame colouration brilliant
yellow, when much sodium is present; not decomposed by hydrochloric
acid. It may be distinguished from other common felspars by its high
degree of fusibility.

_Oligoclase Felspar._--Gives a sodium yellow flame; fusibility, 3.5;
not decomposed by hydrochloric acid.

_Biotite Mica._--With fluxes gives a strong iron reaction of yellowish
red colour; decomposed in concentrated sulphuric acid, leaving a
residue of siliceous matter.

_Muscovite Mica._--When heated in a tube closed at one end, yields
water which often gives fluorine reaction with brazil-wood test paper
by colouring it straw-yellow; it is not decomposed by acids, and
whitens and fuses only on thin edges.

_Kaolin._--Is infusible; gives off water when heated in a closed
tube; and with cobalt nitrate on charcoal, a fine alumina reaction is
obtained.

_Aluminium._--On charcoal, this becomes blue with cobalt nitrate,
though if the surface is fused the reaction is not so clear. Prof. Cole
advises that the soda-residue be dissolved in dilute hydrochloric acid,
then evaporated to dryness, re-dissolved in that acid water, filter
off any silica, and neutralise with ammonia; alumina is precipitated
together with any iron present. The precipitate, if white, or nearly
so, may be tested with cobalt nitrate, and the result is a fine blue
colour.

_Limonite Iron._--Fusibility about 5; yellow and reddish beads;
water given off in closed tube; in reducing flame magnetic residue on
charcoal; soluble in hydrochloric acid after a short time.

_Iron Pyrites._--Fusibility about 2; yellow and red beads; in closed
tube yellow precipitate due to sulphur; magnetic after reduction on
charcoal; insoluble in hydrochloric acid.

_Rock Salt._--Intense yellow sodium flame; fusibility about 1;
microcosmic salt with copper oxide shows strong chlorine reaction--a
fine blue flame surrounding the bead when re-introduced into the flame.
It is soluble in water.

_Selenite_ (_Gypsum_).--Fusibility about 2.5; brilliant flame; in
closed tube it becomes white and opaque and much water is given off;
with soda, on charcoal, sulphur reactions are obtained; soluble in
hydrochloric acid.

_Calcite_ (_Carbonate of Lime_).--Flame glows very strongly; infusible;
effervesces freely in cold hydrochloric acid.

_Dolomite._--Flame, with hydrochloric acid, like calcite; infusible;
effervesces in hot hydrochloric acid.

_Magnesite._--Infusible; with cobalt nitrate a fair magnesia reaction
on charcoal, _i.e._, turns into a dull pink; effervesces in hot
hydrochloric acid.

_Manganese._--With borax in oxidising flame a red-violet bead is
obtained, but with the reducing flame it is colourless.

The above are commonly met with in brick-earths; for other minerals
and substances also found, the reader may be referred to special works
dealing with blowpipe analysis.



CHAPTER VIII.

THE CHEMISTRY OF BRICK-EARTHS (_Continued_).


In this chapter we shall fulfil our promise (_ante_ p. 58) to explain
in an elementary manner the precise meaning of ordinary commercial
chemical analyses of some typical earths used in brickmaking, etc. We
may commence by explaining a few terms used by the chemist.

An _atom_ is the smallest imaginable portion of matter, and all matter
is said to consist of atoms. A _molecule_ is the smallest conceivable
combination of atoms, and every compound substance is ultimately built
up of molecules. An _element_ is a substance that has hitherto defied
the efforts of the chemist to subdivide or split up. Over seventy of
these elementary substances are at present known, and their number is
being constantly added to. Again, by improvement in analytical methods,
a so-called element may be subdivided, and thus removed from the list.
The elements are classified into _metals_ and _non-metals_; and it is
convenient to give each of them a symbol to save trouble in writing,
and to render clearer to the reader the chemical nature of a compound
body. Thus, the symbol for the element aluminium is Al; for silicon Si;
for carbon C; for calcium Ca; for oxygen O; for iron Fe; for hydrogen
H; for chlorine Cl; and so on.

We are taught by chemistry that elements are capable of combining only
in definite proportions, and that each substance possesses a definite
proportion peculiar to itself. That proportion is called the _atomic
weight_ of the element; or, it is the relative weight of the atom of
each substance compared with that of the lightest substance known,
hydrogen.

Thus, the atomic weight of hydrogen being taken as 1, it is found that
an atom of chlorine is 35.5 times as heavy as that, so that the atomic
weight of chlorine is said to be 35.5. Now, in spite of the enormous
difference between the weight of the two elements just mentioned, they
combine in the same proportions by _volume_; and the union is known as
hydrochloric acid, or HCl.

But in certain cases elements do not combine in equal proportions;
for instance, an atom of oxygen will not combine with less than two
of hydrogen. Further, with this we find that the three volumes are
condensed into the space of two volumes--a very common phenomenon in
the chemical combination of gases. The union of hydrogen and oxygen
alluded to forms water, the chemical symbol of which is, consequently,
H2O.

_Chemical affinity_, or _chemical attraction_, is the force which is
exerted between molecules not of the same kind. Thus, in water, which,
as we have seen, is composed of hydrogen and oxygen, it is affinity
which unites these elements, but it is cohesion which binds together
two molecules of water. In compound bodies, cohesion and affinity
operate simultaneously; whilst in simple bodies, or elements, cohesion
alone has to be considered. To affinity are due all the phenomena of
combustion and of chemical combination and decomposition.

Certain gases, such as chlorine and nitrogen, and such substances
as sulphur, carbon, and silicon, with many others, form _acids_ in
conjunction with hydrogen, or hydrogen and oxygen. These combine
with greater or less facility with other elements which do not form
acids, and are termed _bases_. A combination of an acid and a base is
known as a _salt_. Salts the names of which end in _-ide_, such as
chloride, sulphide, etc., are combinations of a metal with a non-metal.
_Monoxide_ means an oxide containing one atom of oxygen; _dioxide_ one
containing two atoms; _protoxide_ means the first oxide, because it
is the first or lowest of the oxides of the given metal in amount of
oxygen present; the highest oxide is often known as _peroxide_. The
terminations _-ous_ and _-ic_ are frequently used for the lower and
higher oxides respectively. Examples:--

  FeO, iron protoxide, or ferrous oxide.
  Fe2O3, iron sesquioxide, or ferric oxide.
  FeS2, iron disulphide.
  Sb2S3, antimony trisulphide.

The following symbols may be indicated as referring to compounds
especially met with in brick-earths:--

  CaO, lime, instead of calcium oxide.
  Al2O3, alumina, instead of aluminium trioxide.
  SiO2, silica, instead of silicon dioxide.
  Na2O, soda, instead of sodium oxide.
  K2O, potash, instead of potassium oxide.
  MgO, magnesia, instead of magnesium oxide.

In _analysing_ a body, the first step consists in determining the
nature of the elementary substances contained therein. That may be
accomplished in the _dry way_ by means of the blowpipe and accessories,
as explained in the last chapter. Such an examination, as previously
remarked, is known as a _qualitative_ analysis. Or, it may be
accomplished in the _wet way_ by ordinary chemical examination. The
next step is to determine the amount of the constituents present, and
that is known as a _quantitative_ analysis. In making a qualitative
analysis, the chemist is assisted by the knowledge that certain basic
substances and certain acids produce peculiar phenomena in the
presence of known substances or preparations termed _reagents_.

There is a great difference between a _chemical compound_ and a simple
_mixture_ of elements; and it is not always easy (_e.g._, some alloys)
to say whether a substance is in the one state or the other. This
distinction is well exemplified by the air we breathe. The chemist
finds by analysis that the air is nearly constant in composition,
containing essentially in 100 parts 76.8 by weight of nitrogen
(including about 1 per cent. of the recently-discovered element,
argon), and 23.2 of oxygen. Small proportions of water vapour, carbon
dioxide, etc., may be ignored for our present purposes. In view of this
comparatively uniform composition, the question at first arises as to
whether the air is, or is not, a chemical compound? The answer is in
the negative, for, amongst other things, it can be shown that the ratio
of 76.8 to 23.2 is not that of the atomic weights of the two elements
present, viz., 14 : 16, nor of any simple multiples of these.

We will now quote a few analyses of well-known earths, and explain each
in turn:


_Chemical Composition of China-clays._[8]

  -----------+---------+---------------+-------------
             | Kaolin. |Kaolin average.|Sandy Kaolin.
             +---------+---------------+-------------
  Silica     | 46.32   |    44.60      |    66.68
  Alumina    | 39.74   |    44.30      |    26.08
  Iron oxide |   .27   |      .20      |     1.26
  Lime       |   .36 } |     1.60      |      .84
  Magnesia   |   .44 } |               |    trace
  Water      | 12.67   |     8.74      |     5.14
  -----------+---------+---------------+-------------

The kaolin alluded to in the first column is a remarkably pure
material, perfectly white, and contains an enormous quantity of
water. It refers to one of the finest washed china-clays in the
market, and is extensively used in porcelain manufacture. It is
quoted here principally to give an idea of what a really pure clay is
like chemically. We notice that, in spite of its relative purity, it
contains .27 per cent. of iron oxide. This could have been well done
without, from the manufacturer’s standpoint, but is of course a very
minute proportion. Small as it is, it must exert a slight amount of
colouring influence. The lime and magnesia are present in slightly
larger proportions, and a little more of either would be advantageous
rather than otherwise, as assisting to flux the material. This is an
earth with which practically anything may be done by judicious blending
and careful preparation.

With reference to the second column, the figures do not refer to any
particular clay, but they have been compiled to show the average
composition of kaolins as used in the market. It will be observed that
the silica and alumina are present in approximately equal proportions,
which is a characteristic of fairly good china-clays. The iron oxide
remains as before, but there is a larger proportion of lime and
magnesia--as much as can be permitted except in a second-rate clay.

The evidence of the third column shows that the sand in the china-clay
is to a large extent quartzose, and this is at the expense of the
alumina. Such a material would be suitable for making a species of
white fire-brick, and it might do for the commoner kinds of china-ware.
The earth is really of the nature of a loam--a sandy clay. There is
too much iron in it for the production of perfectly white goods. The
proportion of lime might be increased to advantage.


_Chemical Composition of Fire-clays from Newcastle-on-Tyne._[9]

  ------------+-------+-------+-------+-------+-------+-------+--------
              |   1   |   2   |   3   |   4   |   5   |   6   |    7
              +-------+-------+-------+-------+-------+-------+--------
  Silica      | 51.10 | 47.55 | 48.55 | 51.11 | 71.28 | 83.29 |  69.25
  Alumina     | 31.35 | 29.50 | 30.25 | 30.40 | 17.75 |  8.10 |  17.90
  Iron oxide  |  4.63 |  9.13 |  4.06 |  4.91 |} 2.43 |  1.88 |   2.97
  Lime        |  1.46 |  1.34 |  1.66 |  1.76 |}      |       |}  1.30
  Magnesia    |  1.54 |   .71 |  1.91 | trace |  2.30 |  2.99 |}
  Water, etc. | 10.47 | 12.01 | 10.67 | 12.29 |  6.94 |  3.64 |   7.58
  ------------+-------+-------+-------+-------+-------+-------+--------

The reader will see at a glance that the range of variation permissible
in fire-clays is very wide. These earths are all found close together,
and are utilised for similar purposes, though often blended to produce
desired results. It will be noticed that one of them (No. 6) contains
as much as 83.29 of silica, whilst another has no more than 47.55 per
cent. The range with reference to alumina is very wide also, from 8.10
percent. (No. 6) to 31.35. The refractory character of any sample of
fire-clay is determined by the proportions in which the silica and
alumina are contained, and by the absence of lime, iron, and other
easily fluxible substances. The proportion of iron discovered in sample
No. 2 is certainly much in excess of the requirements of the material,
as a fire-clay, and this no doubt is tempered by admixture, unless
utilised for inferior goods. The iron oxide in the other samples is
about sufficient for general purposes. The amount of lime present in
all the samples constitutes a good feature; much lime cannot on any
account be allowed in earths for fire-clay goods. With so much iron
present, and the fair proportions of magnesia (except in sample No.
4) these clays may be regarded as typical, with the exception of
No. 6. They have been utilised for many years in the manufacture of
fire-bricks and the like.


_Chemical Composition of Fire-clays, from Welsh localities._

  ------------+-------+-------+-------
              |   1   |   2   |   3
              +-------+-------+-------
  Silica      | 50.35 | 56.90 | 54.80
  Alumina     | 23.50 | 24.90 | 27.60
  Iron oxide  | 10.40 |  2.83 |  2.56
  Soda        |  1.55 |  3.00 |  2.00
  Magnesia    |  1.45 |  1.07 |  1.00
  Water, etc. | 11.85 | 11.60 | 11.80
  ------------+-------+-------+-------

The first thing that will strike the reader on looking at these results
on Welsh materials, is their uniform composition as compared with the
clays from Newcastle. Yet there is as much as 10.40 per cent. of iron
in sample No. 1, which cannot be a first-rate clay. Its proportion of
silica to alumina is, however, excellent, and, as in sample No. 3, the
amount of soda and magnesia is not excessive. The soda in sample No.
2 (which acts somewhat like lime in the kiln) taken together with the
magnesia and iron in the same material, is too much for a first-class
clay, and would have to be suitably modified before good results could
be obtained. On the whole, it is possible that sample No. 3 would yield
the best results from the chemical standpoint.

We should not forget that remarkable substance of which the well-known
Dinas bricks are made. The proportion of silica present ranges from
about 96 to 99 per cent., the remainder consisting principally of
alumina, though traces of iron, lime, and magnesia frequently occur.
There is not, of course, sufficient natural flux for this “clay,” so a
small proportion (2.5 to 3 per cent.) of lime is added, which produces
the desired effect. In other words, if we can obtain a pure siliceous
sand, with hardly any lime, iron or magnesia in it, we have the
material of which the better kinds of fire-bricks are made. Such sandy
earths are not uncommon in the South of England, but strange to relate,
they are not used for the purpose indicated.

The earths from which the superior Stourbridge bricks are made, are
approximately of the following chemical composition:--Silica, 64.10;
alumina, 23.15; iron oxide, 1.85; magnesia, .95; water and loss, 10.00
per cent. It will be observed that the proportion of iron and magnesia
here is very small, whilst lime is altogether absent. It is a most
excellent earth for the purposes for which it is used, and the chemical
results may be taken as a standard for that class of material. Another
Stourbridge earth yields as much as 4.14 per cent. of iron, however,
whilst its proportion of silica is lower, 51.80, and alumina higher,
30.40, which serves to remind us of the variability of even good earths
used in the manufacture of fire-clay goods.

Let us now turn to the consideration of pottery clays, of which the
following results may be taken as typical:--


_Chemical Composition of Pottery Clays._

  ------------+-------+-------+--------
              |   1   |   2   |   3
              +-------+-------+--------
  Silica      | 46.38 | 49.44 | 58.07
  Alumina     | 38.04 | 34.26 | 27.38
  Iron oxide  |  1.04 |  7.74 |  3.30
  Lime        |  1.20 |  1.48 |   .50
  Magnesia    | trace |  1.94 | trace
  Water       | 13.57 |  5.14 | 10.30
  ------------+-------+-------+--------

Some of the chief qualifications, from a chemical point of view, of
earths suitable for making pottery, is the proportion and potentiality
of the colouring matters present. Where the pottery is to be glazed,
that is not so important; but with ordinary unglazed ware, colour
and uniformity are two highly essential desiderata. We know that the
temperature employed will modify the tint, but under similar conditions
the clays alluded to in the above table will give, approximately, the
following results. Sample No. 1 is typical of an excellent blue pottery
clay, which burns white. It contains more alumina than is commonly
met with in such materials, in which respect it differs markedly also
from the fire-clays just described. The proportion of oxide of iron
is very small, not sufficient to perceptibly colour the finished
product, though, no doubt, on careful examination it would be seen
not to be perfectly white. The latitude of the term “white” is pretty
considerable with clayworkers, as the reader is probably aware.

The pottery clay (also used for bricks) referred to in the middle
column, is brown in colour; it is an ordinary kind, used primarily
for black and common red ware. The proportion of iron is high, and
considerable quantities of both lime and magnesia exist. As might
naturally be expected of such material, it will not bear exposure to
great heat, though that might be regarded as a qualification in some
brick and pottery yards.

The proportion of silica is high in sample No. 3, which appertains to
a common yellow clay, with, possibly, some siliceous sand in it. The
amount of alumina is correspondingly low, but the iron oxide is not
excessive--for a common pottery clay. It is used for the manufacture of
coarse ware, and burns yellow.

The chemical composition of earths used for terra-cotta and bricks
of that substance is so variable, that without going into each case
specifically it would be impossible to convey an adequate idea. It may
be stated generally that it is not one whit less important to consider
the composition of the raw earths for ordinary brickmaking, than in
respect of that for high-class bricks and pottery.

An excellent earth, from the neighbourhood of Ruabon, is of the
following composition:--


_Chemical Composition of Ruabon Clay._

  Silica               63.00
  Alumina              20.10
  Sesquioxide of iron   4.84
  Protoxide of iron     1.51
  Potash                2.37
  Soda                  3.10
  Combined water        3.54
  Moisture              1.54

The proportion of silica in this is higher than in many clays used for
brick- or terra-cotta making, but the alkalis, potash and soda, are
in strong force, so that any refractoriness on the part of the silica
is soon subdued in the kiln. The iron, also, is in abundance. The
principal colouring ingredient is the sesquioxide, and we can quite
understand the manufacturer when he informs us that, in spite of the
rich tint of the goods produced, nothing is artificially mixed with
this clay to produce such a result. We may call attention to the method
of expressing the chemical analysis in this case, which might be copied
to advantage. In the first place, the combined and the uncombined
iron are separately shown, or rather the degree of combination is
indicated; and secondly, the proportion of water chemically combined
is differentiated from that which has simply soaked into the clay,
though expelled, following a well-known practice of chemists, prior to
commencing the analysis proper. It is of very little use giving the
amount of water, unless the proportions are divided in this manner. In
the result given above we learn that there is very little chance of the
clay shrinking, as it only contains moisture to the extent of 1.54 per
cent.; but if that had been added to the water combined, we should have
had a result of 5.08 per cent., which is not nearly so clear in its
meaning. We may add that the Ruabon earth referred to is utilised also
in the manufacture of tesselated and encaustic tiles.

In regard to the composition of earths employed in the manufacture of
the commoner kinds of bricks, we may give the following examples:--


_Chemical Composition of Common Brick-earths._

  -------------------+-------+-------+------+---------+----------+------
                     |       |Alumina|      |         |          | Water
                     |Silica.|  and  | Lime.|Magnesia.|Manganese.|  and
                     |       | Iron. |      |         |          | Loss.
                     +-------+-------+------+---------+----------+------
  Reddish-brown      |       |       |      |         |          |
      brick clay     |  52.6 |  30.8 |  3.4 |    2.8  |    1.4   |  9.0
  Red-brick clay     |  50.4 |  24.0 |  2.7 |    1.3  |    ---   | 21.6
  Common brick-earth |  33.0 |  11.2 | 39.8 |    6.0  |    ---   | 10.0
  Sandy-clay (loam)  |  60.2 |  24.0 |  2.4 |    1.6  |    ---   | 11.8
  -------------------+-------+-------+------+---------+----------+------

Reviewing these results, it will be noted that the brown colouring
imparted to the brick in the first-mentioned example is due, to a
large extent, to the presence of manganese, a rather uncommon feature
in brick-earths, except where these have resulted from the denudation
of iron-producing rocks rich in manganese. It will be noticed also
that the proportion of water is not high for a common earth, and it
must be a fairly easy material to deal with. There seem to be some
possibilities in it that might, in competent hands, lead to higher
things. The amount of lime and magnesia is, however, a rather serious
one for a first-class clay.

In regard to the “red-brick” clay, an essential feature is the
comparative absence of lime, and it would, no doubt, make “rubbers”
of an ordinary kind. Unfortunately, in the results given, the iron is
not separated from the alumina, but clearly the latter is very small
in amount, and the results refer to a sandy material. The proportion
of water is disastrous for the employment of this earth by unskilful
hands. In drying, the greatest care would have to be exercised to
prevent undue shrinking, and, in any case, the earth would have to be
very thoroughly incorporated to make a really serviceable brick. It
is with earths of this character that the majority of brickmakers _in
embryo_ come to grief; they know not how to handle them successfully,
and twisting, warping, cracking, and “bursting” follow as a natural
consequence. It is a common and treacherous material, that could only
be made to succeed by perseverance and wide experience.

The “common brick-earth,” as will be seen, contains an abnormal
quantity of lime, and doubtless refers to a marl, though not much
alumina is shown. Malm bricks could be made from it, and the product
would have to be burned at a low temperature. For bricks useful to the
“jerry-builder” this earth could be strongly recommended. It was, no
doubt, mainly derived from limestone rocks; and, judging from the high
proportion of magnesia, probably from within a watershed composed to
some extent of magnesian limestone.

The “sandy-clay” or loam is of a very common type, and produces
light-red bricks. There is much in common between this and the
“red-brick clay” previously referred to.

The practice resorted to in various parts of the world of making bricks
from slate _débris_, although not hitherto adopted to any large extent
in this country, merits some description in this place. Slates may be
regarded as a highly compressed clay, the original structure of which
has been materially modified by the great pressure exerted during their
manufacture in Nature’s laboratory. To all intents and purposes they
are silicates of alumina, _plus_ iron, lime, magnesia, and so on, and
have, practically, the same range of variation as have ordinary clays.
But during their manufacture, and subsequently, certain adventitious
mineral matter has been frequently introduced, as may be gathered from
the following results:--


_Chemical Composition of Slates._

  -------------------------+-------+-------+-------+-------
                           |   1   |   2   |   3   |   4
                           +-------+-------+-------+-------
  Silica                   | 60.50 | 60.15 | 48.00 | 50.88
  Alumina                  | 19.70 | 24.20 | 26.00 | 14.12
  Iron (protoxide)         |  7.83 |  5.83 |  ---  |  9.96
    „  (sesquioxide)       |  ---  |  1.82 |  ---  |  ---
    „                      |  ---  |  ---  | 14.00 |  ---
  Lime                     |  1.12 |  ---  |  4.00 |  8.72
  Magnesia                 |  2.20 |  ---  |  8.00 |  8.67
  Potash                   |  3.18 |  ---  |  ---  |   .88
  Soda                     |  2.20 |  ---  |  ---  |  ---
  Alkalis (not determined) |  ---  |  4.28 |  ---  |  ---
  Carbon dioxide           |  ---  |  ---  |  ---  |  6.47
  Water, &c.               |  3.30 |  3.72 |  ---  |  ---
  -------------------------+-------+-------+-------+-------

_Analysis No. 1_ refers to a blue Welsh roofing slate of Cambrian
age. It is quite certain that the large proportion of alkalis present
would render this material unsuitable for brickmaking, except for the
commonest kinds of bricks. The iron, again, is very large in quantity,
whilst the amount of alumina is low. We could not recommend this slate
for good bricks under any consideration.

_Analysis No. 2_ is of a dark-blue slate from Llangynog, in North
Wales. The amount of iron present is high, but from the low content
of alkalis this material, under proper treatment, should make fairly
good bricks. The ferruginous constituent is too powerful, however, for
fire-bricks to be made of this slate.

_Analysis No. 3_, of a purple slate from Nantlle, shows a remarkable
diminution in silica and a corresponding increase in iron. Lime and
magnesia being present to such an enormous extent, taken in conjunction
with the iron, would render this slate absolutely useless for
brickmaking. There is not a redeeming feature about it.

_Analysis No. 4_, which refers to a green Westmorland slate, has a low
percentage of alumina and very large quantities of iron, lime, and
magnesia. Only bricks of an exceedingly inferior quality could result
from such material.

Summing up the general characteristics of these slates from the
chemical aspect, one would say that none of them are very suitable for
high-class bricks. No. 2 is the best. Several minor differences will
be observed between the results quoted and those referring to ordinary
brick-earths--in particular, the distribution of the alkalis. A general
impression is abroad that any purple slate will do for brickmaking, and
manufacturers do not yet seem to have realised that the chemical nature
of slates is as variable as of brick-earths. That may account for the
difficulties experienced in many cases in turning out a satisfactory
material. The microscope is of much use in this connexion, however,
and the practical effects of chemical analyses are not always as bad
as they seem at first sight.

The remainder of this chapter will be devoted to the consideration of
rarer kinds of brick-earth and other raw earths used principally in the
manufacture of bricks for special purposes, or as pointing to certain
anomalies. As an example of what some manufacturers can do, we may
quote the chemical composition of a peculiar brick-earth employed in
Zurich, in Switzerland:--


_Chemical Composition of Brick-earth, Zurich._

  ----------------------------------------+------
                                   |Yellow| Blue
                                   |Clay. | Clay.
                                   +------+------
  Carbonate of Lime                |23.68 |27.80
      „      „ Magnesia            | --   | 5.7
  Other carbon dioxide             | 2.85 | 1.55
  Silica                           |42.39 |38.25
  Alumina                          |18.16 |12.44
  Iron oxide                       | 3.66 |  .73
  Lime (as silicate)               |  --  | 1.85
  Magnesia                         |  --  |  .15
  Potash                           | 2.14 | 1.54
  Soda                             | 1.27 | 3.05
  Moisture (at 100° C.)            | 1.27 | 1.37
  Water, &c., chemically combined  | 3.85 | 4.72
  ---------------------------------+------+------

Here we have two clays with the carbonates of lime and magnesia
present, in one case of over 35 per cent., and in the other of over
26 per cent. Professor Lunge, of Zurich, states that the bricks made
from them, if burned at the ordinary heat, say a moderate red heat, are
_red_, and do not keep in the air, but crumble away very soon, as the
quicklime slackens on combining with the moisture. When burned at a
bright red heat, about 200° C. above the former, however, they become
nearly white. The lime is then present as a ferri-alumina-calcic
silicate, which causes the red colour of the iron oxide to disappear,
and, at the same time, entirely prevents any action of the moisture,
quicklime being no longer present. We have no hesitation whatever in
saying that most British makers would look down upon raw earths such
as these from Zurich, and yet many millions of really good bricks
have been made from them during the past twenty years, and they are
especially noted for their durability. The crux of the case is the
temperature at which the earths are burned, as the reader has perceived.

Under the heading of “magnesia,” we have said a few words regarding
basic bricks. In this country they have been made primarily from
magnesian limestone, the chemical composition of which is shown in the
following results of analyses:--


_Chemical Composition of Magnesian Limestones._

  ----------------------+------+-------+------+-------
                        |   1  |   2   |   3  |   4
  ----------------------+------+-------+------+-------
  Silica                |  3.6 |  2.53 |   .8 |   --
  Carbonate of lime     | 51.1 | 54.19 | 57.5 | 55.7
      „     „  magnesia | 40.2 | 41.37 | 39.4 | 41.6
  Iron, alumina         |  1.8 |   .30 |   .7 |   .4
  Water, &c.            |  3.3 |  1.61 |  1.6 |  2.3
  ----------------------+------+-------+------+-------

_Analysis No. 1_ refers to the well-known magnesian limestone of
Bolsover.

_Analysis No. 2_ to that from Huddlestone.

_Analysis No. 3_ to that from Roach Abbey.

_Analysis No. 4_ to that from Park Nook.

These results were obtained by Professors Daniell and Wheatstone in
connexion with an enquiry many years ago as to the kind of stone
suitable for the erection of the Houses of Parliament.

Regarding them generally, it may be said that they are remarkable as
not containing much acid, practically the whole substance of the rocks
(except No. 1) being made of the carbonates of lime and magnesia. In
manufacturing bricks of such materials as these, it will be seen that
the ordinary methods of brickmaking would not suffice. On heating
magnesian limestone, the carbonic acid is driven off, leaving the base
behind; it is estimated that the loss of the acid, _plus_ moisture
dried out, leads to its reduction in weight of from 40 to 45 per cent.,
and the shrinkage is from 25 to 35 per cent. If water were mixed with
this material, after calcination, strong chemical reactions would
result, and of such a nature as to render a coherent mass of the kind
required for making bricks impossible. Seeing that water cannot be
employed, crude petroleum oil, coal oil, resin oil, &c., have been
employed, all of them with more or less satisfactory results. The
petroleum, &c., is mixed with the lime, and when the whole is burned
the oil passes off, leaving bricks of solid lime. In manufacture it is
highly essential to see that the lime is well burned, and it must be
fresh, and not have been exposed to a damp atmosphere. An improvement
has been effected by mixing from 5 to 7½ per cent. of burned clay,
which makes the lime harder after burning. An admixture of from 3 to 5
per cent. of iron oxide also consolidates the lime, though it increases
shrinkage. The bricks are commonly made, in the first instance, under
hydraulic pressure.

The diatomaceous earth known as Kieselguhr, which is used in the
manufacture of fire-bricks for chemical works and the like, and which,
for the most part, is of German origin, has the following chemical
composition:--


_Chemical Composition of Kieselguhr._

  Silica           83.8
  Lime               .8
  Magnesia           .7
  Alumina           1.0
  Oxide of Iron     2.1
  Organic matter    4.5
  Water, &c.        7.1

The reader will perceive that this earth is composed very largely of
silica, though there is enough iron, &c., to flux it, at any rate,
without material addition. The product is extremely light, and when
properly made, Kieselguhr bricks are the lightest known. They are
usually of a light yellow tint, with iron spots. The silica is not in a
crystalline form, the bulk of the material being composed of the hard
parts of microscopic plants known as diatoms; it is more like flint.

An earth of a similar character is found in the Isle of Skye, as
previously mentioned, though that burns into a redder colour.

An infusorial earth from Tuscany is composed of silica 55, magnesia 15,
water 14, alumina 12, lime 3, and iron 1 per cent. That also is made
into very light bricks. The general principle underlying the method
of utilising those earths of organic origin is similar to that of the
Dinas bricks, though they do not always require artificial fluxing.

At Saarbrücken, in the Rhenish Province of Germany, a material known as
“iron brick” is manufactured. It is made by mixing equal proportions of
finely-ground red clay-slate with fine clay, and adding 5 per cent. of
iron ore. This mixture is then treated with a 25 per cent. solution of
sulphate of iron, together with a certain quantity of finely divided
iron ore. It is then moulded and baked in a special manner. We do not
intend to describe the chemical composition of the various volcanic
ashes, trass, and other volcanic ejectamenta used for brickmaking on
the Continent in several localities. The materials of which glass-sand
bricks, slag-bricks, &c., are made have no special interest in
connexion with our present subject, their composition naturally varying
according to the particular kinds of “refuse” employed.



CHAPTER IX.

DRYING AND BURNING.


Of the merely mechanical aspects of the operations of drying and
burning bricks, we shall say little or nothing. But there are just a
few points of a more or less scientific nature that offer themselves at
this juncture to which we desire to allude.

The brickmaker hardly needs to be told that if he places his bricks
in the sun to dry, they, or a large percentage of them, will crack,
and become practically worthless from a commercial standpoint. To dry
a brick properly in the open air is a lengthy operation--too lengthy
for many manufacturers, who, in consequence, have had recourse to
artificial drying. Many a brickyard has had to be abandoned from the
inability of the worker to produce bricks that did not crack at some
period of the operation, either in the drying, or burning, or both. And
several manufacturers have their particular methods of “doctoring” the
raw earths to prevent cracking. These are invariably “trade secrets;”
though usually of a very open and transparent character, however, to
the student of the subject.

It is most curious to learn the different reasons for adding this or
that ingredient to the earths to prevent the brick from cracking. One
who in a district has found that the addition of a little sand is
beneficial, imparts that information by degrees, either personally or
through his workmen, and in time it is laid down as a general axiom
that “sand will prevent cracking.” Another has discovered that clay
should be mixed in small quantity to produce the desired result, so
he and his neighbours do that, and pity the ignorance of the “sand
mixers.” A third feels quite certain that crushed brick, or brick dust,
is a good thing; while a fourth will add a little lime. Now, each of
these ingredients is useful in its way; everything depends upon the
class of brick-earth to be dealt with. It may happen that what will,
in a measure, prevent cracking, will be a bad thing in the burning,
and the art of the brickmaker is to know what to do under the varied
conditions.

As a general rule, where care is exercised in the drying, the cracks
arise from the brick-earth being too wet or plastic in the first
place, and it cannot be too well understood that, _cæteris paribus_,
the wetter the earth the more liable it is to crack during drying. The
contraction, even when the unburnt brick is shielded, and in the open
air, often proves too much for the material. Then we have that class of
brick-earth composed of too much clay, and that would be improved by
the addition of sand--just how much depends on the particular earth;
and there is no better method of ascertaining the quantity required
than by subjecting the materials to direct practical experiment in
the kiln. Where no sand is available, it frequently happens that
brick-dust will answer the purpose, though this may be at the expense
of homogeneity in the long run. In the semi-dry process of manufacture
the initial causes of cracking are not present, the block having to
contract so little that it may be taken from the press and stacked in
the kiln for burning. Unless the brick-earth be carefully prepared,
however, the surfaces of the hard blocks produced by that process are
liable to develop minute cracks. And here it may be stated that unless
the clay, with brick dust or other foreign substance, be thoroughly
incorporated prior to being sent under the press, and the whole ground
very fine, it is impossible to prevent cracking during some part of the
process.

Apart from the fierce and variable drying action of the open air, we
have a fruitful source of cracks in the indentations made by stamping
the makers’ name or trade-mark upon the blocks. With bricks burnt
very hard this does not so much matter, but on the commoner kind of
materials one may often perceive minute, hair-like cracks radiating
from the indentations. We presume that in this age of advertising it is
impossible to convince many makers of that fact, yet if full justice is
to be done to the material, it will be better not to make any sharp or
deep marks on the brick.

The commoner kinds of brick-earth, as we have seen, mostly possess
gross particles, grit, pebbles, &c.; these act as so many centres from
which cracks radiate either during the drying or burning, and apart
from their influence in a chemical sense, they are apt to seriously
weaken the brick.

It is truly marvellous to see how little attention many large makers
pay to the initial drying; often the long rows of drying blocks are
left unprotected except for a rude kind of roof placed over them. The
passing shower of rain drives in underneath, and wets the exposed
surfaces, causing the clay to swell. These surfaces, being moister than
the remaining portions of the brick, contract at a different rate,
the centre occasionally being drier than the outside. The unequal
contraction produces minute cracks even in most excellent earths.

Turning to a smaller matter, the hand-barrow coming from the drying
stacks to the kiln is unprotected, which often means that a good brick
is spoilt. Of course, we are not alluding, in this connexion, to what
takes place during clamp stacking; the brick produced by such a process
must take its chance. The method of stacking in the kiln or clamp is
very often responsible for damage to the bricks. A common method is to
build them sloping outwards, and all sorts of strains and stresses are
thus set up, which have their effect in producing lines of weakness, if
not of actual visible cracks.

The “London stock,” if not a thing of beauty, is usually strong, and
that in spite of the “breeze” which forms so many points from whence
cracks radiate. We must not forget, however, that a really good London
stock is, above all things, thoroughly burnt, and that is a set-off
against the numerous and often wide cracks.

We will assume that the brick has been either naturally or artificially
dried, that no cracks have made their appearance, and that it is
properly stacked in the kiln ready for burning. Now comes a most
important part of the process. It is possible that any microscopic
cracks will be closed by fusion or agglutination; but it more
frequently happens that in unskilled hands the kiln is responsible for
many cracked and “starred” bricks. To know exactly how to introduce
the heat so gradually that the bricks shall not be impaired, is an art
begotten only of considerable experience. Even when dealing with one
particular kind of brick-earth, the maker must be careful to notice
the relative moistness of his charge, and vary the mode of procedure
accordingly. Suppose the brick to be as “dry as a bone” before being
put in the kiln, we shall notice a considerable amount of moisture
coming out of it as soon as the fires are alight; and if the heat
is applied too suddenly, the bricks are not improved--they contract
unevenly and too quickly, and warp. When well alight, care should
be taken to keep the temperature as uniform as possible, and when
sufficiently burnt it must be lowered by almost imperceptible degrees.
Above all things, there should not be too great a disparity between
the temperature in the kiln and the outside air when unloading. Except
to those who had minutely studied this matter, such a precaution might
seem superfluous; it may be that no damage caused will be visible to
the naked eye, but the microscope frequently shows flaws due apparently
to this cause. The manufacturer may test this for himself by heating
a good, sound medium burnt brick to the temperature usually found in
his kiln when unloading, and suddenly plunging it in snow. It is not,
perhaps, that any one of these things is especially dangerous to the
brick, but it is the combined effect of all of them trending in the
same direction. We desire to be clearly understood on this point.
The cracks produced may not seriously impair the strength of the
brick; they may be merely superficial, and they mostly are. But they
materially assist the agents of denudation in “scaling” the brick, and
weathering it unevenly. To this we shall return later on.

Let us now say something concerning the superficial changes produced in
bricks by burning. The most important of all is the change of colour,
upon which the sale of the brick depends in ninety-nine cases out of
a hundred. We said a few words on this subject when dealing with the
behaviour of individual minerals in the kiln. The production of an
uniform tint is the main point aimed at; and it may be at once remarked
that unless the brick-earth employed is very homogeneous, or has been
most carefully prepared and thoroughly incorporated, the production of
an uniform colour is impossible. In regard to the tint to be produced,
it should be remembered that the temperature employed in burning is a
most potent factor. It is frequently laid down that such and such a
temperature will form a red brick, and another and higher temperature,
a blue one. That is a most absurd notion. In a general sense the
principle could be correctly applied to a limited district, and with
one class of brick-earth; but it cannot be made to apply all round.
There is nothing like experience in regard to a point like this. In
a general way, of course, a pink, red, or blue tint may be produced
from one brick-earth depending upon the temperature employed; but the
bulk of brick-earths would melt and the whole kiln-full be ruined
in any attempt to attain such a temperature as is used in burning a
sound “Staffordshire blue.” Quite a large number of bricks made in the
Southern half of England, may be described as having been dried in
the kiln only--they cannot be said to be burnt, except that the heat
employed was enough to turn them red, or to make them piebald; the
particles are not agglutinated by fusion, and, indeed, there is often
no trace of the constituents having been melted. On the other hand, we
have red bricks in which the constituents are distinctly agglutinated
by fusion, and the whole burnt thoroughly. The brick-earth of which
these latter are made, would barely turn tint--would certainly not
become red--at so low a temperature as that employed in producing the
red in the non-agglutinated bricks alluded to.

It is not always an easy matter in burning a red brick to obtain two
kilns full of the same tint, even in the same yard. When the employment
of pyrometers becomes more general, that will be considerably
simplified; but it is a difficult matter to get a reliable instrument,
none of the forms hitherto invented being altogether suitable. That by
Professor Roberts-Austen is as good as any. Many manufacturers, we are
sorry to say, place colour before everything else; they even sacrifice
durability to attain a certain tint. And there is much excuse for
them so long as they find a ready sale for the material. When colours
are made from artificially introduced mineral matter (which is not so
often the case as some appear to think) the mineral introduced is,
most commonly, iron; though it will be understood, from what we have
previously said, that it must be used very sparingly.

The ultimate tint assumed by the brick cannot always be judged
beforehand from the colour of the brick-earth. In brickmakers’
language, a red clay is one that produces a red brick, a blue clay
a blue brick, and so on. For the most part, colour depends on the
proportion of hydrated oxide of iron in the clay; if iron is present
in an earth that contains no lime, or similar mineral substance, the
colour produced in the brick at a moderate red heat will be red, and at
the same temperature, with the same brick-earth, the more iron present
the deeper the tint. In an ordinary brick-earth, when more than 10 per
cent. of iron is present, the clay is apt to burn bluish, however, and,
in certain cases, almost black. With a smaller proportion of iron, and
the application of intense heat, the same tint may result, and the
brick become vitrified. A brown colour may frequently be obtained when
the brick-earth has from 2.75 to 4 per cent. of magnesia, or a similar
proportion may be artificially added to the earth.

To obtain a white brick, so that it shall also be of excellent quality,
the pure white clays of Devon and Cornwall are the best, though the
so-called “white” is, in the majority of cases, a light cream colour,
unless, of course, the brick is glazed. In the neighbourhood of
London, a whitish brick results from a mixture of chalk (carbonate
of lime) with clay or loam, and is known as a “malm.” In parts of
Yorkshire, white pressed bricks are manufactured from common red clay
mixed with magnesian lime (made from magnesian limestone) in a slacked
condition. The latter ingredient, on introduction, immediately absorbs
about 40 per cent. of the moisture present in the clay.

Yellow bricks can easily be manufactured from the more impure kaolins;
also from certain clays in Cambridgeshire, Huntingdonshire, Kent, &c.
(gault bricks); “malms” are mostly yellow, though called white.

Laboratory experiments, many years old, show that with white clay as
a basis the following tints may be obtained. Phosphates of lime of
various kinds = very light blue bricks. The phosphates, mixed with a
quarter by weight of alum = brighter blue bricks. A mixture of white
vitriol (sulphate of lime) three-quarters, with borax one-quarter =
light dirty green. Sulphur and tin oxide in equal proportions = yellow.
These experiments are interesting, but the ingredients would, as a
rule, be too expensive for ordinary brick manufacture. They are more
applicable for the production of ornamental tiles.

A time-honoured method of producing black bricks is to make any
ordinary bricks red-hot and to dip them in a cauldron of boiling
coal-tar for a few seconds. It is essential that the brick should be
very hot, or the black staining will rub off. A good test that the
operation has been successful is, that the surface shall be dull black,
not shining. And there are many other ways of obtaining different
tints, the description of which would be beyond the scope of the
present work.

Unless a brick is extremely well burnt it is not uniform in colour
throughout. A considerable proportion of a “draw” is often ruined in
regard to tint by the adoption of an unsuitable form of kiln. Where
the brick is actually burned (as distinguished from being baked),
the contact of the flame from the fires is almost sure to lead to
uncertainty in that respect along the flues. Impurities in the coal,
such as iron pyrite, are the chief delinquents, and there is sure to be
a certain amount of “flash.” In that, as well as in the baking method,
bricks are liable to be discoloured by the bringing out of impurities
which they themselves contain.



CHAPTER X.

THE DURABILITY OF BRICKS.


This is one of the most important parts of our subject, and it may
be approached from several points of view. When a brick decays, its
structure, for the most part, is responsible therefor. A great deal
depends on whether the ingredients forming the brick are merely baked
in the process of manufacture, or whether they are wholly or in part
agglutinated by igneous fusion. A rough and ready plan of determining
this point, in the absence of experience, is by ascertaining the
porosity of the brick. Other things being equal, the absorption test
is undoubtedly the best all-round method of gauging the weathering
qualities of a brick. But there are certain kinds of bricks which defy
that method; an imperfectly burnt one with a vitreous exterior is
especially treacherous in that respect, and, indeed all “vitrified”
bricks are difficult to deal with by the “absorption process.” Again,
a brick cracked all over, not with superficial cracks only, but with
those which go far into the interior, will not yield its quality by
mere immersion in water. The water, it is true, finds its way right
into the brick, but, as often as not, the sides of the cracks are
perfectly vitrified and almost damp proof, so that on lifting the brick
out of the water the latter rolls off as though it were on “a duck’s
back.” Yet such a brick, yielding but the merest fraction as a result
of the immersion, may be utterly worthless when put into a building,
because it would not be strong enough.

Then we have those bricks which are seriously affected chemically, but
which seem fairly good in other respects. They also, in many cases,
defy the efforts of the experimenter in regard to absorption; though
they are nevertheless easily detected as being of bad quality, by
other methods. Such bricks often resist great “crushing weights,” and
generally bear a good character, their subsequent behaviour when put in
the building to the contrary notwithstanding.

In determining the weather-resisting qualities of a brick we have the
following things to consider:--

  1. The chemical composition of the brick.
  2. Its absorptive capacity.
  3. Its minute structure.
  4. Its specific gravity.
  5. Its strength.

The last-mentioned property can often be inferred from a knowledge of
the three preceding ones, and need not, therefore, form the subject
of direct experiment. In spite of that, however, we find that the
“crushing strength” is much more popular than the others. The reason,
so far as brick manufacturers are concerned, is not far to seek.
Architects demand that especial quality. “What is the ‘crushing
strength’ of your bricks?” enquires the architect. And if the maker
does not know, he stands a good chance of losing the order. Figures
are demanded, and if the maker cannot produce a higher figure than his
neighbour, woe betide him. But statistics are ever deceptive, and as
applied to bricks in regard to their strength especially so.

In general, we have to consider whether the brick is strong enough
for the purpose to which it is to be applied; and that depends much
more on the manner in which it is built up, than on the strength of
the individual brick. For ordinary building purposes almost any
kind of brick is, _per se_, strong enough, and a mere inspection of
the specimen suffices to carry conviction as to its suitability or
otherwise in that respect. For certain structures, such as buildings
to carry heavy weights--especially moving weight--for engineering
purposes, and the like, we ought, it is true, to know a little more.
Yet the engineer would be a very poor one who could not tell at sight
whether a brick submitted to him was fit or not for the purpose he has
in view, from the point of view of its weight-carrying properties. In
any case, however, fashion demands the “crushing weight” in figures,
and although such figures are in general of but little practical value,
they must be given.

The principal difficulty the architect and engineer have to contend
with is not lack of strength, but the setting in of decay, and that
even in bricks sometimes of the strongest description. Unless the
strength is going to be maintained, it is of no use whatever, in a
scientific sense, to give it in the first instance.

After these few preliminary observations, it will be well to treat the
subject more systematically.


THE EFFECT OF THE ATMOSPHERE ON BRICKS.

Air is a mixture of gases; dry air consists of at least four of them,
namely, nitrogen, oxygen, carbonic acid, and argon. Of these, by far
the most abundant is nitrogen, present to the extent of about 78 per
cent., then oxygen, 20.96 per cent., argon about 1 per cent., and
carbonic acid 0.04 per cent. Extremely minute quantities of ammonia and
ozone, though practically always present, have been omitted from the
preceding results of analysis of air.

We have been speaking of pure dry air; but the atmosphere is hardly
ever of precisely the same chemical composition in two different
places. By the seaside it has more ozone, and chloride of sodium
is found in particular abundance. In cities, especially where
large factories exist, nitric acid and sulphuric acid appear most
conspicuously, and the proportion of ammonia becomes larger. In the
air of streets and houses, the proportion of oxygen diminishes, whilst
that of carbonic acid increases. Dr. Angus Smith has shown that very
pure air should contain not less than 20.99 per cent. of oxygen, with
0.030 of carbonic acid; but he found impure air in Manchester to have
only 20.21 of oxygen, whilst the proportion of carbonic acid in that
city during fogs was ascertained to rise sometimes to 0.0679, and in
the pit of a theatre to the very large amount of 0.2734. Although these
may seem to be very small percentages, yet the total amount of carbonic
acid in the atmosphere is enormous, and plays a conspicuous part in the
decay of certain kinds of bricks.

Sulphuric acid is found in the air of large cities principally as
a product of combustion, and is, of course, a distinct impurity. A
portion of this acid is free, and a larger quantity is combined. Free
sulphuric acid is very destructive to clay goods in the open; and it
should be remembered that the relative abundance of this impurity
depends on the precise _locale_ in the city. A great deal has been said
and written about the decomposition of the stone of which the Houses of
Parliament are built. The air in the immediate vicinity must be highly
charged with both sulphuric and nitric acid from the proximity of the
busy factories on the opposite banks of the Thames in Lambeth. Had the
Houses of Parliament been erected, say, in Kensington, where but few
factories exist, it is conceivable that the stone would have behaved
much better.

Air in itself, however, has no power to destroy bricks--the various
gases, acids, chlorides, salts, solid carbon, inorganic and organic
dust can do nothing by themselves. But the air is always laden with
vapour, the most important of which is water vapour, which condenses
into rain, hail, snow, and dew. When rain is formed, the drops of water
take up minute quantities of air with its proportion of carbonic acid,
sulphuric acid, or what not, and it is these acids, applied to the
surface of bricks through the medium of rain and moisture generally,
that are liable to do the damage if the nature and composition of the
brick are favourable.

Let us assume that we have a brick composed of a goodly percentage of
carbonate of lime. The carbonic acid in the rain reduces this to a
bi-carbonate, which is soluble in water, and hence the surface of the
brick decays, the rain water washing it away. Other things being equal,
it follows that the same brick will decay most rapidly in a district
where the rainfall is very great and where there is the largest
proportion of these deleterious acids in the air.

Whilst speaking of the various acids which attack and destroy bricks,
we must not forget those formed by the decomposition of organic matter
on the surface of bricks which “vegetate.” The lichens, mosses,
and so forth, growing from cracks in the wall, or spread over on
to the brick from the mortar, yield, on decomposition, some of the
most powerful acids in existence. A brick with a “crumbly” surface
affords good foothold for these plants, and when they die they give
rise to the so-called humus acids--crenic and apocrenic acid--which
undoubtedly do an immense amount of damage. By keeping the surface
of the brick moist, the plants permit the ordinary acids in rain to
do more execution than they otherwise would. Taking two bricks, one
which “vegetates” and one that does not, and exposing them in the same
situation, it will be found that after a smart shower of rain the
surface of the former has become thoroughly soaked, and the vegetation
keeps it so, completely rotting it in time; whereas the surface of the
latter, exposed to the same shower, may be quite dry within an hour or
two after the rain has fallen.

Returning to the subject of rainfall, which exercises such material
influence on the durability of bricks, we may give a few particulars
concerning the distribution of rain in this country. Speaking
generally, the east coast of England is the driest part of the country,
the west coast having the greatest rainfall. The annual quantity at
sea-level ranges from 60 to 80 inches on the west coasts of Ireland and
Scotland, to about 20 inches on the east coast of England.[10] In some
localities, however, the fall is much greater, amounting to 154 inches
on the average of six years at Seathwaite, in Borrowdale, at the height
of 422 feet above the sea.

The quantities which fall in particular showers are often very great,
and this aspect of rainfall also has its interest for us. About London
a fall exceeding an inch in 24 hours is comparatively rare, although
on August 1, 1846, 3.12 inches were collected in St. Paul’s Churchyard
in two hours and seventeen minutes.[11] On our west coasts this amount
is often exceeded. On October 24, 1849, 4.37 inches were collected at
Wastdale Head; June 30, 1881, 4.80 inches at Seathwaite; on April 13,
1878, 4.6 inches fell at Haverstock Hill, London; and a fall of 5.36
inches was recorded from Monmouthshire on the 14th July, 1875.

Taking averages of districts, we may give the following statistics,
referring, of course, to annual rainfall:--

Less than 25 inches = Essex, Suffolk, Norfolk, Cambridgeshire,
Huntingdonshire, Rutland, Middlesex, and parts of Surrey, Oxfordshire,
Buckinghamshire, Bedfordshire, Northamptonshire, Leicestershire,
Nottinghamshire, Lincolnshire, Yorkshire, and Durham. In other words,
with the exception of parts of the North and East Ridings of Yorkshire
and parts of Herts. and Bucks., which have a rainfall of from 25 to 30
inches, the eastern half of England, to the east of a line drawn from
Sunderland to Reading, and then eastwards to the mouth of the Thames,
has only a rainfall of 25 inches, or slightly less, per annum.

Between 30 and 40 inches = Practically the whole of the south coast
from Kent to Devonshire, the whole of Somerset, Wilts., and the west of
England generally, with the exceptions about to be noticed.

Between 40 and 50 inches = A great part of Devon and Cornwall, the
western half of Wales, with the exceptions presently to be given, a
great part of Lancs., and Cumberland.

Between 50 and 75 inches = A small patch in the centre of Devon,
a large strip in West Wales, and an enormous tract of country in
Cumberland, Westmorland, with Lancs. and north-west Yorks.

Above 75 inches = The wettest parts of the country. A small part of
Dartmoor, a region in Wales in the vicinity and to the south-east of
Snowdon, and the Lake District.

With reference to statistics concerning rainfall, it should be borne
in mind that those relating to special districts, especially to hilly
parts of the country, are often very deceptive, and require careful
local study. A slight difference in the physical features of a locality
is often sufficient to lead to considerable variation--the proximity
of a conical hill rising from the plain, the sudden convergence of the
two sides of a valley, or, conversely, the widening of a valley into a
flat stretch of land, all materially affect the local distribution of
rain. A clump of trees situated in proximity to a house will frequently
be the means of a downpour that would otherwise have passed over.
With winding valleys great latitude must be allowed. Then, again,
the geological structure of the locality is an important factor in
determining the amount of moisture delivered at a given spot. Where we
find a thick clay cropping out in the bottom of a valley, with more
or less porous rocks rising on either side of it, we soon ascertain
that the houses on the clay receive more moisture (or the latter is
distributed over a longer period) than those edifices on the hill sides
in the same district.

Our readers could no doubt give us plenty of instances where in a
circumscribed area their bricks have behaved very erratically--the
bricks of a house in one part of the district weathering well, and
in another badly. That may often be due, not only to the actual
distribution of the rain, but to the manner in which the rain or dew
has fallen. If an inch of rain falls in the neighbourhood in one day,
that would not tend to weather the bricks so vigorously as though the
fall had been spread over, say, a week.

A very important aspect of the subject is that which deals with the
“efflorescence” on bricks. This appears to be greatly misunderstood,
being commonly assumed to be due to one set of circumstances
rather than to the conspiracy of several. There are many kinds of
efflorescence, and an explanation of one of them obviously will not
apply to all. The “scum” that appears on the surface of bricks is,
however, to some extent bound up in the composition of the rain in the
particular locality where it occurs. Examined attentively, the commoner
kinds of efflorescence are seen to be minute white and yellowish-white
crystals. The substance of which these are formed has been drawn out
of the brick, or the mortar, or both, and rain has been the principal
agent in accomplishing this work, though its power in that respect must
necessarily vary according to the chemical composition and structure
of the brick or mortar, as compared with the nature of impurities in
the rain. If some substance were present in the rain that could readily
form an alliance with an ingredient of the brick, and the union was
capable of crystallising out, the surface of the brick would naturally
form a convenient spot for the crystallisation to take place. To
prevent it, we ought to know the composition of the air at the spot
where the house is to be erected, and also the chemical and physical
structure of the brick to be employed. That is rather too much to
expect from the manufacturer and architect; but there is a method--we
will not say an infallible one--which may be adopted to get rid of that
particular kind of scum. That method could not always be adopted, as
will be seen. The bricks must be burned more thoroughly, and at a high
temperature; that would lead in most cases to the active employment of
practically all the ingredients of which the bricks are composed, and
the impurities in the rain would, in consequence, stand less chance
of successfully inducing some of them to break their allegiance.
In practice, however, we believe it would be found that the high
temperature requisite to bring about the result just stated would
either tend to spoil the colour of the brick or partially melt it. The
latter could be prevented with due care, but we are afraid the former
could not be so easily dealt with, with the majority of brick-earths.
And if the brick is to be permanently discoloured to prevent
efflorescence, it is better to permit the latter to manifest itself.
The life of the “scum” is very variable; sometimes, after having once
appeared and disappeared, it will never come again. The passing shower
may wash it off (though it is not always so easily removed), and it may
come again and again for years. It behaves very erratically. The amount
of the efflorescence may be such as, in course of time, to lead to the
surface of the brick “bursting” and peeling off, or, on the other hand,
it may be a mere film.

There is one thing in connexion with efflorescence which cannot be
overlooked in regarding its practical effects in the building. In ever
so many cases we find that the scum, or the major part of it, is only
to be found in the neighbourhood of the mortar joints. That is a matter
of direct observation, and we have taken some considerable trouble to
verify it, as it has always been regarded as a point whereon to hinge
a debate. We do not say that in all cases the efflorescence appears
only in the position on the brick just indicated; but it unquestionably
does so in too many instances to enable us to regard its occurrence as
mere accident. Taking a large surface of brickwork just commencing to
show efflorescence, we find that the vicinity of the mortar joints are
the first places, in very many instances, where the nuisance begins to
manifest itself. From thence it spreads over the surface of the brick
until the whole is more or less discoloured.

It seems impossible to deny that the mortar is guilty, to some extent,
in such cases. At the same time, we must confess that we have never
seen the efflorescence spreading over the mortar. It would appear that
something in the mortar enters into chemical alliance with certain
ingredients of the brick, and that neither without the other could
produce the phenomenon alluded to. The remedy suggesting itself most
readily is to chemically analyse the efflorescence, the brick, and the
mortar; supplementing the experiments with a micro-examination to see
how far it is possible to locate the deleterious substances found to
exist, so that they may be removed in the manufacture of the materials,
if that is possible. But information on that head is of the scantiest
description, and much more will have to be done before the question is
definitely settled.

Another kind of “efflorescence” that often appears on bricks in
damp situations is mere vegetable growth, which bears a superficial
resemblance to the crystalline “scum” just described, though it can,
of course, be easily differentiated on examination with a lens. The
damp atmosphere is no doubt largely responsible for this, though
ineffectual damp-courses are contributors. The remedy lies in having a
less absorbent brick--one that will not afford ready foothold to the
vegetation.

The influence of rain on the weathering of bricks may be considered
from yet another standpoint. Where the brick is fairly porous, its
durability is liable to be materially influenced through the agency
of successive frosts. The water finds its way a short distance into
the brick and saturates it. During frost the water is turned into ice
at and near the surface of the brick. In forming, the ice exerts
considerable expansive force, which forces asunder the particles
(sand-grains and the like) of which the brick is composed--that is
to say, near the surface of the brick. The accumulated effects of
successive frosts in this way tends to weather the brick by breaking
up its exposed surfaces. To be materially affected, however, the brick
would have to be of very poor quality, and it will be seen that the
presence of cracks would much facilitate the operation.

The style of a building, the manner of its construction, and especially
the class of metals used for exterior decoration, all assist rain in
its work. A projecting course will have its upper surface washed clean,
whilst the underside remains very dirty--in cities, becoming quite
black. The limit of this dark discolouration is often frayed out by
the irregular action of the rain dripping from the projecting ledge,
assisted by the wind. Where the projection is so designed that the
rain is induced to drain to one point, and then to fall over on to the
wall, an unsightly streak down the latter is the result. The free use
of metal ornaments, railings, for supporting signs, for down-pipes,
&c., is unfortunate in not a few instances. At the point of junction
between the metallic substance and the brick into which it is inserted,
or in the immediate neighbourhood above which it is fastened, the
brickwork is sure to be discoloured. This may arise from the dripping
of rain-water from the metal, or it may be from the decomposition of
the latter, or from both. Iron rust leads to brown streaks, zinc-compo.
to dirty red, and so on.

The action of the wind as affecting the durability of bricks is
sufficiently important to warrant passing allusion. It drives rain and
its deleterious acids farther into the brick than the moisture would
soak in the ordinary way. It leads to wet walls interiorly, unless the
latter are so constructed as to overcome the effects. On the other
hand, a gentle breeze dries moisture on the face of the brickwork. In
cities, wind indirectly assists rain and its impurities by blowing
organic matter from the streets into niches and corners, where it
lodges, and, decomposing, provides powerful acids capable of doing much
work. Discolouration is the chief effect produced on the average brick
through this medium. In certain countries, wind, by driving dust, sand,
&c., acts as a species of sand blast.

Considerable diurnal variations in temperature are known to be
peculiarly destructive to certain kinds of brick and terra-cotta work.
Very porous bricks are not much affected, but the more compact kinds,
and especially terra-cotta blocks, often suffer. These observations
do not so much apply to our own country as to warmer climates; though
we are not altogether without experience here. On being heated these
materials expand; when made loosely, as in rubbers and the like, the
effect of the expansion is not very manifest, because the motion
is absorbed, so to speak, by the brick itself. On the other hand,
increased compactness of the particles leads to a perceptible increase
in the size of the bricks, and when the sun has gone down contraction
takes place as the bricks are cooling. It often happens in hot climates
that the brick or terra-cotta block is unable to part with its heat as
rapidly as the surrounding air becomes cooler, although it tries hard
to do so, and this leads to corners of the brick being broken off, the
physical forces exerted during the struggle doing the damage.

A highly interesting case of the effects of temperature on terra-cotta
was detailed by Mr. T. Mellard Reade, C.E., F.G.S., a few years
ago.[12] He shews that the cumulative effect of small, but repeated
changes of temperature is very striking, and describes the lengthening
of a terra-cotta coping in that connexion. The coping in question,
which was freely exposed to the direct rays of the sun, consisted of
two courses of red Ruabon terra-cotta bricks set in cement upon a
fence wall, built with common bricks in mortar, a brick and a half
in thickness. The courses were level, but, in consequence of the
inclination of the road, the coping stepped down at intervals, so that
the undercourse of bricks of one length was just gripped and held in
position by the top course of the next length of coping. It will be
observed that that form of construction constituted, by liability
to lifting, a more delicate test than ordinarily of any increase of
length, that might take place in the coping. On subsequent examination
of the coping, the end position of one length, abutting against the
next length at the drop in the level, was found to be thrown up into an
arch-shape bend of about 6 feet span; the coping bricks being lifted
in the highest part one inch from their bed. There was a fracture at
the crown of the arch, and another at the foot or springing, but for
a distance of 30 feet the coping was practically one solid continuous
bar. A careful examination shewed that the coping had “grown” about a
quarter of an inch longer than when it was first set, and that this
lengthening, as shewn by movement on the corbel bricks which occur at
intervals, was evenly distributed along a length of 30 feet.

Mr. Mellard Reade tells us that this is by no means an isolated case.
In the neighbourhood of Blundellsands inspection of brick copings
shewed that it was quite a common feature, and he has noted several
instances in which the end brickwork and piers have been badly
fractured by the force of expansion. In a case where the coping was
of blue Staffordshire bricks, the top course in cement and the under
course in mortar, a change in length was clearly shewn by the coping
being lifted off the wall at each of the two ramps which exist in its
length, and the movement was readily measured on the corbel bricks as
in the case previously detailed. In this case the lengthening was also
a quarter of an inch, and was evenly distributed over a considerable
length of coping.

Whilst speaking of changes of temperature in their effect on bricks, we
may allude to the behaviour of the material in severe conflagrations.
A general rule cannot be laid down, because it is customary now-a-days
to use fire-bricks for ordinary building purposes which will withstand
practically any heat to which they may be subjected. Leaving them out
of the question, and referring to ordinary bricks, it may be said
that those of an inferior class frequently become cracked all over
during a fire, or, it may be, by the sudden cooling after the fire has
been put out, or by the sudden lowering of the temperature in them
by the continuous action of the fireman’s hose. All the same, the
average brick withstands heat far better than any kind of granite, or
similar igneous holo-crystalline rock; loosely compacted sandstones
and limestones crumble up on the surface, or flake, or may be utterly
destroyed when subjected to a conflagration that would not have the
slightest effect on bricks.



CHAPTER XI.

THE MICRO-STRUCTURE OF BRICKS.


The reader may be tempted to enquire, What is the use of knowing the
micro-structure of a brick? We have anticipated the question to some
extent in dealing with the structure of brick-earths, but it may be
well to enlarge upon it here. In the first place, the study of the
minute structure enables the manufacturer to ascertain whether the
brick is thoroughly and homogeneously burnt. It tells him whether the
materials mixed together in the earlier stages of manufacture were
thoroughly incorporated or not, whereby, if need be, he can improve
that part of the process. In carefully examining what the average
manufacturer would call a well-burnt brick, the microscope assists us
in perceiving that it is often anything but well burnt, small local
patches--“tears”--of semi-vitrified matter being observed, which
should not exist, of course, in a perfectly homogeneous brick. And if
the brick is not homogeneous, it suffers in respect of its strength
as a whole, and in the majority of cases its colour is not uniform.
To arrive at the cause of this lack of uniformity is to indicate the
manner in which the manufacture of the brick may be improved, and the
microscope often enables us to arrive at a satisfactory solution of the
problem.

From a chemical standpoint we know that a high percentage of iron in
the average brick-earth is not conducive to the production of a good
brick. In the same manner by “rule of thumb” we learn that a high
percentage of lime prevents the manufacture of the raw material into
a fire-brick, unless, indeed, we are making basic bricks. The chemist
tells us also of the respective values of potash and soda. Too much
iron will cause the brick to “run”; salt has a similar effect; but
beyond this the chemist cannot go, except that in the broad sense he
explains what unions take place to produce such results.

The microscope, on the other hand, enables one to see exactly what
has taken place; the deleterious constituents are detected at their
work, and careful chemical investigation teaches us what to add to the
brick-earth to neutralise the effects observed; for it is only from
its effects that the artificial constitution of the brick-earth can be
properly regulated.

The same instrument is extremely useful in all questions concerning
the relations subsisting between a brick and the glaze upon it, the
cause and prevention of the cracking of the latter, and its general
quality from a physical aspect. And, speaking of cracks, we may
again draw attention to the influence these have on the strength and
durability of the brick: many of these minute fissures cannot be seen
by the naked eye. In a similar way can the microscope be made use of
in the manufacture of terra-cotta and faïence. The cracking of glazes
is one of the most troublesome features the high-class brick and
tile manufacturer has to deal with. If the character of the surface
of the brick is not suitable for “taking” the glaze, the maker knows
in a moment; the trouble is where the glaze takes readily and then,
some time after the operation is finished, it becomes covered with
“spider-web” cracks, unsightly and considerably detracting from the
value of the brick. The cause of the cracking is commonly attributed
to the composition of the glaze, and the manner in which the latter is
allowed to cool, and no doubt a great deal is due on both those heads.
At the same time, we know of many instances where the same glaze being
used under similar conditions on two different surfaces of bricks made
from one and the same brick-earth, the glaze cracks in the one case,
and hardly ever in the other. The direction of the cracks points to
their origin, and the character of the surface is brought in guilty.
And yet the average manufacturer would not detect any difference in the
quality of the surface--he could not, without a good lens or low power
objective, perceive the slightest discrepancy.

The ordinary glaze behaves very much like Canada balsam with
reference to surfaces on which it is laid, and something akin to what
petrologists call “perlitic” cracks is produced in the glaze. We can
make these cracks, and imitate the structure artificially, by suitably
distributing the Canada balsam over the surface of a piece of ground
glass, and in other ways. That direct relationship exists between
the cracks and the grain of the surface on which the preparation is
laid, is certain, for we may vary the distribution of the cracks by
varying the grain of the surface. An intelligent appreciation of the
disposition of cracks in glazes should be the means of preventing them
altogether, and not only with bricks, but with faïence and vitrified
work generally, the study may be best carried on by aid of the
microscope.

The microscope, also, may be made use of in identifying bricks in
case of dispute, though its applications in this respect are not so
important as in dealing with building stones.

Questions of durability may frequently be decided on appeal to that
instrument. Take a case in which a brick is known to contain a rather
high percentage of lime: if the lime were in a combined state, the
quality of the brick would not be materially affected; but assuming
it were not so employed, it is possible that in a short space of time
the brick would be thoroughly decomposed by atmospheric agencies. The
microscope tells us at a glance the state in which that and other
ingredients exist, in a well-burnt brick. We draw the line at bricks
intended for the “jerry” builder; they may well be left to take care of
themselves; we allude only to high-class productions in which science
may be some aid to the manufacturer.

And now as to the microscope--for we do not use an ordinary one in
such investigations. The best kinds of microscope are those used
by petrologists in the study of the minute structure of rocks and
minerals. The reader will find these fully described in works specially
devoted to the subject,[13] but we may say a few words thereon.

A common form of “Student’s” petrological microscope, as manufactured
by Swift of London, may be described as follows:--

_Eye Pieces and Objectives._--These need not be expensive, clear
definition being the principal object to aim at; the objectives should
be of low power, 2-inch, 1-inch and ½-inch objectives being plenty
for the purpose. Unless the reader desires to follow the subject
from a purely petrological point of view, to study the development of
trichites, globulites, skeleton crystals, etc., in vitrified bricks,
in such places as these latter have cooled from igneous fusion,
there is no occasion to resort to higher powers. We are far from
saying that the brickmaker of the present day would not derive any
advantage from studying this subject in its higher aspects, for the
origin of crystallization appeals strongly to the imaginative mind,
and is one of the most remarkable problems that Nature offers for our
investigation. But in an elementary treatise of this kind we cannot go
into the matter; and, as previously remarked, low power objectives are
sufficient for our present purpose. The eye-pieces should be fitted
with cross-wires, the use of which will presently be explained.

_The Stage._--In the instrument we are now describing this is
circular with a hole in the middle, and is so arranged as to revolve
horizontally on a collar about an axis, the centre of which comes
exactly underneath the centre of the objective. In other words, a
straight line drawn through the eye-piece down the centre of the barrel
of the microscope, and passing through the objective passes through
that axis. To assist in more accurately centreing than is otherwise
possible (depending on the lenses) with this cheap form of instrument,
a collar with adjustable screws is ordinarily affixed to the lower
part of the barrel of the microscope. The stage, with suitable clips
to hold the object to be examined, is graduated so that on its being
revolved it is easy to ascertain the number of degrees, at any period
of the revolution, through which it has been turned. Thus, it will be
observed that the object revolves with the stage. A pointer is placed
in a suitable position on the frame of the microscope to facilitate the
observation.

_The Polariscope._--This is an indispensable adjunct, for determinative
purposes it is often necessary to observe the object in polarised
light. Briefly, the polariscope consists of two parts--the analyser,
placed in the barrel of the microscope above the objective, and the
polariser, arranged underneath the revolving stage. The analyser is so
fitted that it may be shot in and out of the barrel in order that the
polariser alone may be used, or the latter may be removed, leaving only
the analyser in position, or both may be removed to enable the object
to be examined in ordinary light, either reflected or transmitted. The
lower nicol[14] is made to revolve, and the collar in which it is fixed
is broadly graduated and furnished with a pointer.

_Reflector._--An ordinary reversible and adjustable reflector is
arranged beneath all.

_Accessories._--For the more accurate determination of minerals, a
quartz wedge, a quartz plate, etc., are used by the petrologist, but
the description of these is beyond the scope of the present work.
For examination in reflected light it is highly desirable to have a
“bull’s-eye” condenser.

An ordinary microscope with a revolving stage may be readily converted
to petrological purposes, though it is better to have a special
instrument.

       *       *       *       *       *

The object to be examined may be in the form of (_a_) a fragment of the
brick, or (_b_) a very thin slice of the same.

The fragment may be securely clipped and held in position on the
stage, the “bull’s-eye” condenser being brought into use to throw
a strong light on the part immediately under the objective. The
polarising apparatus is no use for this, and may be thrown out of
gear. A very low power should be employed. The observation may be
directed towards ascertaining how far the fragments composing the
brick are agglutinated, and their size may be noted. Anything like a
discolouration should be specially observed, and a minute description
jotted down. In bricks that have not been burnt very hard, and in
those that have merely been baked, we shall often be able to detect
particles of mineral matter which further investigation, after the
manner presently to be described, shows are opaque. Different forms
of iron, iron pyrite, fragments of clay that have merely been dried
in the process of baking, and minute pieces of chalk (now converted
into lime) are amongst the most prominent opaque substances met with
in common bricks. These may generally be differentiated and determined
at sight, and bricks thus composed are never of good quality, though
the ingredients have been ground very fine, and there may be nothing
superficially to find fault with. Their bad qualities are usually
brought out in the weathering. A great deal may, therefore, be learned
from a careful examination of fragments in this manner.

In regard to the examination of very thin slices, that is in the
majority of instances the most instructive, and, if we may say so, the
most interesting method of investigation, though it must always go hand
in hand with the other. The slice of the brick is so thin that the
bulk of the constituents is rendered transparent, or semi-transparent.
The preparation of such slices[15] is not difficult, but demands
some experience; those who have neither the time nor patience to
make them will find it convenient to send the fragments of brick to
Damon, of Weymouth, or some other first-class dealer in geological
and mineralogical specimens. The price charged, per slide, is usually
1s. 6d. At the same time, the student will find it eminently to his
advantage to prepare the slices himself. In the process he will learn
much that escapes attention when the work is done by another.

The thin slice mounted on a slip of glass is placed on the stage of
the microscope and firmly clipped, as with the fragment. The reflector
is brought into position, and a beam of light thrown through the
slice--the thin section is now being examined in transmitted light. At
first it will be convenient to study it with the polariser and analyser
thrown out of position. A certain proportion of the constituents is
found to be opaque, and should be examined in reflected light, as
above described. The remainder are more or less transparent, and some
of the grains will, possibly, be coloured. We notice the way in which
the whole of the fragments are bound together--say, by some opaque
mineral such as iron--or whether they seem to be partially or wholly
fused together. In the case of a vitrified brick, the latter phenomenon
is most usual, and we shall find that although crystalline fragments
have been melted, or partially fused, there is commonly a centre or
nucleus of each fragment in its original condition remaining, which
passes through insensible gradations from the crystalline to the
non-crystalline, or amorphous state. This latter circumstance may
be ascertained by using the polariscope. Ignoring the opaque matter
adverted to, we shall then see that what was transparent in ordinary
light appears, for the most part, to be opaque in polarised light.
Those portions which still let the light through are truly crystalline,
and by revolving the stage we notice that they frequently change
tint, becoming alternately light and dark. In that brick where the
particles are agglutinated by igneous fusion, we shall observe the
light decreasing in intensity from the crystalline portion (forming the
nucleus, as it were, of each particle) outwards, and where the crystal
fragment has been melted, so as to become fused to its neighbour, the
periphery, or rather what was originally the boundary of the fragment,
is quite dark. Polarised light cannot pass through non-crystalline
matter, and in being melted that portion of the crystal fragment
had passed from the crystalline to the non-crystalline stage. It is
very easy, therefore, to determine how far the fragments composing
a vitrified brick have been melted down and fused together; but to
observe the phenomena under the most favourable conditions, the brick
must be thoroughly well-burnt, and the section taken, by preference,
from near the outside surface of the brick.

In some instances, partial fusion is so well exemplified (especially
in bricks from fairly pure china clay), and the brick, after being
burnt, has been permitted to cool so slowly, that devitrification
has set in, when we are presented with aggregates of crystallites
closely resembling the “felspathic matter” of petrologists. That is
a circumstance which the maker should note well, for he has burnt
the brick to the best advantage, and it is not then so brittle as it
might have been had more “glass” made its appearance in the section.
Prolonged heat, just above the agglutinating point, has accomplished
this, and the microscope here clearly shows the advantage of allowing
the kiln to cool slowly, and to permit the lapse of several days in the
operation.



CHAPTER XII.

THE MICRO-STRUCTURE OF BRICKS (_Continued_).


Turning now to the actual appearance of minerals commonly found in
bricks as they are examined under the microscope, we may remind the
reader, that the physical aspect of the majority of them has already
been described in those chapters dealing with the “Mineral Constitution
of Brick Earths” and “Minerals: their behaviour in the Kiln,” and the
particulars that follow may be read in conjunction with what was there
said.

It will be convenient now to describe the appearance of certain
well-known minerals, as they are seen (A) in reflected light and (B)
in thin sections in transmitted light, whilst the latter will be
subdivided into _1_ denoting the phenomena observed in ordinary light,
and _2_ in polarised light. To save repetition, the letters and figures
will be used to denote the methods of examination as indicated.

QUARTZ.--Present in nearly all rubber-bricks, and in the vast majority
of common stocks, as well as in vitrified goods and fire-bricks. In the
last mentioned, the grains are usually partially agglutinated, and are
extremely minute.

A. As more or less rounded, or sub-angular fragments, white and
crystalline, like clear window glass.

B. 1--Clear white, often broken up by thin hair-like lines running
in various directions, and rows and patches of minute specks, which,
as previously remarked, have been shown to contain fluid, &c. 2--On
revolving the stage of the microscope, the crystals are usually
seen to present beautiful, clear transparent colours, which in
characteristic sections are very vivid--red, blue, yellow, &c.

FLINT.--Found in the same class of bricks as quartz.

A. Bluish horn colour; irregular fragments and splinters.

B. 1--Translucent; often melted more thoroughly than quartz in hard
burnt bricks; colourless. 2--Opaque unless in some such form as
chalcedony, when an extremely minute granular aspect results, becoming
slightly transparent. Melted portions always opaque.

FELSPAR.--The alteration which the different kinds of felspar have
undergone in a hard burnt brick, when present, render it almost
impossible to recognise them specifically.

A. Milk white, or more rarely light pink; the mineral, even when red in
the raw earths, becomes white on the application of moderate heat, as
in the burning of common bricks. It is often closely fractured, and but
rarely powdered.

B. The characteristic parallel lines of the triclinic varieties may
often be observed, especially in rubber bricks; but great heat, such
as leads to partial peripheral fusion, frequently obliterates them
to a large extent, and in a well-burnt brick it is quite impossible
in the majority of cases to determine whether the felspars present
are triclinic or monoclinic. More particularly is this the case when
the mineral has been more or less decomposed prior to its having been
burnt. The bulk of the fragments of the mineral can only be alluded to
in the general term “felspars,” and in ordinary light these are opaque
or “fleecy,” whilst in polarised light minute portions may be found to
be slightly birefringent. In a decomposed state it forms a prominent
constituent of brick-earths in the first place, and that is precisely
the material which most readily agglutinates in presence of a suitable
flux. Crystallites are not uncommon in the melted peripheries, as may
be seen in a hard-burnt brick in ordinary light.

MICA.--In minute flakes, shining, or glistening, and commonly black,
silvery or bronze-coloured.

A. Detected at once by its thin shining scales, which frequently have
not suffered much in the kiln except near the outside of the brick.

B. 1--The darker micas are usually citron coloured or light brown,
and unless cut parallel to the cleavage of the mineral, exhibit a
number of closely-set parallel lines, the fragments being much “frayed
out” and “ragged” at the edges. 2--Using one nicol only, the mineral
changes from dark to light on the revolution of the stage, and is said
(in common with other minerals exhibiting a similar property) to be
dichroic. With both nicols in position but little further difference is
noted, except that in changing tint the whole is darker. Vivid colours
are not observed except in yellows and browns. Muscovite mica is often
quite white and transparent.

IRON.--Common except in white bricks made from the purest china-clays.

A. Brown or reddish-brown specks; sometimes as blue black films in
fire-bricks; dull and frequently powdery in common bricks. Surrounding,
film-like, grains of mineral matter of which the brick is composed.
A grain of quartz, for instance, is frequently seen enveloped by a
film of red iron. Other metallic iron is more lustrous and whiter than
magnetite when seen in reflected light, but such unaltered particles of
the mineral could only occur in a brick that had not been subjected to
great heat.

B. Opaque either in 1 or 2.

IRON PYRITE only occurs as such in bricks that have not been thoroughly
burnt, or in common “baked” bricks. Higher temperatures lead to the
separation of the iron from the sulphur and the general incorporation
of both in the agglutination of the brick during partial fusion.

A. Brassy yellow particles.

B. Opaque both in 1 and 2.

CALCITE.--Not found in burnt bricks, nor indeed in any except those
that have been sun-dried, or have been subjected to very little heat.
Small pellets of lime are of common occurrence in poorly-burnt bricks.
In reflected light such pellets are generally of a dirty white tint;
opaque in transmitted light.

DOLOMITE.--Practically the same observations apply as to calcite,
crystals of dolomite not being found except in sun-dried bricks and
the like. Under the action of much heat the mineral, like calcite, is
reduced to lime.

SELENITE.--This is not rare in the commoner class of bricks, though
the application of much heat reduces it to the state of powder. In
reflected light it is found to be present as extremely minute specks or
“tears” of whitish powdery plaster. Opaque, of course, in transmitted
light.

The description of the micro-appearance of many other minerals which
occur but rarely in bricks does not fall within the scope of the
present elementary treatise; for practical purposes they may be
ignored.



CHAPTER XIII.

ABSORPTION.


The advantage of knowing the relative absorptive capacity of bricks has
been stated in these pages in divers connexions. The means of arriving
at the total capacity for absorption of water, as generally practised
by experimenters, are very incomplete and founded on an erroneous
principle. It is admitted by all that absorption is one of the very
best tests as to the quality of a brick, but such tests are meaningless
unless they imitate one or other or several of the influences to which
the brick would be subjected on being used in the building, or other
structure.

A common method is to weigh the brick when dry and then to immerse
it in water for periods varying from one to three days, subsequently
re-weighing it, the difference in weight between the dry and wet states
being termed the brick’s “absorptive capacity.”

Mr. Heinrich Ries remarks[16] that the absorption is determined by
weighing the thoroughly dry samples, immersing in clean water from
48 to 72 hours, then wiping dry and weighing again. Vitrified bricks
should not show a gain in weight of over 2 per cent. There are cases
where bricks of apparently good quality shew a greater absorption than
this, but they have great toughness and refractory qualities. Bricks
made from fire-clays which will not vitrify so easily will, naturally,
show higher absorption.

Again, Mr. E. S. Fickes, of Steubenville, Ohio, has recently made[17] a
large series of valuable tests of both paving and building bricks, in
which he shews the connexion between the power of absorption and the
strength of the materials experimented with. Mr. Fickes’ more important
conclusions are:--

1. The strength of the building brick, both transverse and crushing,
varies in tolerably close inverse ratio with the quantity of water
absorbed in twenty-four hours. The strongest bricks absorb the least
water.

2. Good building bricks absorb from 6 to 12 per cent. in 24 hours, and
with no greater absorption than 12 per cent. will ordinarily show from
7,000 to 10,000 or more pounds per square inch of ultimate crushing
strength.

3. Poor building bricks will absorb one-seventh to one-fourth of their
weight of water in 24 hours, and average a little more than one-half
the transverse and crushing strength of good bricks.

4. An immersed brick is nearly saturated in the first hour of
immersion, and in the remaining 23 hours the absorption is only
five-tenths to eight-tenths of 1 per cent. of its weight, as a rule.

These experiments are of much interest and are probably approximately
correct; but we venture to think that if the absorption experiments had
been carried out in a different manner, the results would have been
still more valuable.

Long before the publication of the results of the last mentioned series
of experiments, the present writer had discovered the close connexion
which subsists between the relative absorptive capacity of bricks and
their strength; a slight correction must be applied for specific
gravity. We are not prepared to enter into this subject at any length,
but it may be observed that we should not have arrived at such close
results had we experimented in the same way as the American authors
just quoted (or others, for the matter of that).

When you completely immerse a brick in water you prevent the escape of
air to a very large extent from the pores in the interior of the brick.
An old-fashioned way of overcoming this difficulty, was to place the
brick in the receiver of an air-pump and exhaust the air, subsequently
immersing the brick. This latter method certainly possessed the merit
of enabling the experimenter to arrive at total absorption very
rapidly, but it did not imitate natural processes any more than does
the thorough immersion of the brick in water.

A writer in the _Builder_ of May 25th, 1895, p. 397, experimented as
follows:--The bricks were placed in water in a large vessel, on edge,
supported where necessary by flat blocks, to bring the uppermost face
of each brick about ¼-inch above the surface of the water. Experience
had shewn that by completely immersing a brick, the air did not get
an opportunity of escaping from its pores with the same facility as
when one surface was left out of water. This disability, it was found,
materially impaired the results of the rate of absorption (rate, as
well as total tests, being carried out). By arranging the experiments
in the manner described, there can be no doubt that each brick absorbed
the maximum quantity of water possible; at any rate, there was no
water-pressure from above to retard the expulsion of the air.

The tests in the last-mentioned case extended over one week, the
relative absorption being taken at intervals of 1 second, 1 minute, 30
minutes, 1 day, and at the end of the week. It was found that English
vitrified bricks absorbed from 1.16 to about 1.85 per cent. in one
week; white glazed and good red and blue facing bricks from 5.31 to
10.34 per cent. in one week; wire cut facers and rubbers, with white
gaults, imbibed as much as from 12.93 to 20.50 per cent. of their dry
weight in one week. The rate of percolation suggested many interesting
problems, not the least important being the effect of chemical
decomposition in prolonged immersions, whereby after being quiescent
for a few days (after taking in the water for a few hours), absorption
“burst out” again and continued to the end of the week. One thing is
very apparent from this, namely, that for the lower grade brick even an
immersion for one week is not sufficient for practical purposes. The
writer remarks, “some of the red bricks from Bracknell, being placed
in the vicinity of the white gault bricks (in the water), discoloured
the latter to such an extent as to disfigure them. It was not merely a
surface colouration; it extended to at least ¼-in. into the interior.
The red colouring matter was iron, but there was not enough of it by
weight dissolved to materially interfere with the experiments. This
very clearly shews, however, the folly of erecting a building coursed
with white and red bricks, when both are very absorbent and the red has
so little hold of the iron of which it is partly composed--unsightly
stains are bound to appear.”

This question of the solubility of certain ingredients of bricks, has
not received the attention it deserves; and closely connected with
that is gradual decomposition, whereby the brick becomes more and more
porous--a potent factor in its ultimate destruction.



CHAPTER XIV.

STRENGTH OF BRICKS.


A very great deal is known concerning the strength of bricks. In
addition to the innumerable experiments carried out by public bodies,
we have the results of painstaking investigation by professors in
universities and colleges, and the results carried out for and
published by brickmakers themselves. Yet another large series of
results have been published from time to time by professional
journals, and it is, indeed, to these that we must look (at any
rate in Britain) for anything like detailed work. The “Minutes of
Proceedings of the Institution of Civil Engineers,” the “Transactions
of the Royal Institute of British Architects,” the “Proceedings” of
several allied provincial architectural societies, the “Builder,” the
“British Clayworker,” builders’ “Price Books,” and several engineering
“Handbooks,” have all contributed to our knowledge in regard to the
strength of bricks. Of works consecrated entirely to the subject there
are none--applied to British materials; but we have that excellent
text-book by Professor Unwin, F.R.S., “The Testing of Materials of
Construction,” and the important work by Mr. David Kirkaldy, both of
the greatest possible value as being the results, largely, of original
work. The experiments of recent years have been made almost exclusively
by Mr. David Kirkaldy at his works in Southwark; by Professor W. C.
Unwin at the Central Institution of the City and Guilds of London
Institute; and by the Yorkshire College, Leeds.

With such a wealth of information a whole treatise might profitably
be written, but it will be understood that in a small work like the
present we can only give a comparatively few results, prefaced by
observations to impart a general idea.

With the strength of brickwork, it is different, and it would seem
rather remarkable, at first sight, that architects and engineers, who
are every day using thousands of bricks, should have been at little
pains to ascertain the “safe load” which this or that brick pier or
wall would carry. Experience is, of course, of great value in all work
of that description; but there is always the lurking suspicion that the
engineer is making his piers too big, and that the architect is by no
means running the thing close. The real reason why so little has been
done to test the strength of brickwork is the difficulty in getting
machines of such capacity as would crush sufficiently large masses.
Small piers have been built from time to time, and bricks embedded in
putty for mortar have served their purpose, but practically nothing
of a really serious nature was carried out in Britain until a few
months ago. The science committee of the Institute of Architects, well
knowing the advantage of information as to the strength of brickwork,
have partially carried out a most elaborate series of experiments,
the first fruits of which have already been published, but it would
be out of place to allude to them here. When the remaining brickwork
shall have been built long enough at the experimental station, the
final experiments will be made, and the results will, we have no doubt,
be the most important contribution to our knowledge concerning the
strength of brickwork that has ever been published in the kingdom.

But we must give our attention solely to the strength of bricks. To
begin with, we must deprecate the idea that experiments as at present
carried out give anything like the actual strength of bricks--the
results are generally either too high or too low. Neither are the
results comparative, except to a limited extent. One kind of brick has
a “frog” on one side, another is recessed on both sides, a third is
stamped with the maker’s name, or some device by way of trade mark,
a fourth is as flat on all sides as may be, a fifth is pressed, a
sixth is hand made, and a seventh wire-cut, and there are many other
varieties of make. With such different kinds it is next to impossible
to arrive at comparative data that shall be of much use for working
purposes. Again, the whole brick may be subject to the experiment, or
only the half-brick. The faces placed between the dies of the crushing
machine may not be flat, and they are most frequently irregular. If
the dies are applied to such bricks it is evident that corners will
be broken off before the brick has really suffered much, and that to
get the best result the faces must either be made perfectly true and
parallel to each other, or some other method adopted to put matters
right. That commonly employed is to place some yielding substance
between the faces and the surface of the dies. Sometimes thin sheets
of lead or pine wood are inserted. Professor Unwin has the faces of
the brick made smooth and parallel by means of plaster of Paris, and
the brick is then crushed between two pieces of millboard or between
the iron pressure-plates, one plate having an arrangement to allow for
any slight want of parallelism between the two surfaces of the brick
applied to the plates.

Now it will be obvious, what with the difference in the shape and the
various modes of experimenting, that the results are by no means
comparative unless the precise facts are given; and when they are, it
is but rarely that you can find more than half-a-dozen or so kinds
of bricks of each category that offer all the elements necessary for
comparison. So that, with all the wealth of information, we are by no
means laden with much that is of actual comparative value, and if the
experiments and their results are not comparative, of what use are
they? So long as experimenters are each allowed a different method of
research, and so long as makers will have partial or whole “frogs,”
will stamp their names or initials, or will produce plain bricks only,
so long will it be impossible to arrive at the best results that are
really attainable. What we want is a government testing station as
they have in Germany; or, at least, the mode of experimenting should
be under some central control. The experimenter, further, should
select the samples to be crushed, and should be at liberty to publish
all results obtained. At present, if the brickmaker does not like
the results arrived at, he, of course, does not publish them. And,
if he has had a number of experiments carried out from time to time,
he will, usually, quote only the highest results on his bricks. That
is perfectly natural, and would be understood as “business.” All
brickmakers may not do that, and a few may publish every or average
results (we do not mean of one set of experiments, on say six bricks)
of different experiments, but we fancy they are very rare. Therefore,
in a matter so important to the architect and the engineer, and indeed
to the general public, from the point of view of safety, we maintain
that the whole thing should be carried out under some central control,
as on the continent.

And now to proceed with the description of results on a few typical
bricks. Glancing at table I, we may say that the strength of bricks
as a whole is often quoted as here given, and has done duty for many
years as the average strength of bricks. These bricks were crushed in a
Clayton machine, and all were bedded upon a thickness of felt and laid
upon an iron faced plate, and the experiments were conducted by the
Metropolitan Board of Works.


STRENGTH OF BRICKS.--I.

  -------------------------------------------------
          Description.        | Pressure in tons to
                              +----------+---------
                              |   Crack. | Crush.
  ----------------------------+----------+---------
  Four white bricks, each     |   16.25  | 41.00
  Three  „     „      „       |   17.05  | 41.05
  Red bricks, ordinary        |   13.00  | 26.25
  Red bricks, not well burned |   13.75  | 25.05
  Best Paviours               |   14.00  | 23.00
  Grey Stocks, London         |   12.00  | 14.00
  ----------------------------+----------+---------

Turning to the second table, compiled for the most part from
brickmakers’ circulars, and from the original results obtained for
the late Building Exhibition, at the Agricultural Hall, all the
experiments, we believe, having been carried out by Mr. David Kirkaldy,
it will be noted that great variation in strength is apparent,
following the different kinds of bricks. The highest result, 1064.2
tons per square foot, was obtained on a blue Staffordshire brick,
though that is very closely run by bricks made from slate débris
(1056.2 tons) from South Wales. The lowest result, 139.5 tons per
square foot, was from a Worcester brick.


STRENGTH OF BRICKS.--II.

  -------------------+----------------+--------------------+------------------
                     |                |                    |  Mean stress of
                     |                |     Dimensions,    |  six samples in
     Locality.       |  Description.  |       Inches.      |tons per square ft
                     |                |                    +---------+--------
                     |                |                    | Cracked | Crushed
  -------------------+----------------+--------------------+---------+--------
  West Bromwich      |     Blue       |  2.74, 9.03 × 4.36 |   548.6 | 1064.2
                     |                |                    |         |
    „     „          | Blue (another  |} 2.80, 8.75 × 4.12 |   260.7 |  651.0
                     |    make)       |}                   |         |
                     |                |                    |         |
    „     „          | White glazed,  |}                   |         |
                     |   “Terra       |}                   |         |
                     |   Metallic,”   |} 3.10, 8.80 × 4.22}|   225.0 |  273.7
                     |   recessed     |} 3.16, 8.70 × 4.34}|         |
                     |   both sides   |}                   |         |
                     |                |                    |         |
    „     „          | Blue vitrified |  2.55, 9.03 × 4.30 |   245.1 |  654.9
                     |                |                    |         |
                    {| “Pressed,”     |}                   |         |
  Worcester         {|   recessed top |} 3.20, 9.14 × 4.50 |    65.0 |  139.5
                    {|   and bottom   |}                   |         |
                     |                |                    |         |
      „              | “Builders.”    |}                   |         |
                     |   recessed top |} 3.20, 9.30 × 4.50 |    56.1 |  155.5
                     |    and bottom  |}                   |         |
                     |                |                    |         |
  Saltley,           | Red, recessed  |} 3.20, 8.90 × 4.35}|   138.7 |  180.5
  Birmingham         |   one side     |} 3.25, 8.95 × 4.40}|         |
                     |                |                    |         |
  Rowley Regis,      | Blue vitrified |} 2.85, 8.75 × 4.20 |   385.6 |  722.7
  Staffs.            |   no recess    |}                   |         |
                     |                |                    |         |
  Leicester          | Red, recessed  |} 2.65, 8.90 × 4.25}|   105.9 |  150.6
                     |   both sides   |} 2.75, 9.10 × 4.36}|         |
                     |                |                    |         |
  Napton-on-the-Hill,|  Light brown,  |} 2.85, 8.92 × 4.20}|   131.6 |  303.9
  Rugby              |    wire cut    |} 2.90, 9.10 × 4.25}|         |
                     |                |                    |         |
  Ruabon             | Red, no        |} 3.10, 8.75 × 4.28}|   439.2 |  676.8
                     |        recess  |} 3.15, 8.73 × 4.29}|         |
                     |                |                    |         |
     „               | Blue, no       |} 3.02, 8.99 × 4.37}|   358.9 |  561.2
                     |        recess  |} 3.01, 8.95 × 4.36}|         |
                     |                |                    |         |
  Glogue, Whitland,  | Slate débris   |  2.33, 8.70 × 4.25 |   556.4 | 1056.2
  S. Wales           |                |                    |         |
                     |                |                    |         |
  Ravenhead, St.     | Red, brown     |} 2.90, 9.00 × 4.20}|   215.8 |  354.7
  Helens, Lancs.     |   wire cut     |} 2.90, 8.90 × 4.27}|         |
                     |                |                    |         |
  Earith, St. Ives,  | Yellow, wire   |} 2.50, 8.70 × 4.10}|   135.9 |  178.8
  Hunts.             |   cut          |} 2.50, 8.80 × 4.20}|         |
                     |                |                    |         |
  Gillingham, Dorset | Red, wire cut  |} 2.60, 8.90 × 4.30}|   159.5 |  261.7
                     |                |} 2.60, 8.90 × 4.25}|         |
                     |                |                    |         |
  Newton Abbot,      | Vitrified      |} 2.80, 8.90 × 4.35}|    --   |  445.2
  Devon              |   “granite”    |} 2.80, 9.10 × 4.55}|         |
  -------------------+----------------+--------------------+---------+--------

Table III. is by Professor Unwin,[18] and records the strength of
several well-known bricks. Professor Unwin’s mode of experimenting we
have already alluded to.


STRENGTH OF BRICKS.--III.

  ---------------+----------------+-----------+----------+--------+------------
                 |                |Cracked, at|Crushed at|        |
    Description. |  Dimensions.   | tons per  | tons per | Colour.| Remarks.
                 |    Inches.     |  sq. ft.  |  sq. ft. |        |
  ---------------+----------------+-----------+----------+--------+------------
  London stock   |4.6 × 4.1 × 2.4 |    128    |    177   | Yellow |Half brick
    „      „     |4.6 × 4.0 × 2.45|    133    |    181   |    „   |    „
    „      „     |9.2 × 4.1 × 2.8 |     --    |    129   |    „   |
    „      „     |8.9 × 4.2 × 2.3 |     --    |    113   |    „   |
    „      „     |8.9 × 4.25 × 2.5|     --    |    103   |    „   |
  Aylesford,     |8.9 × 4.4 × 2.7 |     48    |    183   |  Pink  |
    common       |                |           |          |        |
      „      „   |8.9 × 4.4 × 2.7 |    111    |    228   |    „   |
      „   pressed|9.1 × 4.3 × 2.7 |     71    |    141   |   Red  |Deep frog
  Rugby, common  |9.5 × 4.2 × 2.9 |    158    |    190   |    „   |{ Between  }
    „      „     |9.0 × 4.2 × 3.0 |     --    |    120   |    „   |{ pine bds.}
  Lodge Colliery,|                |           |          |        |
    Notts        |9.0 × 4.2 × 3.4 |    127    |    159   |    „   |
    „       „    |9.0 × 4.2 × 3.25|     55    |    122   |    „   |
  Digby Colliery,|                |           |          |        |
    Notts        |9.3 × 4.1 × 3.25|    248    |   [353]  |    „   |Not crushed
    „       „    |4.6 × 4.2 × 3.2 |    414    |    414   |    „   |Half brick
  Ruabon, pressed|8.8 × 4.3 × 2.7 |    361    |   [361]  |    „   |Not crushed
  Grantham, wire |9.2 × 4.4 × 3.2 |     --    |     83   |    „   |
    cut          |                |           |          |        |
  Leicester,     |4.4 × 4.1 × 2.6 |    251    |    337   |Pale red|Half brick
      „  „  „    |4.3 × 4.1 × 2.6 |    109    |    308   |    „   |    „
      „  „  „    |9.06 × 4.2 × 2.8|    115    |    229   |    „   |
  Cranleigh,     |4.7 × 4.6 × 2.5 |    149    |    181   |    „   |Half brick
    pressed      |                |           |          |        |   frog.
      „  „       |4.6 × 4.6 × 2.5 |    165    |    237   |    „   |„   „   „
  Candy, pressed |8.8 × 4.3 × 2.8 |     80    |    381   |   --   |
  Gault, wire cut|8.7 × 4.1 × 3.0 |    111    |    173   |  White |
  „   „          |4.4 × 4.2 × 2.5 |    119    |    145   |    „   |Half brick
  „   „          |8.7 × 4.1 × 2.9 |    --     |    169   |    „   |
  Staffordshire  |                |           |          |        |
    blue, common |4.5 × 4.3 × 3.0 |    216    |    464   |  Blue  |    „
    „    „    „  |4.3 × 4.2 × 3.0 |    152    |    386   |    „   |    „
    „    „    „  |8.9 × 4.3 × 3.1 |    240    |   [353]  |    „   |Not crushed
  Staffordshire  |                |           |          |        |
    blue, pressed|9.0 × 4.3 × 3.1 |     --    |    275   |    „   |
  Glazed brick   |8.8 × 4.4 × 3.3 |     69    |    166   |   --   |Frog.
    „      „     |8.9 × 4.4 × 2.9 |    166    |    174   |   --   |
  ---------------+----------------+-----------+----------+--------+------------

Table No. III. is specially instructive as indicating the relative
strength of several well-known bricks, the experiments being carried
out solely for scientific purposes. Yet the figures must not be taken
too seriously. Glancing at those relating to “London Stocks,” we find
the strength varied from 103 tons per square foot to 181 tons. But
more recent experiments made by Professor Unwin[19] on some London
Stocks from Sittingbourne, in Kent, shewed that with four samples one
crushed at 60.76 tons per square foot and another gave out 94.6 tons,
the mean strength of the four yielding 84.27 tons per square foot.
With such heterogeneous materials as London Stocks, we ought not to be
surprised at these results, but they form a striking commentary on the
value of general statements concerning the strength of bricks of varied
character going by the same name in the market.

When we consider the strength of homogeneous bricks, and especially
where these latter are made of thick marine clays, or where the
relative proportions of earths employed are carefully attended
to in the raw material, the results appear to be more generally
applicable--as far as they go.

With ordinary Gault bricks we find a range in strength from 145 tons to
173 tons per square foot; but Professor Unwin,[20] in his more recent
experiments, finds that of four Gault bricks, one reached as high as
197.6 tons per square foot, and he gives 182.2 tons as the average
strength.

To shew the absurdity of alluding to the strength of “blue
Staffordshire” bricks, without also giving the precise locale of the
samples dealt with, the reader is requested to refer to Table III.,
where the figures indicate a range from 275 tons to 464 tons per square
foot, and to compare them with the results on Staffordshire bricks as
stated in Table II., where we find a range from 651 tons to 1,064.2
tons per square foot. Of what value can a single formula be which gives
the strength of Staffordshire bricks as a whole as based on such widely
divergent figures as these? Professor Unwin, in his recent series of
experiments alluded to, finds that with four Staffordshire blue bricks,
the weakest gave a result of 564.8 tons per square foot, and the
strongest 788 tons; the mean of the four being 701.1 tons per square
foot.

The results on the Leicester “reds” are no more encouraging; the
figures in the foregoing tables are 150.6 tons, 229 tons, 308 tons, and
337 tons per square foot. Similarly, Professor Unwin has more recently
found that the Leicester “reds” from Elliston, near Leicester, bear a
crushing strain varying from 311.4 tons to 591.4 tons per square foot
in four samples.

From the foregoing it will appear to the reader that average results
are of very little value to the architect or engineer, unless--(1)
the brickyard is mentioned from which the bricks experimented with
came; (2) the particular class of brick from that yard; (3) the method
of experimenting, as to whether any substance was placed between the
dies of the press and the brick to be crushed, and if so, what; (4) if
recessed or initialled; (5) whether machine or hand made, and (6) as to
whether the surfaces of the bricks were concave, convex, or flat.

Results on bricks not localised are not of much value, and it is
absolutely useless for working purposes to give in one figure the
strength of “London Stocks,” “Staffordshire blues,” “Leicestershire
reds,” and the like. In a general way, of course, it will be admitted
that the “Staffordshire blue” is a stronger brick than the “London
Stock,” and so forth; but that is as much as can be permitted--it is of
no practical use to give relative figures in general terms.

It frequently happens that the capacity of the machine used for testing
the strength of bricks is not enough for those bricks having a very
high resistance to crushing. In the recent experiments by Professor
Unwin, more than once alluded to in this article, it was found
necessary to experiment with half-bricks only, and he ascertained that
bricks tested as half-bricks shew about 25 per cent. less resistance
per square foot than when tested as whole bricks.

Further observations on strength are made under the next heading in
connexion with other forms of testing the value and physical properties
of bricks.



CHAPTER XV.

ABRASION, SPECIFIC GRAVITY.


_Abrasion._--In this country it is not customary to test bricks and
stone by means of the abrasion process, though many English materials
have been dealt with in this manner on the continent.

Abrasion tests are of special value in regard to paving bricks, and
this mode of experiment is largely carried out in the United States.
As Mr. H. Ries remarks,[21] the abrasion test approximates closely the
conditions under which the paving brick is used, and is, therefore, an
important one. The usual method of conducting this test is to put the
bricks in an ordinary “foundry rattler,” filling it about one-third
full. It is then rotated at the rate of about 30 revolutions per
minute, and about 1,000 turns are sufficient. The bricks are weighed
before and after to determine loss by abrasion.

A more recent modification is to line the “rattler” with the bricks to
be tested and then put in loose scrap iron. This is claimed to give
more accurate results, and avoids loss by chipping due to the bricks
knocking against each other, as in the previous method, although
that has been somewhat obviated by Professor Orton, jun., by the
introduction of a few billets of wood into the rattler.

The abrasion test may also be made by putting the weighed bricks on
a grinding table covered with sand and water, and noting the weight
before and after grinding. This last method seems to us to be decidedly
the best, provided the bricks be weighted, that the weight is constant,
that the feed of sand and water is uniform, and that the bricks to be
tested are placed equidistant from the centre of the turning table. If
this last point be not attended to, it will be obvious that in course
of the revolutions the sand will tend to accumulate towards the centre
of the table, and the bricks placed in that vicinity would receive
more than their fair share of abrasion, as compared with those bricks
situated near the edge of the table. Conversely, those bricks near
the periphery would be subjected to greater grinding action, from the
circumstance that the table would move faster underneath them than
under those bricks nearer the centre of the table.

The bricks should certainly be weighted in such abrasion tests, and
it seems desirable that the weights should be so adjusted that the
weight of the brick is also taken into account. It is obvious that the
abrading action of, say, street traffic, will be the same on a brick,
no matter what the latter weighs, depending on the area of surface
exposed to traffic. And if we experiment with one brick, weighing say 7
lbs., and another weighing 14 lbs., the greater weight of the latter,
will (_cæteris paribus_), by the abrasion tests as usually adopted,
give a much higher result than would the lighter brick. On the other
hand, if the 7 lbs. brick be weighted another 7 lbs., then the results
would be strictly comparable, provided always that the area exposed to
abrasion in each case be the same, and that the other conditions we
have laid down are strictly observed.

Knowing as we do that the rough and ready method of “rattling” cannot
possibly give truly comparative results, we do not intend to enlarge
much on the results of the American tests; but the following are
suggestive as shewing the connexion between the tests for absorption,
rattling, and strength combined.

Some valuable and interesting tests were recently made by the Ohio
Geological Survey, to determine the relative merits of fire-clays and
shales for the manufacture of paving bricks, as well as the influence,
if any, of the method of manufacture adopted. Twenty-two varieties of
shale bricks, or bricks the largest constituent of which is shale, were
grouped together: fifteen varieties of fire-clay brick; four varieties
composed of shale and fire-clay mixed in equal proportion; and three
varieties made from Ohio River sedimentary clays. The averages of these
four classes of results were as follow:--

  --------------------------------------------------
    TESTS OF FIRE-CLAY AND SHALES.--PAVING BRICKS.
  ----------+-----------+---------+-----------------
            |Absorption.|Rattling.|    Crushing.
            +-----------+---------+--------+--------
            |           |         |Square  | Cubic
            |           |         |Inches. | Inches.
            |           |         +--------+--------
  Shales    |   1.17    | 17.61   | 7,307  |  1,764
  Fire-clay |   1.62    | 17.32   | 6,876  |  1,678
  Mixture   |   1.44    | 18.72   | 5,788  |  1,400
  River Clay|   1.36    | 19.02   | 4,605  |  1,176
  ----------+-----------+---------+--------+--------

From a series of tests recently made by Mr. Fickes,[22] the following
factors were educed:--

1. A brick which stands the “rattling” test well, has ample crushing
strength and rarely chips under less than 5,000 lbs. per square inch,
or crushes under less than 10,000 lbs. The crushing strength tends to
vary with the resistance to abrasion, however, but more slowly and
irregularly.

2. The transverse strength also tends to vary with the resistance to
abrasion, but more slowly and irregularly.

3. The toughest bricks usually absorb the least water.

_Specific Gravity._--The practical value of knowing the specific
gravity of a brick has, perhaps, been a little over-rated by writers on
the subject. At the same time we do not deny that there is some use in
ascertaining this property. Foremost, we have to mention its value in
conjunction with absorption in arriving at a rough and ready means of
gauging the strength of a brick, without having actual recourse to the
crushing machine. It appears to us, however, that the specific gravity
of bricks is rarely quoted in a proper manner, and until there is one
uniform method, the results will always be at a discount. We allude to
the fact that some experimenters take the specific gravity of a porous
brick, without stating whether the amount of water absorbed, during the
process, was taken into account in arriving at the specific gravity
or not. Theoretically, of course, the substance to be dealt with is
non-porous, and experimenters, worthy the name, either render the
brick waterproof, or, ascertaining the amount of water the brick has
absorbed, take that into consideration in calculating results.

The writer is in the habit of quoting the specific gravity in two ways,
viz.: (_a_) the true specific gravity, and (_b_) the specific gravity
of the particles. In an elementary treatise like the present, however,
it is not desirable to enlarge on this subject.


THE END.



FOOTNOTES


[1] This, and all other technical terms used, will be explained in an
alphabetical glossary at the end of the book.

[2] “Canal and River Engineering,” p. 315.

[3] See, Geikie’s “Text Book of Geology,” 1882, p. 72.

[4] Information on this subject will be found in Mr. J. H. Collins’
work, “The Hensbarrow Granite District.” Truro, 1878.

[5] “Text Book of Geology,” 1882, p. 85.

[6] “Aids in Practical Geology,” 1893, page 36.

[7] See E. S. Dana, “Minerals and How to Study Them,” 1895, p. 154.

[8] Consult “Applications of Geology,” etc., by Prof. Ansted, 1865, p.
116, _et seq._

[9] “Industrial Resources of the Tyne, Wear and Tees,” 1864, p. 204.

[10] R. H. Scott, “Elementary Meteorology,” 1883, p. 137.

[11] Report of British Association for 1846, Part II., p. 17.

[12] Geological Magazine, N.S., Dec. III., Vol. V, 1888, pp. 26 _et
seq._

[13] Such as “The Study of Rocks,” by F. Rutley: “Aids in Practical
Geology,” by Prof. Grenville Cole; “Tables for the Determination of
the Rock-forming Minerals,” by Prof. Lœwinson Lessing; “Petrology for
Students,” by A. Harker; and especially “Microscopic Physiography of
the Rockmaking Minerals,” by Rosenbusch (transl. Iddings).

[14] Consult the works on petrology previously mentioned.

[15] The mode of preparation of thin rock sections for examination by
the microscope is described in much detail in the works of Mr. Rutley
and Professor Cole previously alluded to; also in “Outlines of Field
Geology,” by Sir Archibald Geikie, 1882, p. 202 _et seq._

[16] 16th Ann. Rep. U. S. Geol. Surv. (1894–95), pt. IV., p. 532.

[17] 16th Ann. Rep. U. S. Geol. Surv. (1894–95), pt. IV., p. 539.

[18] “Testing of Materials of Construction,” 1888, p. 438.

[19] _British Clayworker_, April, 1896, Supplement, p. iv.

[20] _Op. cit._ p. iv.

[21] 16th Ann. Rep. U.S. Geol. Surv. Pt. IV., 1895, p. 532.

[22] _Engineering News_ (U.S.), Dec. 13th, 1894.



INDEX, &c.


  Abrasion tests, 146

  Absorption of bricks, 132

  Acids defined, 76

  Actinolite, 69

  Air, chemical composition of, 105

  Albite felspar, chemical composition, 34

  Almandite, 68

  Aluminium, under blowpipe, 73

  Anorthite felspar, chemical composition, 34

  Aragonite, 49


  Bases defined, 76

  Basic bricks, 90
    dolomite for, 55
    magnesite for, 56

  Biotite mica, 43
    under blowpipe, 73

  Black bricks, 101

  Blowpipe, 58

  Blue bricks, 101

  Bluish-black brick-earths, 27

  Boulder clay, 50

  Bourges, Oxford clay of, 24

  Bovey Heathfield clays, 20

  Bracknell bricks, 135

  Brick earths, artificial mixing, 42
    artificial mixtures, 94, 95
    bluish-black, 27
    boulder clay, 50
    brown, 27
    chalk pebbles in, 50
    changes in character on being dug into, 2, 5, 10
    chemical composition of, 23, 52, 83, 84, 85
    chemistry of, 58, 75
    chert in, 41, 42
    coprolites found in, 51
    Cornwall, 35
    Crayford, 1
    Devon, 35
    Erith, 1
    estuarine, 21
    fluviatile--Chapter I., 1–16
    fossil shells in, 50
    Ilford, 1
    Kimeridge clay, 26
    lacustrine--Chapter II., 17–21
    Lincolnshire, 21
    London clay, 33
    marine, 22
    mineral constitution, 28
    minerals found in (see Kaolin, Felspar, Quartz, Flint, Mica, Iron,
          Calcite, Aragonite, Selenite, Dolomite, Salt, etc.).
    Northamptonshire, 21
    North-Eastern France, 20
    Oxford clay, 33
    Reading mottled clay, 19
    of river terraces, 12
    salt in, 25
    sea-shore, 25
    section of fluviatile brick-earths, 10
    several kinds of fluviatile, discussed, 14, 15
    Switzerland, 89
    Thames Valley, 1, 2, 3, 4, 5
    value of chemical analyses of, 28, 29

  Brickmaking: earths suitable for (see Brick-earths)

  Bricks, abrasion tests, 146
    absorption of, 132
    basic, 90
    Bracknell, 135
    colour of, 100
    Dinas, 81
    discolouration of, 114
    durability of, 103
    effect of conflagrations on, 117
    efflorescence on, 110
    London stock, 97
    micro-structure of, 29, 118, 128
    rubber, 29
    specific gravity, 146, 149
    Staffordshire blue, 99
    Stourbridge, 82
    strength of, 136
    vegetable growth on, 113
    weathering of, 105, 113

  British Museum, fossils in, from brick-earths, 3, 26

  Bronzite, 69

  Brown brick-earths, 27

  Burning bricks, 94
    changes produced by, 98
    temperature, 89


  Calc spar, 49

  Calcite, 39, 49
    behaviour in kiln, 39
    micro-structure of, 131
    under blow-pipe, 74

  Californian magnesite, 56

  Carbon dioxide in quartz, 41

  Carbonate of lime (see Calcite, Aragonite)

  Chalk in brick-earths, 1
    mixed with brick-earths, 53
    pebbles in brick-earths, 50

  Chateauroux, Oxford clay of, 24

  Chemical affinity, 76
    analyses of brick-earths, 28
    analysis, 77
    composition of air, 106
    composition of brick-earths, Dinas, 52
    composition, china clays, 78
    composition of fire-clays, 80
    composition of Kieselguhr, 92
    composition of magnesian limestones, 90
    composition, pottery clays, 82
    composition of slates, 87
    disintegration of rocks, 20
    re-agents, 60, 63, 71

  Chemistry of brick-earths, 58, 75

  Chert, 41

  Cheshire, salt in clays in, 25

  China-clays, behaviour in the kiln, 36
    chemical composition, 78
    Cornwall, 35, 36, 37
    Devon, 35

  China-clay (see Kaolin and Felspar)
    thickness of, 38

  China-stone, decomposed, 37

  Colour in the kiln, 98, 99

  Colouring matter of bricks, 45
    of bricks, 53

  Colour of bricks (see Blue, Black, etc.).

  Coprolites: impure varieties of phosphate of lime
    found in brick-earths, 51

  Cornish granite, 35

  Cornwall, china-clays, 35, 36, 37

  Cracks formed in bricks, 52

  Crayford, brick-earth at, 1


  Dartmoor granite, 55

  Denudation, agents of, described, 6, 7
    of sea-cliffs, 22

  Devon, china-clays, 35

  Diatomaceous earth, 42, 91

  Dinas bricks, 52, 81

  Discolouration of bricks, 114, 135

  Dolomite in brick-earths, 55
    micro-structure of, 131
    under blow-pipe, 74

  Drying bricks, 94

  Durability of bricks, 103


  Efflorescence on bricks, 110

  Electric furnace, 33

  Elephants’ remains, found in brick-earth, 2, 3

  Erith, brick-earth at, 1

  Estuarine brick-earths, 21

  Expansion of bricks and variations of temperature, 115


  Felspar, 34
    chemical composition of, 34
    micro-structure of, 129
    under blow-pipe, 73

  Ferruginous matter (see Iron)

  Fire-bricks, Dinas, 52
    earths suitable for making, 21
    effect of lime in, 53
    Kieselguhr for, 42, 91
    strength of, 136

  Fire-clays, chemical composition of, 80, 81
    Newcastle-on-Tyne, 80
    tests, 148
    Welsh localities, 81

  Fishes, fossil, 25

  Flint, 39, 41
    behaviour of in the kiln, 42, 43
    implement: an implement, or tool, made of flint--in the sense
          indicated in this work an implement made by pre-historic man.
    implements, found in brick-earths, 3, 5
    micro-structure of, 129
    origin of, 41

  Fluid inclusions in quartz, 41

  Fluorine in clays, 59

  Fluviatile brick-earths: brick-earths that have been deposited in
          rivers

  Fossil shells, carbonate of lime in, 50
    shells found in brick-earths, 4
    sponges, in flint, 42

  Fusion of brick-earths in the kiln, 29, 31


  Gault clay, 51

  Glaze, micro-structure of, 119, 120

  Glazing, salt, 57

  Granite, Cornish, 35
    Dartmoor, 35

  Granites, weathering of, 36

  Greece, magnesite in, 56

  Green bricks, 101

  Grizzly bear’s remains found in brick-earth, 2

  Gypsum in brick-earths, 54
    under blow-pipe, 74


  Heat, bricks affected by, 117

  Hippopotamus remains found in brick-earth, 2


  Ilford, brick-earth at, 1

  Infusorial earth, Tuscany, 92

  Iron, 44
    a constituent of brick-earths, 44
    behaviour in the kiln, 45
    bricks, Saarbrücken, 92
    micro-structure of, 130
    mode of occurrence in brick-earths, 45
    under blow-pipe, 73, 74
    vapour in the kiln, 46
    pyrites, 46, 131
    pyrites, behaviour in the kiln, 48
    pyrites, under blow-pipe, 74
    pyrites, weathering of in bricks, 48


  Jurassic estuarine clays, 21


  Kangaroo rats, fossil, 25

  Kaolin: a hydrous silicate of alumina, derived chiefly from the
          decomposition of felspars

  Kaolin, 31
    behaviour in the kiln, 32, 33
    chemical composition of, 78
    micro-structure, 32, 33
    under blow-pipe, 73

  “Kaolinised” matter, 33

  Kilns, temperature in, 98

  Kieselguhr: a diatomaceous earth

  Kieselguhr, 91
    chemical composition of, 92
    of the Isle of Skye, 42

  Kimeridge clay brick-earth, 26


  Labradorite felspar, chemical composition, 34

  Lacustrine brick-earth: that laid down or deposited in lakes
    brick-earths--Chapter II., 17–21
    brick-earths, formation of, 17, 18

  Lime, builder’s, 52
    in bricks, 52
    in manufacture of fire-bricks, 53

  Limestone, a flux, 54

  Limonite, under blow-pipe, 73

  Lincolnshire brick-earths, 21

  Loam: sandy clay

  London stock bricks, 97, 140


  Magnesian limestones, chemical composition of, 90
    limestone (see Dolomite)

  Magnesite, 55
    behaviour in the kiln, 56
    Californian, 56
    in Greece, 56
    Styrian, 56
    under blow-pipe, 74

  Malachite, 67

  Malm bricks, 53, 86

  Manganese, under blow-pipe, 74

  Marcasite, 46, 47
    behaviour in the kiln, 48
    weathering of, 47, 48

  Marine brick-earths: those laid down or deposited on the sea-floor

  Marine brick-earths, not so variable in character as river,
          lacustrine, or estuarine, 23
    brick-earths--Chapter III., 22–27
    brick-earths, origin of, 23

  Marl: clay containing much lime.

  Mica, 43
    behaviour in the kiln, 44
    micro-structure of, 130
    under blow-pipe, 73

  Microscopes, 121

  Microscopes, use of, 59
    useful in analysing earths, 30

  Micro structure as a means of determining fusibility of minerals and
          brick-earths, 43
    of bricks, 118, 128

  Minerals, their behaviour in the kiln (see Kaolin, Felspar, Quartz,
          Flint, Mica, Iron, Calcite, Aragonite, Selenite, Dolomite,
          Salt, etc.).
    behaviour under the blow-pipe (see Quartz, Felspar, Mica,
          Calcite, etc.).

  Mississippi, sediments of the Delta of, 7

  Mortar and Scum, 112

  Muscovite mica, 43
    mica, under blow-pipe, 73

  Musk-sheep remains, found in brick-earth, 2, 3


  Natrolite, 68

  Newcastle-on-Tyne fire-clays, 80

  Newton Abbot, clays near, 20

  Northamptonshire brick-earths, 21


  Oligoclase felspar, chemical composition, 34
    felspar, under blow-pipe, 73

  Origin of fluviatile brick-earths--Chapter I., 1–16

  Orthoclase, 69
    felspar, chemical composition, 34
    felspar, under blow-pipe, 73

  Overburden: the material for the most part useless, overlying the good
          brick-earth, sand, limestone, or other rock for which the pit
          or quarry was exploited, and which, in the majority of cases,
          has to be removed to obtain the material sought for.

  Oxford clay, 24, 25, 33

  Oxidising flame, 65


  Paving bricks, tests, 148

  Peterborough bricks, 24
    clays, 51

  Plants, fossil, 21, 26

  Platinum wire, 60, 63, 66

  Plesiosaurus, fossil reptile, 26

  Porcelain earths, 30

  Pottery clays, chemical composition, 82

  Power of transport of sediment by rivers, 8

  Pumice for brickmaking, 93

  Pyrite, 46
    behaviour in the kiln, 48
    under blow-pipe, 74

  Pyrometers, 99, 100


  Quartz, behaviour in the kiln, 39, 42, 43
    cavities in, 40, 41
    characters of, 39
    imperishable, 40
    micro-structure of, 128
    occurrence in brick-earths, 40
    under blowpipe, 72
    vein, 40


  Race: concretions of carbonate of lime, commonly found in brick-earths
          in brick-earths, 50

  Rainfall and the durability of bricks, 108

  “Rattling” tests, 148

  Reading mottled clay, 18

  Red brick, 100

  Red brick clay, 86

  Red bricks, colouring matter of, 45

  Reducing flame, 65

  Refractory minerals (see Calcite, Dolomite, Magnesite, Quartz, etc.).

  Reindeer remains, found in brick-earth, 2, 3

  Reptiles, fossil, 25

  Rhinoceros’ remains, found in brick-earth, 2, 3

  River deposits, typical section, 10, 11

  River terraces, brick-earths of, 12, 13

  Rock crystal, 40

  Rock salt, under blow-pipe, 74

  Ruabon bricks and terra-cotta, 84
    terra-cotta, 116

  Rubbers, 29, 86


  Saarbrücken “iron bricks,” 92

  Salt, a powerful flux, 57
    behaviour in the kiln, 57
    glazing, 57
    in brick-earths, 25, 27, 56
    in quartz crystals, 41
    under blow-pipe, 74

  Schorl, 40

  Scum, 110

  Sea-shore, brick-earths from the, 25

  Selenite, 54, 131
    under blow-pipe, 74

  Septaria: tabular or rounded concretions of argillaceous limestone,
          commonly found in clays

  Septaria, 51

  Shrinkage of brick-earth in the kiln, 85

  Silica, behaviour in the kiln, 28
    group of minerals, 39

  Slates, chemical composition of, 87
    débris for brickmaking, 87
    refuse, 47
    used in brickmaking, 47

  Snails, found in brick-earth, 4

  Specific gravity of bricks, 146, 149

  Stacking in the kiln, 97

  Staffordshire blue bricks, 99

  Stibnite, 68, 70

  Stocks, London, 140

  Stoneware, earths for making, 21

  Stourbridge bricks, 82

  Strength of bricks, 136

  Styrian magnesite, 56

  Swiss brick-earths, 89


  Temperature and weathering of bricks, 115
    in kilns, 98

  Terra-cotta earths, 19, 30
    earth, chemical composition of, 84
    expansion of in weathering, 115
    Ruabon, 116

  Tests for bricks (see strength, absorption, specific gravity, chemical
          composition, micro-structure, etc.).

  Thames, mineral salts in solution in the, 7

  Thames Valley brick-earths, 1, 2, 3, 4, 5

  Tuscany, infusorial earth, 92


  Variable character of brick-earths, 10

  Variability in character of marine brick-earths, 23, 24
    of estuarine brick-earths, 21
    of lacustrine brick-earths, 17

  Vegetable growth on bricks, 113

  Vein quartz, 40

  Volcanic ejectamenta for brickmaking, 93


  Wales, fire-clays of, 81

  Warping, 98

  “Weathering” agents which affect bricks, 6

  Weathering of brick-earths, 27

  White bricks, 100


  Yellow bricks, 101


[Illustration]



Transcriber’s Notes


As Footnote 1 states, technical terms are explained in the Index at the
end of the book.

Punctuation and spelling were made consistent when a predominant
preference was found in the original book; otherwise they were not
changed.

Inconsistent hyphenation was not changed.

Simple typographical errors were corrected; unbalanced quotation
marks were remedied when the change was obvious, and otherwise left
unbalanced.

Figures 1 and 4 were repositioned slightly in the paragraphs that
reference them, thereby splitting those paragraphs.

The uncaptioned illustration above the second advertisement is an image
of that advertisement. The uncaptioned illustration at the end of the
book is decorative.

In this Plain Text version of the eBook, the chemical formulas on pages
76–77 are shown unsubscripted, e.g., H2O.

The original book used middle-dots to represent decimal points; this
eBook uses baseline periods.

The index was not checked for proper alphabetization or correct page
references.





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