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Title: A Handbook of Laboratory Glass-Blowing
Author: Bolas, Bernard D.
Language: English
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A HANDBOOK OF LABORATORY GLASS-BLOWING
_To my Friends
Eric Reid
and
Sidney Wilkinson_
A Handbook of Laboratory Glass-Blowing
BY
BERNARD D. BOLAS
WITH NUMEROUS DIAGRAMS IN THE TEXT
BY NAOMI BOLAS
[Illustration]
LONDON
GEORGE ROUTLEDGE & SONS, LTD
NEW YORK: E. P. DUTTON & CO.
1921
CONTENTS
CHAP. PAGE
I. Introduction and Preliminary Remarks--General Principles
to be observed in Glass Working--Choice of Apparatus--Tools
and Appliances--Glass 1
II. Easy Examples of Laboratory Glass-Blowing--Cutting and
Sealing Tubes, Tubes for High Temperature
Experiments--Thermometer-Bulbs, Bulbs of Special Glass,
Pipettes, Absorption-Bulbs or Washing Bulbs--Joining Tubes,
Branches, Exhaustion-Branches, Branches of Dissimilar Glass,
Blowing Bulbs, A Thistle Funnel, Cracking and Breaking Glass,
Leading and Direction of Cracks--Use of Glass Rod or Strips
of Window-Glass, Joining Rod, Feet and Supports--Gripping
Devices for use in Corrosive Solutions--The Building up of
Special Forms from Solid Glass 10
III. Internal Seals, Air-Traps, Spray Arresters,
Filter-Pumps--Sprays, Condensers; plain, double surface, and
spherical--Soxhlet Tubes and Fat Extraction Apparatus--Vacuum
Tubes, Electrode Work, Enclosed Thermometers, Alarm
Thermometers ... Recording Thermometers, "Spinning" Glass 32
IV. Glass, its Composition and
Characteristics--Annealing--Drilling, Grinding, and Shaping
Glass by methods other than Fusion--Stopcocks--Marking
Glass--Calibration and Graduation of
Apparatus--Thermometers--Exhaustion of Apparatus--Joining
Glass and Metal--Silvering Glass 55
V. Extemporised Glass-Blowing Apparatus--The use of Oil or
other Fuels--Making Small Rods and Tubes from Glass
Scraps--The Examination of Manufactured Apparatus with a view
to Discovering the Methods used in Manufacture--Summary of
Conditions necessary for Successful Glass-Blowing 80
Index 105
PREFACE
To cover the whole field of glass-blowing in a small handbook would be
impossible. To attempt even a complete outline of the methods used in
making commercial apparatus would involve more than could be undertaken
without omitting the essential details of manipulation that a novice
needs. I have, therefore, confined myself as far as possible to such
work as will find practical application in the laboratory and will, I
hope, prove of value to those whose interests lie therein.
The method of treatment and somewhat disjointed style of writing have
been chosen solely with the view to economy of space without the undue
sacrifice of clearness.
BERNARD D. BOLAS.
Handbook of Laboratory Glass-Blowing
CHAPTER I
Introduction and Preliminary Remarks--General Principles to
be observed in Glass Working--Choice of Apparatus--Tools and
Appliances--Glass.
Glass-blowing is neither very easy nor very difficult; there are
operations so easy that the youngest laboratory boy should be able to
repeat them successfully after once having been shown the way, there are
operations so difficult that years are needed to train eye and hand and
judgment to carry them out; but the greater number of scientific needs
lie between these two extremes. Yet a surprisingly large number of
scientific workers fail even to join a glass tube or make a T piece that
will not crack spontaneously, and the fault is rather one of
understanding than of lack of ability to carry out the necessary
manipulation.
In following the scheme of instruction adopted in this handbook, it will
be well for the student to pay particular attention to the reason given
for each detail of the desirable procedure, and, as far as may be, to
memorise it. Once having mastered the underlying reason, he can evolve
schemes of manipulation to suit his own particular needs, although, as a
rule, those given in the following pages will be found to embody the
result of many years' experience.
There is a wide choice of apparatus, from a simple mouth-blowpipe and a
candle flame to a power-driven blower and a multiple-jet heating device.
All are useful, and all have their special applications, but, for the
present, we will consider the ordinary types of bellows and blowpipes,
such as one usually finds in a chemical or physical laboratory.
The usual, or Herepath, type of gas blowpipe consists of an outer tube
through which coal gas can be passed and an inner tube through which a
stream of air may be blown. Such a blowpipe is shown in section by Fig.
1. It is desirable to have the three centring screws as shown, in order
to adjust the position of the air jet and obtain a well-shaped flame,
but these screws are sometimes omitted. Fig. 1, _a_ and _b_ show the
effects of defective centring of the air jet, _c_ shows the effect of
dirt or roughness in the inside of the air jet, _d_ shows a satisfactory
flame.
[Illustration: Fig 1]
For many purposes, it is an advantage to have what is sometimes known
as a "quick-change" blowpipe; that is one in which jets of varying size
may be brought into position without stopping the work for more than a
fraction of a second. Such a device is made by Messrs. Letcher, and is
shown by _e_, and in section by _f_ Fig. 1. It is only necessary to
rotate the desired jet into position in order to connect it with both
gas and air supplies. A small bye-pass ignites the gas, and adjustment
of gas and air may be made by a partial rotation of the cylinder which
carries the jets.
For specially heavy work, where it is needed to heat a large mass of
glass, a multiple blowpipe jet of the pattern invented by my father,
Thomas Bolas, as the result of a suggestion derived from a study of the
jet used in Griffin's gas furnace, is of considerable value. This jet
consists of a block of metal in which are drilled seven holes, one being
central and the other six arranged in a close circle around the central
hole. To each of these holes is a communication way leading to the gas
supply, and an air jet is arranged centrally in each. Each hole has also
an extension tube fitted into it, the whole effect being that of seven
blowpipes. In order to provide a final adjustment for the flame, a
perforated plate having seven holes which correspond in size and
position to the outer tubes is arranged to slide on parallel guides in
front of these outer tubes.
[Illustration: Fig. 2]
The next piece of apparatus for consideration is the bellows, of which
there are three or more types on the market, although all consist of two
essential parts, the blower or bellows proper and the wind chamber or
reservoir. Two patterns are shown in Fig. 2; _a_, is the form which is
commonly used by jewellers and metal workers to supply the air blast
necessary for heating small furnaces. Such a bellows may be obtained at
almost any jewellers' supply dealer in Clerkenwell, but it not
infrequently happens that the spring in the wind chamber is too strong
for glass-blowing, and hence the air supply tends to vary in pressure.
This can be improved by fitting a weaker spring, but an easier way and
one that usually gives fairly satisfactory results, is to place an
ordinary screw-clip on the rubber tube leading from the bellows to the
blowpipe, and to tighten this until an even blast is obtained.
Another form of bellows, made by Messrs. Fletcher and Co., and common in
most laboratories, is shown by _b_; the wind chamber consists of a disc
of india-rubber clamped under a circular frame or tied on to a circular
rim. This form is shown by Fig. 2, _b_.
The third form, and one which my own experience has caused me to prefer
to any other, is cylindrical, and stands inside the pedestal of the
blowpipe-table. A blowpipe-table of this description is made by Enfer of
Paris.
There is no need, however, to purchase an expensive table for laboratory
use. All the work described in this book can quite well be done with a
simple foot bellows and a quick-change blowpipe. Nearly all of it can be
done with a single jet blowpipe, such as that described first, or even
with the still simpler apparatus mentioned on page 84, but I do not
advise the beginner to practise with quite so simple a form at first,
and for that reason have postponed a description of it until the last
chapter.
Glass-blowers' tools and appliances are many and various, quite a number
of them are better rejected than used, but there are a few essentials.
These are,--file, glass-knife, small turn-pin, large turn-pin, carbon
cones, carbon plate, rubber tube of small diameter, various sizes of
corks, and an asbestos heat reflector. For ordinary work, an annealing
oven is not necessary, but one is described on page 60 in connection
with the special cases where annealing is desirable.
Fig. 3 illustrates the tools and appliances. _a_ is an end view of the
desirable form of file, and shows the best method of grinding the edges
in order to obtain a highly satisfactory tool. _b_ is a glass knife,
shown both in perspective and end view, it is made of glass-hard steel
and should be sharpened on a rough stone, such as a scythe-stone, in
order to give a slightly irregular edge. _c_ is a small turn-pin which
may be made by flattening and filing the end of a six-inch nail. _d_ is
the large turn-pin and consists of a polished iron spike, about five
inches long and a quarter of an inch diameter at its largest part. This
should be mounted in a wooden handle. _e_ and _f_ are carbon cones. A
thin rubber tube is also useful; it may be attached to the work and
serve as a blowing tube, thus obviating the necessity of moving the work
to the mouth when internal air pressure is to be applied. In order to
avoid undue repetition, the uses of these tools and appliances will be
described as they occur.
[Illustration: Fig. 3]
Glass, as usually supplied by chemical apparatus dealers is of the
composition known as "soda-glass." They also supply "hard" or
"combustion" glass, but this is only used for special purposes, as it is
too infusible for convenient working in the ordinary blowpipe flame.
Soda-glass consists primarily of silicate of sodium with smaller
quantities of silicate of aluminum and potassium. Its exact composition
varies. It is not blackened, as lead glass is, by exposure to the
reducing gases which are present in the blue cone of a blowpipe flame,
and hence is easier for a beginner to work without producing
discolouration.
Further notes on glasses will be found on page 55, but for ordinary
purposes soda-glass will probably be used.
CHAPTER II
Easy Examples of Laboratory Glass-Blowing--Cutting and
Sealing Tubes for Various Purposes; Test-Tubes,
Pressure-Tubes, Tubes for High Temperature
Experiments--Thermometer-Bulbs, Bulbs of Special Glass,
Pipettes, Absorption-Bulbs or Washing-Bulbs--Joining Tubes;
Branches, Exhaustion-Branches, Branches of Dissimilar
Glass--Blowing Bulbs; A Thistle Funnel; Cracking and
Breaking Glass; Leading and Direction of Cracks--Use of
Glass Rod or Strips of Window-Glass; Joining Rod, Feet and
Supports--Gripping Devices for use in Corrosive
Solutions--The Building Up of Special Forms from Solid
Glass.
Perhaps the most common need of the glass-blower whose work is connected
with that of the laboratory is for a sealed tube; and the sealing of a
tube is an excellent preliminary exercise in glass-blowing.
We will assume that the student has adjusted the blowpipe to give a
flame similar to that shown in _d_, Fig. 1, and that he has learned to
maintain a steady blast of air with the bellows; further, we will assume
that the tube he wishes to seal is of moderate size, say not more than
half an inch in diameter and with walls of from one-tenth to one-fifth
of an inch thick.
[Illustration: Fig. 4]
A convenient length of tube for the first trial is about one foot; this
should be cut off from the longer piece, in which it is usually
supplied, as follows:--lay the tube on a flat surface and make a deep
cut with the edge of a file. Do not "saw" the file to and fro over the
glass. If the file edge has been ground as shown in _a_, Fig. 3, such a
procedure will be quite unnecessary and only involve undue wear; one
movement with sufficient pressure to make the file "bite" will give a
deep cut. Now rotate the tube through about one-eighth of a turn and
make another cut in continuation of the first. Take the tube in the
hands, as shown in _a_, Fig. 4, and apply pressure with the thumbs, at
the same time straining at the ends. The tube should break easily. If it
does not, do not strain too hard, as it may shatter and cause serious
injuries to the hands, but repeat the operation with the file and so
deepen the original cuts. In holding a tube for breaking, it is
important to place the hands as shown in sketch, as this method is least
likely to cause shattering and also minimises the risk of injury even if
the tube should shatter. To cut a large tube, or one having very thick
walls, it is better to avoid straining altogether and to break by
applying a small bead of intensely heated glass to the file cut. If the
walls are very thin, a glass-blower's knife should be used instead of a
file. The tube and glass-blower's knife should be held in the hand, and
the tube rotated against the edge of the knife; this will not produce a
deep cut, but is less likely to break the tube. A bead of hot glass
should be used to complete the work.
The next operation is to heat the glass tube in the middle; this must be
done gradually and evenly; that is to say the tube must be rotated
during heating and held some considerable distance in front of the flame
at first; otherwise the outer surface of the glass will expand before
the interior is affected and the tube will break. From two to five
minutes, heating at a distance of about eight inches in front of the
flame will be found sufficient in most cases, and another minute should
be taken in bringing the tube into the flame. Gradual heating is
important, but even heating is still more important and this can only be
obtained by uniform and steady rotation. Until the student can rotate a
tube steadily _without thinking about it_, real progress in
glass-blowing is impossible.
When the tube is in the flame it must be held just in front of the blue
cone and rotated until the glass is soft enough to permit the ends to be
drawn apart. Continue to separate the ends and, at the same time, move
the tube very slightly along its own axis, so that the flame tends to
play a little more on the thicker part than on the drawn-out portion. If
this is done carefully, the drawn-out portion can be separated off,
leaving only a slight "bleb" on the portion it is desired to seal. This
is illustrated by _b_, Fig. 4.
To convert the seal at _b_, Fig. 4., into the ordinary form of test-tube
seal, it is only necessary to heat the "bleb" a little more strongly,
blow gently into the tube until the thick portion is slightly expanded,
re-heat the whole of the rounded end until it is beginning to collapse,
and give a final shaping by careful blowing after it has commenced to
cool. In each case the glass must be removed from the flame before
blowing. The finished seal is shown by _c_, Fig. 4. If desired, the open
end may now be finished by heating and rotating the soft glass against
the large turn-pin, as illustrated in _d_, but the turn-pin must not be
allowed to become too hot, as if this happens it will stick to the
glass. After turning out the end, the lip of glass must be heated to
redness and allowed to cool without coming in contact with anything;
otherwise it will be in a condition of strain and liable to crack
spontaneously. The finished test-tube is shown by _e_.
When it is necessary to seal a substance inside a glass tube, the bottom
of the tube is first closed, as explained above, and allowed to cool;
the substance, if a solid, is now introduced, but should not come to
within less than two inches of the point where the second seal is to be
made. If the substance is a liquid it can more conveniently be
introduced at a later stage.
Now bring the tube into the blowpipe flame gradually, and rotate it,
while heating, at the place where it is to be closed. Allow the glass to
soften and commence to run together until the diameter of the tube is
reduced to about half its original size. Remove from the flame and draw
the ends apart, this should give a long, thick extension as shown by
_f_, Fig. 4. If any liquid is to be introduced, it may now be done by
inserting a thin rubber or other tube through the opening and running
the liquid in. A glass tube should be used with caution for introducing
the liquid, as any hard substance will tend to scratch the inside of the
glass and cause cracking. The final closure is made by melting the
drawn-out extension in the blowpipe flame; the finished seal being shown
by _g_, Fig. 4.
If the sealed tube has to stand internal pressure, it is desirable to
allow the glass to thicken somewhat more before drawing out, and the
bottom seal should also be made thicker. For such a tube, and especially
when it has to stand heating, as in a Carius determination of chlorine,
each seal should be cooled very slowly by rotating it in a gas flame
until the surface is covered with a thick layer of soot, and it should
then be placed aside in a position where the hot glass will not come in
contact with anything, and where it will be screened from all draughts.
_Joining Tube._--We will now consider the various forms of join in glass
tubing which are met with in the laboratory. First, as being easiest, we
will deal with the end-to-end joining of two tubes of similar glass.
_a_, _b_, and _c_, Fig. 5, illustrate this. One end of one of the tubes
should be closed, a lip should be turned out on each of the ends to be
joined, and both lips heated simultaneously until the glass is
thoroughly soft. Now bring the lips together gently, until they are in
contact at all points and there are no places at which air can escape;
remove from the flame, and blow slowly and very cautiously until the
joint is expanded as shown in _b_, Fig. 5. Reheat in the flame until
the glass has run down to rather less than the original diameter of the
tube, and give a final shaping by re-blowing. The chief factors of
success in making such a join are, thorough heating of the glass before
bringing the two tubes together, and avoidance of hard or sudden blowing
when expanding the joint. The finished work is shown by _c_, Fig. 5.
[Illustration: Fig. 5]
To join a small glass tube to the end of a large one, the large tube
should first be sealed, a small spot on the extreme end of the seal
heated, and air pressure used to expand the heated spot as shown in _d_.
This expanded spot is then re-heated and blown out until it bursts as
shown in _e_, the thin fragments of glass are removed and the end of the
small tube turned out as shown in _f_. After this the procedure is
similar to that used in jointing two tubes of equal size.
When these two forms of joint have been mastered, a T piece will present
but little difficulty. It is made in three stages as shown in Fig. 5,
and the procedure is similar to that used in joining a large and small
tube. Care should be taken to avoid softening the top of the "T" too
much, or the glass will bend and distort the finished work; although a
slight bend can be rectified by re-heating and bending back. Local
re-heating is often useful in giving the joint its final shape.
An exhaustion branch is often made by a totally different method. This
method is shown by _g_, _h_, and _i_, Fig. 5; _g_ is the tube on which
the branch is to be made. The end of a rod of similar glass should be
heated until a mass of thoroughly liquid glass has collected, as shown,
and at the same time a spot should be heated on that part of the tube
where it is desired to make the branch. The mass of hot glass on the rod
is now brought in contact with the heated spot on the tube and expanded
by blowing as shown by _h_. The air pressure in the tube is still
maintained while the rod is drawn away as shown by _i_. This will give a
hollow branch which may be cut off at any desired point, and is then
ready for connection to the vacuum pump.
If the rod used is of a dissimilar glass, the branch should be blown
much thinner. Such a branch will often serve as a useful basis for
joining two tubes of different composition, as the ordinary type of
branch is more liable to crack when made with two glasses having
different coefficients of expansion.
_Blowing Bulbs._--A bulb may be blown on a closed tube such as that
shown by _c_, Fig. 5, by rotating it in the blowpipe flame until the end
is softened, removing it from the flame and blowing cautiously. It is
desirable to continue the rotation during blowing. In the case of a very
small tube, it is sufficient to melt the end without previous sealing,
rotate it in the flame until enough glass has collected, remove from the
flame and blow while keeping the tube in rotation.
_Thermometer Bulbs._--If the thermometer is to be filled with mercury,
it is desirable to use a rubber bulb for blowing, as moisture is liable
to condense inside the tube when the mouth is used, and this moisture
will cause the mercury thread to break. In any case, a slight pressure
should be maintained inside the thermometer tube while it is in the
flame; otherwise the fine capillary tube will close and it will be very
difficult to expand the heated glass into a bulb.
_Large Bulbs._--When a large bulb is needed on a small or medium sized
tube, it is often necessary to provide more glass than would be obtained
if the bulb were blown in the ordinary way. One method is to expand the
tube in successive stages along its axis, as shown by _a_, Fig. 6. These
expanded portions are then re-heated, so that they run together into one
hollow mass from which the bulb is blown; _b_ and _c_, illustrate this.
Another method, and one which is useful for very large bulbs, is to
fuse on a length of large, thick-walled, tubing. The heat reflector,
_g_, Fig. 3, should be used, if necessary, when making large bulbs. It
consists of a sheet of asbestos mounted in a foot, and is used by being
placed close to the mass of glass on the side away from the blowpipe
flame while the glass is being heated.
[Illustration: Fig. 6]
_Bulbs of Dissimilar Glass._--These may be made by the second method
given under "Large Bulbs," but the joint should be blown as thin as
possible. Further instructions in the use of unlike glasses are given on
page 94.
_A Bulb in the Middle of a Tube._--Unless the bulb is to be quite small,
it will be necessary to join in a piece of thick glass tubing, or to
draw the thin tube out from a larger piece, thus leaving a thick mass in
the middle as shown by _d_, Fig. 6. This mass of glass should now be
rotated in the blowpipe flame until it is quite soft and on the point of
running together. Considerable practice will be necessary before the two
ends of the tube can be rotated at the same speed and without
"wobbling," but this power must be acquired. When the glass is
thoroughly hot, remove from the flame, hold in a horizontal position,
and expand by blowing. It is essential to continue the rotation while
this is done. Should one part of the bulb tend to expand more than the
other, turn the expanded part to the bottom, pause for about a second,
both in rotating and blowing, in order that the lower portion may be
cooled by ascending air-currents; then continue blowing and turning as
before.
_Absorption Bulbs or Washing Bulbs._--These are made by an elaboration
of the processes given in the last paragraph, _g_, _h_, and _i_, Fig. 6,
illustrate this.
_A Thistle Funnel._--This is made by blowing a fairly thick-walled bulb
on a glass tube, bursting a hole by heating and blowing, and enlarging
the burst-out part by heating and rotating against a turn-pin.
_Bending Glass Tube._--Small tubing may be bent in a flat flame gas
burner and offers no special difficulty. Large or thin-walled tubing
should be heated in the blowpipe flame and a slight bend made; another
zone of the tube, just touching the first bend, should now be heated and
another slight bend made. In this way it is possible to avoid flattening
and a bend having any required angle can gradually be produced. A final
shaping of the bend may be made by heating in a large blowpipe flame and
expanding slightly by air pressure.
_Glass Spirals._--If a tube is heated by means of a long, flat-flame
burner, the softened tube may be wound on to an iron mandrel which has
previously been covered with asbestos. The mandrel should be made
slightly conical in order to facilitate withdrawal. It is desirable to
heat the surface of the asbestos almost to redness by means of a second
burner, and thus avoid undue chilling of the glass and the consequent
production of internal strain.
[Illustration: Fig. 7]
_A Thermo-Regulator for Gas._--Fig. 7, _a-e_, shows an easily
constructed thermo-regulator. The mercury reservoir, _a_, and the upper
part, _b_, are made by joining two larger pieces of tubing on to the
capillary. The gas inlet passes through a rubber stopper, in order to
allow of adjustment for depth of insertion, and the bye-pass branches,
_d_ and _e_, are connected by a piece of rubber tubing which can be
compressed by means of a screw clip, thus providing a means of
regulating the bye-pass.
_Use of Glass Rod._--Apart from its most common laboratory use for
stirring; glass rod may be used in building up such articles as
insulating feet for electrical apparatus or acid-resisting cages for
chemical purposes. Such a cage is shown by _f_, _g_ and _h_, Fig. 7.
Further, by an elaboration of the method of making an exhaustion branch,
given on page 18, blown articles may also be constructed from rod. Note
the added parts of _e_, Fig. 9.
_A Simple Foot._--The form of foot shown by Fig. 7, _k_, is easy to make
and has many uses. First join a glass rod to a length of glass tubing as
shown (the joint should be expanded slightly by blowing), cut off the
tube and heat the piece remaining on the rod until it can be turned out
as shown by _i_. This should be done with the large turn-pin, and care
should be taken not to heat the supporting rod too strongly, otherwise
the piece of tube will become bent and distorted; it is better to
commence by heating the edge of the piece of tube and turn out a lip,
then extend the heating by degrees and turn out more and more until the
foot looks like that shown by _i_.
We now need to make three projections of glass rod. These are produced
as follows:--Heat the end of the glass rod until a thoroughly melted
mass of glass has accumulated (the rod must be rotated while this is
being done, otherwise the glass will drop off); when sufficient melted
glass has been obtained, the edge of the turned-out foot should be
heated to dull redness over about one-third of its circumference, and
the melted glass on the rod should be drawn along the heated portion
until both are so completely in contact as to form one mass of
semi-fluid glass. The rod should now be drawn away slowly, and, finally,
separated by melting off, thus producing a flat projection. A repetition
of the process will give the other two projections, and the finished
foot may be adjusted to stand upright by heating the projections
slightly and standing it on the carbon plate mentioned on page 7. After
the foot is adjusted it should be annealed slightly by heating to just
below the softening point of the glass and then rotating in a smoky gas
flame until it is covered with a deposit of carbon, after which it
should be allowed to cool in a place free from draughts and where the
hot glass will not come in contact with anything. The finished foot is
shown by _k_, Fig. 7.
_Building up from Glass Rod._--A glass skeleton-work can be constructed
from rod without much difficulty, and is sometimes useful as a container
for a substance which has to be treated with acid, or for similar
purposes. The method is almost sufficiently explained by the
illustration in Fig. 7; _f_ shows the initial stage, _g_ the method of
construction of the net-work, and _h_ the finished container. It is
convenient to introduce the substance at the stage indicated by _g_. The
important points to observe in making this contrivance are that the
glass rod must be kept hot by working while it is actually in the flame,
and that the skeleton must be made as thin as possible with the
avoidance of heavy masses of glass at any place. If these details are
neglected it will be almost certain to crack.
_Stirrers._--These are usually made from glass rod, and no special
instructions are necessary for their construction, except that the glass
should be in a thoroughly fused condition before making any joins and
the finished join should be annealed slightly by covering with a deposit
of soot, as explained on page 16. The flat ends shown in _a_, Fig. 8,
are made by squeezing the soft glass rod between two pieces of carbon,
and should be re-heated to dull redness after shaping. Fig. 8 also shows
various forms of stirrer.
In order to carry out stirring operations in the presence of a gas or
mixture of gases other than air, some form of gland or seal may be
necessary where the stirrer passes through the bearing in which it runs.
A flask to which is fitted a stirrer and gas seal is shown in section by
_b_, Fig. 8. The liquid used in this seal may be mercury, petroleum, or
any other that the experimental conditions indicate.
[Illustration: Fig. 8]
If the bearing for a stirrer is made of glass tube, it is desirable to
lubricate rather freely; otherwise heat will be produced by the
friction of the stirrer and the tube will probably crack. Such
lubrication may be supplied by turning out the top of the bearing tube
and filling the turned-out portion with petroleum jelly mixed with a
small quantity of finely ground or, better, colloidal graphite, and the
bearing should also be lubricated with the same composition. Care
should be taken not to employ so soft a lubricant or so large an excess
as to cause it to run down the stirrer into the liquid which is being
stirred.
_Leading a Crack._--It sometimes happens that a large bulb or specially
thin-walled tube has to be divided. In such a case it is scarcely
practicable to use the method recommended for small tubes on page 12,
but it is quite easy to lead a crack in any desired direction. A
convenient starting point is a file cut; this is touched with hot glass
until a crack is initiated. A small flame or a bead of hot glass is now
used to heat the article at a point about a quarter of an inch from the
end of the crack and in whatever direction it has to be led. The crack
will now extend towards the source of heat, which should be moved
farther away as the crack advances. In this manner a crack may be caused
to take any desired path and can be led round a large bulb.
_Cutting Glass with the Diamond._--Slips of window-glass can be used in
place of glass rod for some purposes, and as cutting them involves the
use of the glaziers' diamond or a wheel-cutter, they may well be
mentioned under this heading.
In cutting a sheet of glass with the diamond, one needs a flat surface
on which to rest the glass, and a rule against which to guide the
diamond. The diamond should be held in an almost vertical position, and
drawn over the surface of the glass with slight pressure. While this is
being done the angle of the diamond should be changed by bringing the
top of the handle forward until the sound changes from one of scratching
to a clear singing note. When this happens the diamond is cutting. A few
trials will teach the student the correct angle for the diamond with
which he works, and the glass, if properly cut, will break easily. If
the cut fails it is better to turn the glass over and make a
corresponding cut on the other side rather than make any attempt to
improve the original cut. The diamond is seldom used for cutting small
glass tubes.
The use of the wheel-cutter calls for no special mention as it will cut
at any angle, although the pressure required is somewhat greater than
that needed by most diamonds.
CHAPTER III
Internal Seals, Air-Traps, Spray Arresters,
Filter-Pumps--Sprays, Condensers; Plain, Double Surface, and
Spherical--Soxhlet Tubes and Fat Extraction
Apparatus--Vacuum Tubes, Electrode Work, Enclosed
Thermometers, Alarm Thermometers, Recording Thermometers,
"Spinning" Glass.
_Internal Seals._--It is convenient to class those cases in which a
glass tube passes through the wall of another tube or bulb under the
heading of "Internal Seals." These are met with in barometers, spray
arresters, and filter pumps, in condensers and some forms of vacuum
tube. The two principal methods of making such seals will be considered
first and their special application afterwards.
_An Air Trap on a Barometer Tube._--This involves the use of the first
method, and is perhaps the simplest example that can be given. Fig. 9,
_a_, _a1_ and _a2_, show the stages by which this form of internal seal
is made. For the first trials, it is well to work with fairly
thick-walled tubing, which should be cut into two pieces, each being
about eight inches long.
[Illustration: Fig. 9]
First seal the end of one tube as described on page 13, heat the sealed
end and expand to a thick walled bulb. Fuse the end of the other tube,
attach a piece of glass rod to serve as a handle, and draw out; cut off
the drawn-out portion: leaving an end like _a_.
Now heat a small spot at the end of the bulb, blow, burst out, and
remove the thin fragments of glass. Heat a zone on the other tube at the
point where the drawn-out portion commences and expand as shown by _a1_.
The next stage is to join the tubes. Heat the ragged edges of the
burst-out portion until they are thoroughly rounded. At the same time
heat the drawn-out tube to just below softening point. Then, while the
rounded edges of the burst-out portion are still soft, insert the other
tube; rotate the join in the blowpipe flame until it is quite soft, and
expand by blowing. If necessary, re-heat and expand again. The finished
seal, which should be slightly annealed by smoking in a sooty flame, is
shown by _a2_.
_A Spray Arrester._--This is made by the second method, in which the
piece of tube which projects inside the bulb is fused in position first
and the outer tube is then joined on. The various stages of making are
illustrated by _b_, _b1_ and _b2_, Fig. 9.
A bulb is blown between two tubes by the method given on page 22, the
larger tube is then cut off and the small piece of tube introduced into
the bulb after having been shaped as shown in by _b_, Fig. 9. The
opening in the bulb is sealed as shown by _b1_. The sealed part is now
heated and the bulb inclined downwards until the inner tube comes in
contact with the seal and is fused in position. This operation requires
some practice in order to prevent the inner tube either falling through
the soft glass or becoming unsymmetrical. The end of the bulb, where the
inner tube comes in contact with it, is now perforated by heating and
blowing, thus giving the form shown by _b2_, and the outer tube is
joined on. The finished spray arrester is shown by _b3_. Practice alone
will give the power to produce a symmetrical and stable piece of work.
_Two Forms of Filter Pump._--That illustrated by _d_, Fig. 9, is made by
the method explained under "An Air Trap on a Barometer Tube." That
illustrated by _c_ is made by the method explained under "A Spray
Arrester." No new manipulation is involved, and the construction should
be clear from a study of the drawings.
_Multiple and Branched Internal Seals._--A fuller consideration of these
will be found on page 39, but one general principle may well be borne in
mind; that, as far as is possible, a tube having both ends fastened
inside another tube or bulb should be curved or have a spiral or bulb at
some point in its length, otherwise any expansion or contraction will
put great strain on the joints.
_Sprays._--A spray which is easy to make, easy to adjust, and easy to
clean after use is shown by _e_, Fig. 9. The opening on the top of the
bulb is made by melting on a bead of glass, expanding, bursting, and
fusing the ragged edges. The two branches which form the spray producing
junction are made by the method used for an exhaustion branch and
described on page 18.
A spray which can be introduced through the neck of a bottle is shown by
_h_, Fig. 9. The various stages in making this are illustrated by _f_,
and _g_. If the inner tube is made by drawing out from a larger piece of
glass so that two supporting pieces are left on each side of the place
where it is intended to make the final bend, that bend can be made in a
flat-flame gas burner without causing the inner tube to come in contact
with the walls of the outer tube. Care must be taken when joining on the
side piece that the inner tube is not heated enough to fuse it. The
small hole in the side of the outer tube is produced by heating and
bursting.
_A Liebig's Condenser._--This consists of a straight glass tube passing
through an outer cooling jacket. In practice it is better to make the
jacket as a separate piece, and to effect a water-tight junction by
means of two short rubber tubes. It may, however, be made with two
internal seals of the class described under "A Spray Arrester." There is
much less risk of these seals cracking if the inner tube is made in the
form of a spiral or has a number of bulbs blown on it in order to give a
certain amount of elasticity.
_A Double-Surface Condenser._--Fig. 10 shows a condenser of this nature
which is supplied by Messrs. Baird and Tatlock. It may be built up in
stages as shown by _a_, _b_, and _c_, but the work involved requires
considerable skill, and the majority of laboratory workers will find it
cheaper to buy than to make.
[Illustration: Fig. 10]
_A Spherical Condenser._--Such a condenser as that shown by _f_, Fig 10,
involves a method which may find application in a number of cases. The
outer bulb is blown from a thick piece of tubing which has been inserted
in a smaller piece (see _d_, Fig. 6); then the inner bulb by similar
method. It is now necessary to introduce the smaller bulb into the
larger, and for this purpose the larger bulb must be cut into halves. A
small but deep cut is made with the file or glass-blowers' knife in the
middle of the larger bulb, and at right angles to the axis of the tube
on which it is blown. A minute bead of intensely heated glass is now
brought in contact with the cut in order to start a crack. This crack
may now be led round the bulb as described on page 30. If the work is
carried out with care, it is possible to obtain the bulb in two halves
as shown by _d_, and these two halves will correspond so exactly that
when the cut edges are placed in contact they will be almost air-tight.
The two tubes from the smaller bulb should be cut to such a length that
they will just rest inside the larger, and the ends should be expanded.
Place the inner bulb in position and fit the two halves of the outer
bulb together, taking great care not to chip the edges. If the length of
the tubes on the inner bulb has been adjusted properly, the inner bulb
will be supported in position by their contact with the tubes on the
outer bulb. Now rotate the cracked portion of the outer bulb in front
of a blowpipe flame and press the halves together very gently as the
glass softens. Expand slightly by blowing if necessary. If a small
pin-hole develops at the joint it is sometimes possible to close this
with a bead of hot glass; but if the bulb has been cut properly there
should be no pin-holes formed. The condenser is finished by joining on
the side tubes and sealing the inner tube through by the methods already
given. In order to blow bulbs large enough to make a useful condenser,
it will be convenient to employ the multiple-jet blowpipe described on
page 4.
_A Soxhlet-Tube or Extraction Apparatus._--This involves the
construction of a re-entrant join where the syphon flows into the lower
tube. It is of considerable value as an exercise and the complete
apparatus is easy to make.
A large tube is sealed at the bottom and the top is lipped, as in making
a test-tube. A smaller tube is then joined on by a method similar to
that given on page 18, but without making a perforation in the bottom of
the large tube. Heating and expanding by air pressure, first through the
large tube, then through the smaller tube and then again through the
large tube, will give a satisfactory finish to this part of the work.
[Illustration: Fig. 11]
The syphon tube is now joined on to the large tube as shown by _a_, Fig.
11, care being taken to seal the other end of the syphon tube before
joining. The details of the final and re-entrant joint of the syphon
tube are shown at the lower part of _a_. This join is made by expanding
the sealed end of the syphon tube into a small, thick-walled bulb, and
the bottom of this bulb is burst out by local heating and blowing; the
fragments of glass are removed and the edges made smooth by melting. A
similar operation is carried out on the side of the tube to which the
syphon tube is to be joined. This stage is shown by _a_. Now heat the
syphon tube at the upper bend until it is flexible, and press the bulb
at its end into the opening on the side of the other tube. Hold the
glass thus until the syphon is no longer flexible. The final join is
made by heating the two contacting surfaces, if necessary pressing the
edges in contact with the end of a turn-pin, fusing together and
expanding. The finished apparatus is shown by _c_.
_Electrodes._--A thin platinum wire may be sealed into a capillary tube
without any special precautions being necessary. The capillary tube may
be drawn out from the side of a larger tube by heating a spot on the
glass, touching with a glass rod and drawing the rod away; or the
exhaustion branch described on page 18 may be used for the introduction
of an electrode. It is convenient sometimes to carry out the exhaustion
through the same tube that will afterwards serve for the electrode. The
electrode wire is laid inside the branch before connecting to the
exhaustion pump. When exhaustion is completed the tube is heated until
the soft glass flows round the platinum and makes the seal air-tight.
The branch is now cut off close to the seal on the pump side, a loop is
made in the projecting end of the platinum wire, and the seal is
finished by melting the cut-off end.
Platinum is usually employed for such work, but if care is taken to
avoid oxidation it is not impossible to make fairly satisfactory seals
with clean iron or nickel wire. Hard rods of fine graphite, such as are
used in some pencils, may also be sealed into glass, but it seems
probable that air would diffuse through the graphite in the course of
time.
Another method for the introduction of an electrode is illustrated by
_d_, _e_, _f_ and _g_, Fig. 11. In this case the bulb or thin-walled
tube into which the electrode is to be sealed is perforated by a quick
stab with an intensely heated wire--preferably of platinum--which is
then withdrawn before the glass has had time to harden, and thus a
minute circular hole is made. The electrode is coated with a layer of
similar glass, or of the specially made enamel which is sold for this
purpose, inserted into the bulb or tube by any convenient opening, and
adjusted by careful shaking until the platinum wire projects through the
small hole. The bulb or tube is then fused to the coating of the
electrode and the whole spot expanded slightly by blowing. The
appearance of the finished seal is shown by _g_. It is well to anneal
slightly by smoking.
_Thermometers._--Apart from the notes on page 20 with respect to the
blowing of a suitable bulb on capillary tubing there is little to say in
connection with the glass working needed in making a plain thermometer.
The size desirable for the bulb will be determined by the bore of the
capillary tube, the coefficient of expansion of the liquid used for
filling, and the range of temperature for which the thermometer is
intended.
Filling may be carried out as follows:--Fit a small funnel to the open
end of the capillary by means of a rubber tube, and pour into the funnel
rather more than enough of the liquid to be used than is required to
fill the bulb. Mercury or alcohol will be used in practice, most
probably. Warm the bulb until a few air bubbles have escaped through
the liquid and then allow to cool. This will suck a certain amount of
liquid into the bulb. Now heat the bulb again, and at the same time heat
the capillary tube over a second burner. The liquid will boil and sweep
out the residual air, but it is necessary to heat the capillary tube as
well in order to prevent condensation. Allow the bulb and tube to cool,
then repeat the heating once more. By this time the bulb and tube should
be free from air, and cooling should give a completely filled
thermometer. Remove the funnel and heat the thermometer to a few degrees
above the maximum temperature for which it is to be used; the mercury or
other filling liquid will overflow from the top, and, as the temperature
falls, will recede, thus allowing the end of the capillary to be drawn
out. Reheat again until the liquid rises to the top of the tube, then
seal by means of the blowpipe flame. The thermometer is now finished
except for graduation; this is dealt with on page 75.
_An Alarm Thermometer._--A thermometer which will complete an electric
circuit when a certain temperature is reached may be made by sealing an
electrode in the bulb and introducing a wire into the top, which in this
case is not sealed. Naturally, this thermometer will be filled with
mercury. There is considerable difficulty in filling such a bulb without
causing it to crack.
Several elaborations of this form are made, in which electrodes are
sealed through the walls of the capillary tube, thus making it possible
to detect electrically the variation of temperature when it exceeds any
given limits.
_An Enclosed or Floating Thermometer._--The construction of this type of
thermometer is shown by _h_ and _i_, Fig 11. It is made in the following
stages:--A bulb is blown on the drawn-out end of a thin-walled tube as
shown by _h_. A small bulb is blown on the end of a capillary tube,
burst, and turned out to form a lip which will rest in the drawn-out
part of the thin-walled tube but is just too large to enter the bulb.
The capillary tube is introduced and sealed in position, care being
taken to expand the joint a little. The thermometer is filled and the
top of the capillary tube closed by the use of a small blowpipe flame. A
paper scale having the necessary graduations is inserted, and the top
of the outer tube is closed as shown by _i_.
_A Maximum and Minimum Thermometer._--If a small dumb-bell-shaped rod of
glass or metal is introduced into the capillary tube of a horizontally
placed, mercury-filled thermometer in such a position that the rising
mercury column will come in contact with it, the rod will be pushed
forward. When the mercury falls again the rod will be left behind and
thus indicate the maximum temperature attained. If a similar
dumb-bell-shaped rod is introduced into an alcohol-filled thermometer
and pushed down until it is within the alcohol column, it will be drawn
down by surface tension as the column falls; but the rising column will
flow passed it without causing any displacement; thus the minimum
temperature will be recorded.
Six's combined maximum and minimum thermometer is shown by _b_, Fig. 11.
In this case both maximum and minimum records are obtained from a
mercury column, although the thermometer bulb is filled with alcohol. It
is an advantage to make the dumb-bell-shaped rods of iron, as the
thermometer can then be reset by the use of a small magnet, another
advantage consequent on the use of metal being that the rods can be
easily adjusted, by slight bending, so as to remain stationary in the
tubes when the thermometer is hanging vertically, and yet to move with
sufficient freedom to yield to the pressure of the recording column.
The thermometer may be filled by the following method:--When the
straight tube has been made the first dumb-bell is introduced and shaken
down well towards the lower bulb, the tube is now bent to its final
shape and the whole thermometer filled with alcohol as described on page
44. Now heat the thermometer to a little above the maximum temperature
that it is intended to record, and pour clean mercury into the open bulb
while holding the thermometer vertically. Allow to cool, and the mercury
will be sucked down. The second dumb-bell is now introduced, sufficient
alcohol being allowed to remain in the open bulb to about half fill it,
and the alcohol in this bulb is boiled to expel air. The tube through
which the bulb was filled in now sealed.
_Clinical Thermometers._--The clinical thermometer is a maximum
thermometer of a different type. In this case there is a constriction
of the bore at a point just above the bulb. When the mercury in the bulb
commences to contract, the mercury column breaks at the constriction and
remains stationary in the tube, thus showing the maximum temperature to
which it has risen.
_Vacuum Tubes._--There are so many forms of these that it is scarcely
practicable or desirable to give detailed instructions for making them;
but an application of the various methods of glass-working which have
already been explained should enable the student to construct most of
the simpler varieties. An interesting vacuum tube is made which has no
electrodes, but contains a quantity of mercury. When the tube is rocked
so as to cause friction between the mercury and the glass sufficient
charge is produced to cause the tube to glow.
_A Sprengel Pump._--This, in its simplest form, is illustrated by _a_,
Fig. 12. Such a form, although highly satisfactory in action, needs
constant watching while in action, as should the mercury funnel become
empty air will enter the exhausted vessel. Obviously, the fall-tube must
be made not less than thirty inches long; the measurement being taken
from the junction of the exhaustion branch with the fall-tube to the top
of the turned-up end.
[Illustration: Fig. 12]
_A Macleod Pump._--One form of this is illustrated by _b_, Fig. 12. It
has the advantage that the mercury reservoir may be allowed to become
empty without affecting the vacuum in the vessel being exhausted.
_"Spinning" Glass._--By the use of suitable appliances, it is quite
possible to draw out a continuous thread of glass, which is so thin as
to have almost the flexibility and apparent softness of woollen fibre; a
mass of such threads constitutes the "glass wool" of commerce.
The appliances necessary are:--a blowpipe capable of giving a
well-formed flame of about six or eight inches in length, a wheel of
from eighteen inches to three feet in diameter and having a flat rim of
about three inches wide, and a device for rotating the wheel at a speed
of about three hundred revolutions per minute.
A very satisfactory arrangement may be made from an old bicycle; the
back wheel having the tyre removed and a flat rim of tin fastened on in
its place. The chain drive should be retained, but one of the cranks
removed and a handle substituted for the remaining pedal. The whole
device is shown by Fig. 13.
[Illustration: Fig. 13]
The procedure in "spinning" glass is as follows:--First melt the end of
a glass rod and obtain a large mass of thoroughly softened glass, now
spin the wheel at such a speed that its own momentum will keep it
spinning for several seconds. Touch the end of the melted rod with
another piece of glass and, without withdrawing the original rod from
the blowpipe flame, draw out a thread of molten glass and twist it round
the spinning wheel. If this is done properly, the thread of glass will
grip on the flat rim, and by continuing to turn the wheel by hand it is
possible to draw out a continuous thread from the melted rod, which must
be advanced in the blowpipe flame as it is drawn on the wheel. If the
rod is not advanced sufficiently the thread will melt off, if it is
advanced too much, so as to heat the thick part and allow the glass to
become too cool at the point of drawing out, then the thread will become
too thick, but it is easy after a little practice to obtain the right
conditions. Practice is necessary also in order to find the right speed
for the wheel.
When sufficient glass has been "spun," the whole "hank" of thin thread
may be removed by drawing the thumb-nail across the wheel at any point
on its flat rim, thus breaking the threads, and allowing the "hank" to
open.
_Brushes for Use with Strong Acids._--Glass wool, if of fine enough
texture to be highly flexible, can be used to make acid-resisting
brushes. A convenient method for mounting the spun glass is to melt the
ends of the threads together into a bead, and then to fuse the bead on
to a rod; thus giving a brush. If a pointed brush is necessary, the
point may be ground on an ordinary grindstone or carborundum wheel by
pressing the loose end of the spun glass against the grinding wheel with
a thin piece of cardboard.
When using brushes of this description, it is well to bear in mind the
fact that there is always a liability of a few threads of glass breaking
off during use.
CHAPTER IV
Glass, Its Composition and Characteristics. Annealing.
Drilling, Grinding, and Shaping Glass by methods other than
Fusion. Stopcocks. Marking Glass. Calibration and Graduation
of Apparatus. Thermometers. Exhaustion of Apparatus. Joining
Glass and Metal. Silvering Glass.
There are three kinds of glass rod and tubing which are easily
obtainable; these are soda-glass, which is that usually supplied by
chemical apparatus dealers when no particular glass is specified;
combustion-glass, which is supplied for work requiring a glass that does
not so easily soften or fuse as soda-glass; and lead-glass, which is
less common. There are also resistance-glass, made for use where very
slight solubility in water or other solutions is desirable, and a number
of other special glasses; but of these soda-glass, combustion-glass,
lead-glass, and resistance-glass are the most important to the
glass-blower whose work is connected with laboratory needs.
_Soda-Glass._--Consists chiefly of sodium silicate, but contains
smaller quantities of aluminum silicate, and often of calcium silicate;
there may also be traces of several other compounds.
The ordinary soda-glass tubing melts easily in the blowpipe flame, it
has not a long intermediate or viscous stage during fusion, but becomes
highly fluid rather suddenly; it does not blacken in the reducing flame.
Bad soda-glass or that which has been kept for many years, tends to
devitrify when worked. That is to say the glass becomes more or less
crystalline and infusible while it is in the flame; and in this case it
is often impossible to do good work with that particular sample of
glass; although the devitrification may sometimes be remedied by heating
the devitrified glass to a higher temperature. The presence of aluminum
compounds appears to have some influence on the tendency of the glass to
resist devitrification. Soda-glass, as a rule, is more liable to crack
by sudden heating than lead-glass, and articles made from soda-glass
often tend to crack spontaneously if badly made or, in the case of
heavier and thicker articles, if insufficiently annealed.
_Combustion-Glass._--Is usually a glass containing more calcium silicate
and potassium silicate than the ordinary "soft" soda-glass. It is much
less fusible than ordinary soda-glass, and passes through a longer
intermediate or viscous stage when heated. Such a glass is not very
suitable for use with the blowpipe owing to the difficulty experienced
in obtaining a sufficiently high temperature. If, however, a certain
amount of oxygen is mixed with the air used in producing the blowpipe
flame this difficulty is minimised.
_Resistance-Glass._--May contain zinc, magnesium, and other substances.
As a rule it is harder than ordinary soda-glass, and less suitable for
working in the blowpipe flame. It should have very little tendency to
dissolve in water, and hence is used when traces of alkali or silicates
would prove injurious in the solutions for which the glass vessels are
to be used.
_Lead-Glass._--This, or "flint" glass as it is often called from the
fact that silica in the form of crushed and calcined flint was often
used in making the English lead-glasses, contains a considerable
proportion of lead silicate. Such a glass has, usually, a particularly
bright appearance, a high refractive index, and is specially suitable
for the production of the heavy "cut-glass" ware.
Lead-glass tubing is easy to work in the blowpipe flame, melts easily,
but does not become fluid quite so suddenly as most soda-glasses;
articles made from it are remarkably stable and free from tendency to
spontaneous cracking, although, as is essential for all the heavy or
"glass-house" work, the massive articles need annealing in the oven.
The two chief disadvantages of lead-glass for laboratory work are that
it is blackened by the reducing gases if held too near to the blue cone
of the blowpipe flame, and that it is rather easily attacked by chemical
reagents; thus ammonium sulphide will cause blackening.
The effect of the reducing flame on lead is not altogether a
disadvantage, however; because a little care in adjusting the blowpipe
and a little care in holding the glass in the right position will enable
the student to work lead-glass without producing the faintest trace of
blackening. This, in addition to being a valuable exercise in
manipulation, will teach him to keep his blowpipe in good order, and
prove a useful aid in his early efforts to judge as to the condition of
the flame. It prevents discouragement if the student does his
preliminary work with the soda-glass, but he should certainly make
experiments with lead-glass as soon as he has acquired reasonable
dexterity with soda-glass.
_Annealing._--Annealing is a process by which any condition of strain
which has been set up in a glass article, either by rapid cooling of one
part while another part still remains hot, or by the application of
mechanical stress after cooling is relieved. Annealing is carried out by
subjecting the article to a temperature just below the softening point
of the glass, maintaining that temperature until the whole article has
become heated through the thicker part, and then reducing the
temperature very gradually; thus avoiding any marked cooling of the
thinner and outer parts first.
For thin glass apparatus of the lamp-blown or blowpipe-made variety in
which there are no marked difference of thickness, such as joins on
tubes, ordinary seals, bulbs, etc., there is little need for annealing;
and even those having rather marked changes of thickness, such as
filter pumps, can be annealed sufficiently by taking care that the last
step in making is heating to just below visible redness in the blowpipe
flame and then rotating in a sooty gas flame until covered with a
deposit of carbon. The article should then be allowed to cool in a place
free from draughts and where the hot glass will not come in contact with
anything.
A few of the blowpipe-made articles, such, for example, as glass
stopcocks, need more careful annealing, and for this purpose a small
sheet-iron oven which can be heated to dull redness over a collection of
gas burners will serve. Better still, a small clay muffle can be used.
In either case, the article to be annealed should be laid on a clean,
smooth, fireclay surface, the temperature should be maintained at a very
dull red for two or three hours and then reduced steadily until the oven
is cold. This cooling should take anything from three to twelve hours,
according to the nature of the article to be annealed. A thick article,
or one having great irregularities in thickness will need much longer
annealing than one thinner or more regular. As a rule, soda-glass will
need more annealing than lead-glass.
_Drilling Glass._--Small holes may be drilled in glass by means of a rod
of hard steel which has been broken off, thus giving a more or less
irregular and crystalline end.
There are several conditions necessary to enable the drilling of small
holes to be carried out successfully:--the first of these is that the
"drill" should be driven at a high speed. This may be done by means of a
geared hand-drill such as the American pattern drill, although a
somewhat higher speed than this will give is even more satisfactory. The
second condition is that the pressure on the drill is neither too light
nor too heavy; this is conveniently regulated by hand. The third
condition is that the drill be prevented from "straying" over the
surface of the glass; for this purpose a small metal guide is useful.
The fourth condition is that a suitable lubricant be used; a strong
solution of camphor in oil of turpentine is perhaps the most suitable.
For commercial work, a diamond drill is often used, but this is scarcely
necessary for the occasional work of a laboratory.
_Larger Holes in Glass._--The method of drilling with a hard steel rod
is not highly satisfactory for anything but small holes. When a larger
hole, say one of an eighth of an inch or more, is needed it is better to
use a copper or brass tube. This tube may be held in an American
hand-drill, but a mixture of carborundum or emery and water is supplied
to the rotating end. Tube or drill must be lifted at frequent intervals
in order to allow a fresh supply of the grinding material to reach the
end. In this case, also, a guide is quite essential in the early stages
of drilling; otherwise the end of the tube will stray. The speed of
cutting may be increased slightly by making a number of radial slots in
the end of the tube; these serve to hold a supply of the grinding
material.
_Grinding Lenses._--This is scarcely within the scope of a book on
glass-blowing for laboratory purposes, but it may be said that the lens
may be ground by means of a permutating mould of hard lead or
type-metal. The rough shaping is done with coarse carborundum or emery,
and successive stages are carried on with finer and finer material. The
last polishing is by the use of jewellers' rouge on the mould, now
lined with a fine textile.
_Filing Glass._--If a new file, thoroughly lubricated with a solution of
camphor in oil of turpentine, is used, there is but little difficulty in
filing the softer glasses. A slow movement of the file, without
excessive pressure but without allowing the file to slip, is desirable.
After a time the cutting edges of the file teeth will wear down and it
will be necessary to replace the file by another.
_Grinding Stoppers._--This is, perhaps, the most common form of grinding
that the laboratory worker will need to perform, and for that reason,
rather full details of the procedure are desirable.
A very crude form of ground-in stopper may be made by drawing out the
neck and the mass of glass which is intended to form the stopper to
approximately corresponding angles, wetting the surfaces with a mixture
of the abrasive material and water, and grinding the stopper in by hand.
Frequent lifting of the stopper is necessary during grinding, in order
to allow fresh supplies of abrasive material to reach the contacts. When
an approximate fit is obtained, the coarse abrasive should be washed
off, care being taken that the washing is complete, and a finer abrasive
substituted. After a while, this is replaced in its turn by a still
finer grinding material.
Such a method of grinding may give a satisfactory stoppering if the
angles of the plug and socket correspond very closely before grinding is
commenced; but if there is a wide difference in the original angles,
then no amount of grinding by this method will produce a good result.
The reason for this is that the plug will become so worn in the
preliminary grinding as to assume the form of a highly truncated cone;
the socket will assume a reverse form, and the end result will be a
loose-fitting plug and socket.
Satisfactory grinding may be carried out by the use of copper or
type-metal cones for the preliminary shaping. Such cones should be
mounted on a mandrel which will fit into the chuck of the American
hand-drill and turned on the lathe to the desirable angle for
stoppering. A number of these cones will be necessary. A number of
similar moulds, that is to say blocks of type-metal or hard lead in
which is a hole corresponding in size and angle to the plug desired,
should be made also. These must be rotated, either in the lathe or by
other means, and are used for the preliminary shaping of the plug. If
but few plugs are to be ground it is unnecessary to provide a means of
rotating the moulds, as the plug may be held in the hand and ground into
the mould in a manner similar to that used in the first method of
stoppering.
[Illustration: Fig. 14]
When the socket and plug have been ground, by the successive use of
cones and moulds, to the desired angle, so that they correspond almost
exactly, the plug is given its final fitting into the socket by
grinding-in with a fine abrasive, in the manner first described.
_Stopcocks._--Although it would be more strictly in keeping with the
form of this book to divide the making of stopcocks into two parts;
shaping by heat and grinding, we will consider the whole operation here,
and take for our example a simple stopcock such as that illustrated by
Fig. 14.
The "blank," _f_, that is the socket before grinding, is made by drawing
out a piece of fairly thick-walled tubing into the form shown by _a_.
Two zones on this tube are then heated by means of a small, pointed
flame, and the tube is compressed along its axis, thus producing two
raised rings as shown by _b_. Two zones, slightly towards the outer
sides of these two raised rings are heated and the tube is drawn while
air pressure is maintained within. This produces two thin-walled bulbs
or extensions similar to those shown by _c_. One of these extensions is
now broken off by means of a sharp blow with the edge of a file or
other piece of metal, and the edges of the broken glass are rounded in
the flame. The other extension is left to serve as a handle. We have now
a piece of glass like that shown by _d_. Now heat a spot on the side of
this, medially between the raised rings, until the glass is on the point
of becoming deformed, and bring the intensely heated end of a smaller
tube in contact with the heated spot. Without disturbing the relative
positions of the two tubes, press the smaller tube down on a thin steel
wire, so that the wire passes along the tube and enters the soft glass;
thus forming a projection inside the sockets as shown by _e_. The wire
must be withdrawn, again immediately. When the wire has been withdrawn,
heat the place where it entered to dull redness, in order to relieve any
strain; break off the thin extension, which up to the present has served
as a handle, round off the broken edges in the flame, and join on and
indent a similar piece of small tubing to the opposite side of the
socket; the socket at this stage being shown by _f_. The "blank" for the
socket is now completed, but it must be heated to dull redness in order
to relieve strain and be placed in an annealing oven, where it should
be annealed for some hours.
The "blank" for the plug offers no special difficulty; it is made by
heating a glass rod and compressing it axially until a mass having the
form shown by _g_, Fig. 14, is produced; the end of this is heated
intensely and brought in contact with the rather less heated side of a
glass tube which has been drawn to the shape desired for the handle;
when contact is made a slight air pressure is maintained in the glass
tube, thus producing a hollow join. The ends of the tube are sealed and
the bottom of the plug is drawn off, thus giving the finished "blank" as
shown by _h_. This blank is now held in a pair of asbestos-covered
tongs, heated to dull redness all over, and transferred to the annealing
oven.
When cold, the socket is ground out by the second method given under
"Grinding Stoppers"; that is to say, by means of type-metal or copper
cone, and the plug is ground to fit in a corresponding mould. When the
fit is almost perfect, the transverse hole is drilled in the plug, and
the final finishing is made with fine abrasive powder. Great care must
be taken in the final grinding that there is no accumulation of
abrasive material in the transverse hole of the plug; if this is allowed
to occur there will be a ring ground out of the socket where the holes
move, and the tightness of the finished stopcock will be lost.
_Marking Glass._--As a preliminary to a consideration of the methods of
graduating and calibrating glass apparatus, it is convenient to consider
the various methods which are available for marking glass. Among these
are, the writing diamond, the carborundum or abrasive pencil, the
cutting-wheel, and etching by means of hydrofluoric acid. Each produces
a different class of marking and each is worthy of independent
consideration.
_The Writing Diamond._--This is the name given to a small irregular
fragment of "bort" which is usually mounted in a thin brass rod. Such a
diamond, if properly selected, has none of the characteristics of a
cutting diamond; although one occasionally finds so-called "writing
diamonds" which will produce a definite cut. These should be rejected.
The writing diamond is used in much the same way as a pencil, but is
held more perpendicularly to the object, and a certain amount of
pressure is necessary. The mark produced is a thin scratch which,
although fairly definite, lacks breadth, and this is a disadvantage
where the marking has to be read at a distance. This disadvantage may to
some extent be overcome by making a number of parallel scratches.
_The Abrasive Pencil._--A rod of carborundum composition may be ground
or filed to a point, and this forms a very useful pencil for general
work. The marking produced is rather less definite than that produced by
a writing diamond, but has the advantage of being broader.
_The Cutting Wheel._--"Cutting" in this case is scarcely the ideal
expression, it should rather be "grinding," but "cutting" is more
commonly used. Exceedingly good graduations may be made by the edge of a
small, thin, abrasive wheel which is mounted on the end of a small
mandrel and driven by a flexible shaft from an electric motor or any
other convenient source of power. The depth of the mark can be
controlled, and very light pressure will suffice.
_Etching._--This is often the quickest and easiest way of marking glass
apparatus. The object to be marked should first be warmed and coated
very thoroughly with a thin film of paraffin wax. When cold, the marking
is made through the paraffin wax by means of a needle point, and the
object is then exposed to the action of hydrofluoric acid. If a shallow
but clearly visible marking is desired, it is well to use the vapour of
the acid; this may be done by bending up a sheet-lead trough on which
the object can rest with the marked surface downwards. A little of the
commercial hydrofluoric acid, or a mixture of a fluoride and sulphuric
acid, is distributed over the bottom of the trough, and the whole
arrangement is allowed to stand for about an hour. The object is washed
thoroughly and the paraffin wax removed, either by melting and wiping
off or by the use of a solvent, and the marking is finished.
If a deep marking is desired, in order that it may afterwards be filled
with some pigment, a better result is obtained by the use of liquid
commercial hydrofluoric acid, which is a solution of hydrogen fluoride
in water. The acid is mopped on to the object after the markings have
been made on the paraffin wax film, and allowed to remain in contact for
a few minutes. It is advantageous to repeat the mopping-on process at
intervals during the etching.
In all cases where hydrofluoric acid is used, or stored, it is of great
importance to keep it well away from any optical instruments, as the
most minute trace of vapour in the air will produce a highly destructive
corrosion of any glass surfaces.
_Methods of Calibration._--In the case of apparatus for volumetric work,
this is usually carried out by weighing, although some of the smaller
subdivisions are often made by measurement. When the subdivisions are
made in this way it is of importance to see that the walls of the tube
or vessel to be calibrated are parallel. Great errors arise in some of
the commercial apparatus from neglect of this precaution. A convenient
method of testing for parallelism, in the case of a wide tube, is to
close one end and to weigh in successive quantities of mercury. An
observation of the length occupied by each successive quantity will
indicate any change in the bore. In the case of capillary tubes, it is
convenient to introduce an unweighed quantity of mercury, measure its
length accurately, and then to move it along the tube in stages, either
by tilting the tube or by the application of air pressure. A measurement
of the length at each stage will indicate whether the bore is
approximately parallel or not. Neither of these methods is to be relied
on without a careful examination of the tube, as it may happen that
there are local irregularities in the bore which compensate for each
other, and do not, therefore, affect the volume of a given length.
Obviously, the smaller the quantity of mercury with which the test is
carried out and the greater the number of observations made, the less
risk will there be of such an error. A liquid, such as water or alcohol,
which wets the glass is not suitable for such a test, unless special
precautions are taken.
When, however, a pipette or burette has to be calibrated to deliver a
certain volume of water, the final calibration must be made with this
liquid. Thus, the burette would first be calibrated by weighing in
definite quantities of mercury of say 13.54 grammes (1 cc at 15°C.),
each of the 1 cc divisions should be marked by some temporary marking.
The burette is now filled with a solution of potassium bichromate and
sulphuric acid and allowed to soak for some time; the bichromate is
washed out and distilled water is put in. Successive quantities of water
are run out of the jet, a fixed time being allowed for draining, and the
weights of the quantities delivered are noted. This procedure will give
the necessary data for altering the marking so that it may correspond to
1 cc _delivered_. Each 1 cc division is now divided into tenths by the
method described below. A final verification of the markings should be
made when the subdivision is completed.
_Subdivision of Graduations._--Mark out the spaces to be subdivided on a
sheet of paper. Take a reliable ruler on which any convenient length is
divided into the desired number and place it across the lines at such an
angle that the limits noted on the rule exactly bridge the gap. Now draw
parallel lines through the markings.
_Copying a Scale._--When a scale has been prepared on paper and it is
necessary to copy that scale on the waxed-glass surface for etching, a
convenient method is to employ a long wooden bar having a sharp needle
passing through it at either end. The scale and object to be marked are
fastened in line with one another, and the caliper bar is used from step
to step. The mark is made by moving the bar through a minute portion of
a circle, which provided that the bar is two or three feet in length,
will not introduce any perceptible error in a scale of say a quarter of
an inch in width. The arrangement is shown by Fig. 15.
[Illustration: Fig. 15]
_Graduating a Thermometer._--Assuming that the thermometer has been made
of carefully selected tubing in which the bore is parallel and free from
any small irregularities, we have only to fix the freezing point and
boiling point. The intervening space may then be divided into 100 (if
the thermometer is to be Centigrade) or 180 (if Fahrenheit). This
division may be carried out by the method given under "Subdivisions of
Graduations." A thermometer should not be calibrated until some weeks
after making, as the glass bulb tends to contract.
_Joining Glass and Metal._--It sometimes happens that one needs to make
a more permanent and less flexible joint between a glass and metal tube
than can be obtained by means of a rubber tube. To this end, any one of
three slightly different methods may be employed. In the method of
Chatelier one first coats the glass with platinum or silver, which may
be done by moistening the glass with platinum chloride or silver nitrate
and then heating to redness; a layer of copper is then deposited
electrolytically on the treated surface of the glass, and soldering is
carried out in the usual manner.
McKelvy and Taylor call attention to two other methods in the _Journal
of the Chemical Society_ for September, 1920. In one of these methods
the glass is coated with platinum by covering it with a suspension of
platinum chloride in oil of lavender and heating until the oil is burnt
off. The metal tube is then tinned on its inner side and soldered to the
prepared glass, slightly acid zinc chloride being used as a flux.
In the second method, a joint is made by means of the Kraus flux, which
consists of equal weights of zinc oxide, borax, and powdered soda-glass
fused together. This is coated on the inner surface of the metal tube,
and the hot glass tube, which has had the end slightly flanged to give
support, is inserted. Fusion of the flux is completed by heating the
outside of metal tube.
_Silvering Glass._--In all cases where it is intended to deposit a
silver mirror on a glass surface, thorough cleaning is essential.
Prolonged soaking in a hot solution of potassium bichromate which has
been acidified with sulphuric acid will often prove useful. The glass
should then be washed thoroughly, rinsed in distilled water, and the
solution should then be used.
There are many formulæ for the silvering solution, but that used in
Martin's method may be given:--
A--Nitrate of Silver 40 grammes
Distilled Water 1000 c. cm.
B--Nitrate of Ammonium 60 grammes
Distilled Water 1000 c. cm.
C--Pure Caustic Potash 100 grammes
Distilled Water 1000 c. cm.
D--Pure Sugar Candy 100 grammes
Distilled Water 1000 c. cm.
Dissolve and add:--
Tartaric Acid 23 grammes
Boil for ten minutes, and when cool add:--
Alcohol 200 c. cm.
Distilled Water to 2000 c. cm.
For use take equal parts of A and B. Mix together also equal parts of C
and D in another vessel. Then mix both liquids together in the silvering
vessel and suspend the glass to be silvered face downwards in the
solution. Or if a vessel has to be silvered on the inside, the solution
is poured in. In this case, the deposition of silver may be hastened by
immersing the vessel to be silvered in warm water.
In working with a silver solution containing ammonia or ammonium salts
there is sometimes the possibility of forming an explosive silver
compound. It is well, therefore, to avoid keeping such solutions longer
than is necessary, and to bear in mind that any deposit formed by
solutions containing both silver and ammonia may have explosive
properties, especially when dry.
CHAPTER V
Extemporised Glass-Blowing Apparatus--The Use of Oil or
other Fuels--Making Small Rods and Tubes from Glass
Scrap--The Examination of Manufactured Apparatus with the
View to Discovering the Methods Used in Manufacture--Summary
of Conditions Necessary for Successful Glass-Blowing.
If, in the early stages of his study of glass-blowing, the student
should attempt to work with the very simplest appliances, it is probable
that his progress will be hindered; the use of the apparatus will
require an undue amount of care and his attention will be distracted
from the actual manipulation of the glass. The case is widely different
after he has acquired a certain facility in glass-blowing.
_A Simple Form of Blowpipe._--Although there are even more simple forms
than that described here, we are not concerned with them. The form
described is the simplest with which any considerable amount of
glass-blowing can be carried out with certainty.
This form consists of a tube through which air may be blown with the
mouth, a condensation chamber in which any moisture from the breath can
condense, a blowpipe jet, a supporting piece and a source of flame.
The tube, condensation chamber, and jet are combined in the ordinary
Black's blowpipe, such as is used for blowpipe tests in qualitative
analysis; it consists of a conical tin tube having a mouthpiece at the
small end and a side tube which carries a brass jet. A support for such
a blowpipe may be cut out of a piece of brass or tin-plate, and should
be fastened to a small, flat, wooden board. A source of flame may
consist of an ordinary brass elbow, such as is used on gas fittings, and
into which a piece of thin brass tube (the body of a fish-tail burner
from which the perforated non-metallic plug has been removed will serve
quite well) has been fitted. It is an advantage to flatten the brass
tube somewhat and to file the flattened end to a slope which corresponds
with the angle at which the blowpipe jet enters the burner. The whole
source of the flame should be mounted on a separate base, in order that
it may be moved while adjusting the apparatus to the best relative
positions of flame and blowpipe jet. The complete apparatus is shown by
_a_, Fig. 16.
[Illustration: Fig. 16]
In order to take full advantage of this blowpipe, it is desirable that
the student should learn to maintain a steady steam of air with his
mouth and, at the same time, be able to breathe. This requires a little
practice.
As a first exercise in breathing, before trying to breathe while using
the mouth blowpipe, the student should close his mouth and inflate his
cheeks with air; now, still keeping his cheeks tightly inflated, he
should attempt to breathe through the nose. At first, this may be found
rather difficult, but it becomes remarkably easy after a little
practice. When he has mastered this, the student may practise the same
operation, but with the blowpipe. It is important to bear in mind that
the cheeks, not the lungs, form the reservoir for air used in
maintaining the blowpipe flame. After a while, the student will find
that he can maintain a steady air pressure and yet breathe with complete
comfort.
In adjusting the flame, care should be taken not to blow so hard as to
produce a ragged and noisy cone of fire. A small jet, such as that
commonly used on a mouth blowpipe, will with care give a pointed and
quiet flame, having an appearance similar to that shown in the
illustration.
With a blowpipe like this, it is quite easy to seal glass tubes up to an
inch in diameter, to join tubes up to half an inch in diameter, to bend
tubes, to blow small bulbs, and to make the simpler forms of internal
seal; but the provision for condensation of moisture is not ideal, and
prolonged use of such a blowpipe also tends to produce undue fatigue.
_A Mouth Blowpipe With an Expanding Reservoir._--This form of blowpipe
can be made to give most excellent results; it is highly portable, and
does not produce nearly so much fatigue when used continuously as the
blowpipe described in the last section. Various slight modifications
have been made in its construction during the last eighty years, but
that described below will be found quite satisfactory.
The apparatus consists of a tube through which air is blown from the
mouth, a valve through which the air passes into an expanding reservoir,
and a blowpipe jet in communication with the reservoir.
In making the valve, several essentials have to be remembered; it must
allow a free passage of air into the reservoir, it must open easily, and
must close quickly. A satisfactory form of valve is that shown by _b_,
Fig. 16. The moving part consists of a light glass bulb of about
three-eights of an inch diameter and having a glass stem of rather under
one-eighth diameter and about an inch and a half long. This stem rests
in a guide at the end of a brass tube, the bulb contacting against the
other end which is approximately shaped. The bulb and its seating are
ground air-tight. A very light spring holds the bulb in position.
This valve is fitted into a metal or glass T piece, one limb of which
leads to the air reservoir and the other limb leads to the blowpipe jet;
the limb containing the valve leads to the tube through which the air is
blown in.
A convenient reservoir may be made from a fairly large football bladder.
A network of string should be fitted over the outside of the bladder and
the strings should terminate in a hook on which a weight can be hung, in
order to provide a means of adjusting the pressure at which the air is
delivered to the jet. This bladder should be washed out and allowed to
drain after use.
The air tube which passes from the valve to the mouth may conveniently
be made of brass, but, in order to avoid the continued contact of metal
with the lips of the operator, it should be fitted with a non-metallic
mouthpiece. It is an advantage from the point of view of portability to
have the air tube easily detachable from the T piece containing the
valve.
The blowpipe jets, of which there may be several with advantage, may be
made of glass tubing, bent to the most convenient angle and having an
enlargement or bulb at some point in the tube. This bulb serves as a
final condensing place for any traces of moisture that may escape from
the larger reservoir.
The whole device, blowing tube, reservoir, and T piece may be fastened
to a clamp, so that it can be secured on the edge of any table where
blowpipe work is to be carried out. If the blowpipe is to be used with
gas, the form of burner described under. "A Simple Form of Blowpipe"
will be found quite satisfactory.
_The Use of Oil, or Other Non-Gaseous Fuels._--Although gas, when
available, is usually preferred on account of its convenience, there are
several other fuels which give a hotter flame. They have, also, the
additional advantage of not requiring any connecting pipes; but each has
its own disadvantage.
One liquid fuel deserves special mention as being rather less desirable
than the others; this is alcohol. Although very convenient in use, it
has the disadvantage of being rather too highly inflammable and capable
of burning without a wick, thus involving a certain fire risk; the flame
is scarcely visible in a bright light, and the heat given by either the
ordinary flame or the blowpipe flame produced from alcohol is
considerably less than that from a similar flame in which coal gas is
used. For small work, however, the facility with which a spirit lamp may
be lighted may more than counterbalance these disadvantages at times.
_Paraffin Wax._--Where there is no coal gas available and the blowpipe
is only required at intervals, and especially where high portability is
required, there are few fuels so convenient as paraffin wax. This may be
obtained in pieces of a satisfactory size by cutting paraffin candles,
from which the wick has been withdrawn, into lengths of about half an
inch. These cut pieces have the advantage over any oily fuel, such as
colza oil, that they can be wrapped in paper or carried in a cardboard
box; further they will keep indefinitely, even in the presence of air,
without undergoing any perceptible change.
_Forms of Lamp for Paraffin Wax._--Probably, the best form is that
devised by Thomas Bolas, and described by him in the _Journal of the
Society of Arts_, December 2nd, 1898. This lamp consists of a small open
tray of iron, through which pass three or more flat tubes, and between
these tubes are placed small flat pieces of wick, the fit being such
that the pieces of wick may be adjusted easily by means of a pair of
pointed tweezers.
The flame thus obtained, instead of having one large hollow, is broken
or divided so that the combustion is concentrated into a smaller area,
and the air blast, which is directed across the flame, carries the flame
with it in a more complete manner than is the case with the ordinary
flame; a more thorough combustion being realised by this arrangement.
Another advantage is the ease with which the wick may be changed and a
larger or smaller wick inserted to suit the flame to any size of air
jet.
This form of lamp may be used for oily fuel, although it is specially
suitable for paraffin wax.
Two small pieces of bent tin-plate may be used as side covers, and these
serve to adjust the flame within certain limits. A tin-plate cover which
fits easily over the whole lamp serves as an extinguisher. The complete
lamp is shown by _d_, Fig. 16, and this figure shows also a quick-change
air-jet device, the whole arrangement forming a blowpipe for use where a
non-gaseous fuel is to be employed.
Although the lamp just described is desirable when complete control over
the size of the flame is necessary, and if the ideal conditions and
maximum heat are to be obtained, yet a simpler form of lamp will be
found to give very good results. Such a lamp may consist of a flat tin
tray, having a diameter of about three and a half inches and a depth of
about one inch. In this tray is a tin support for the wick, and the wick
itself may consist of a bundle of soft cotton, for example, a loosely
rolled piece of cotton cloth, but in either case the top of the wick
should be cut to approximately the same angle as that at which the
blowpipe jet meets the flame.
In using paraffin wax as a fuel, it is necessary to see that sufficient
wax reaches the wick to prevent charring during the first few minutes
before the bulk of the wax is melted.
_Animal and Vegetable Oils._--Almost any oil may be used as a fuel, but
many tend to become hard and gummy if allowed to stand in the air for
any considerable time. When this happens, the wick becomes clogged and
it is impossible to obtain a good flame. A number of the oils tend,
also, to produce rather strongly smelling smoke.
_A Flame-Guard for Use With Non-Gaseous Fuels._--In order to avoid the
eye-strain produced by the luminous base of the flame from a wick
burning paraffin wax or oil, it is often advantageous to make a small
tunnel of tin-plate, which can be rested on the sides of the lamp and
rises over the top of the wick. Such a flame guard is shown by _e_, Fig.
16.
_Small Rods and Tubes from Glass Scrap_:--It is scarcely practicable to
make small quantities of good glass with the blowpipe flame as the only
source of heat, but it is less difficult to make small rods or tubes
from glass scrap, and the ability to do this is sometimes of
considerable value when a small tube has to be joined on to some special
piece of apparatus made of glass of unknown composition. It may be
possible to obtain some fragments of similar glass, either from a broken
part of the apparatus or from a similar piece, and from these fragments
small tubes or rods can be made.
The fragments of glass may be melted together on the end of a clay
pipe-stem, care being taken to avoid trapping air bubbles as fresh
fragments are added to the molten mass. When a sufficient quantity of
glass has been accumulated, the viscous mass may be drawn out into a rod
by bringing another pipe-stem into contact with the hot mass, rotating
both pipe-stems steadily, and separating them until a rod of the desired
size has been obtained.
If, on the other hand, it is desired to produce a tube from the mass of
heated glass, the mass should be blown hollow before the pipe-stems
supporting it are separated.
_Methods of Manufacture._--When the student has familiarised himself
with the more common operations and processes used in glass-blowing, he
will be in a position to increase his skill and knowledge of special
methods by a critical examination of various examples of commercial
work. There are few exercises more valuable than such an examination,
combined with an attempt to reconstruct the stages and the methods by
which the article chosen for examination was made.
Obviously, it is impossible to give full details of all constructions in
a small text-book; but it is easy to give an example of the
constructional methods employed in the making of almost any piece of
light blown-glass apparatus, and these methods should prove of special
value when apparatus of a new pattern has to be evolved for the purposes
of research. That is to say, one designs the apparatus required, applies
known methods of construction as far as possible, and, by the
examination of commercial apparatus having similar features, evolves the
new methods required. For an exercise in such a process of
reconstruction we may well take an ordinary commercial vacuum tube, such
as that shown by _a_, Fig. 17.
[Illustration: Fig. 17]
In the tube from which this drawing was made, it was found that the
spiral in the middle bulb was of a slightly yellowish colour and gave a
green fluorescence when the electric discharge was passed through the
tube; that is to say, the spiral is made of uranium-glass, which is
usually a soda-glass containing trace of uranium, and hence differing
slightly in composition from the ordinary glasses. The two enclosed
tubes which are bent into a series of S bends gave a pink fluorescence,
which indicates lead-glass; and the remainder of the tube fluoresced
with an apple-green colour; this suggests ordinary soda-glass. We have,
therefore, a piece of apparatus in which three dissimilar glasses are
joined, while, at the same time, that apparatus contains a number of
internal seals, and it is not probable that the dissimilar glasses will
have their coefficients of expansion so nearly alike as to permit of a
stable internal seal being made if one part of the seal consists of a
glass differing from that of the other part.
These considerations lead us to a closer examination of the joins where
the dissimilar glasses are introduced, and we find that in no case is
the internal seal made between dissimilar glasses, but that a soda-glass
extension is joined on to both the uranium-glass tube and the lead-glass
tubes at a point about half an inch before the internal seal commences.
Careful examination of these joins shows that the change from one glass
to another is not abrupt but gradual. Such a transitional joint may be
made by taking a length of soda-glass tubing, sealing the end and fusing
a minute bead of the other glass on to the sealed end, the end is then
expanded and another bead of the other glass added, this bead is
expanded and the operation is repeated, thus building up a tube, and,
finally, the tube of the other glass is joined on to the end of this.
We are now concerned with the question of the insertion of the
uranium-glass spiral into the bulb (see p. 38). Obviously the spiral is
too large to pass through the necks of the bulb, and it is difficult to
imagine that the spiral was obtained by the insertion of a length of
straight tubing which was bent after entering the bulb; therefore, the
only remaining method is that the spiral was made first and the
soda-glass extensions fastened on, and that the bulb was blown, cut in
halves and the spiral inserted, and the two halves were then rejoined.
That this was actually the case is confirmed by traces of a join which
are just visible round the middle of the bulb. The insertion of the
spiral and the making of the first internal seal are shown by _b_, and
_c_.
There is one detail in making the second join of the spiral to the bulb
which calls for attention, and the small branch, similar to an
exhaustion branch, at the side of the bulb provides a clue to this. If
an attempt were made to complete the second internal seal through a
closed bulb it would be impossible to obtain a good result, as the
air-pressure in the bulb would not be under control when once union was
effected, and further heating of the air in the bulb would cause
expansion and perforate the wall near the second internal seal; we
therefore make a small branch which can be left open and through which
such air-pressure as may be found necessary can be maintained.
The third join, by which the lead-glass tube is joined to the soda-glass
is made in stages similar to those in which the soda-glass and
uranium-glass were joined; but the internal seal is most conveniently
made by sliding a length of tubing over the lead-glass and fusing this
tubing to the large diameter soda-glass tube to which the lead-glass is
already joined. The first stage of this operation is illustrated by _d_.
When this seal is completed, the end of the soda-glass tube is drawn off
and sealed as shown in _e_, and at this stage a side tube or branch is
joined on. The sealed end of the outer and large diameter soda-glass
tube is heated until it contracts and fuses to the enlargement that has
previously been joined to the lead-glass tube, and the end is burst out
as shown in _f_. Another length of soda-glass is then joined on to the
burst-out end, and this length of soda-glass tubing is drawn out to a
thin-walled contraction; the non-contracted part is expanded to form the
bulb, and a small exhaustion branch made on the side, the drawn-out
portion being cut off, and an electrode, previously prepared by coating
a part of its length with a suitable enamel, is introduced. The tube is
tilted to keep the electrode away from the drawn-out end, which is
melted off and sealed. A small perforation is made with a hot platinum
or iron wire in the sealed end, the electrode is shaken into position,
and the sealing is completed as explained on page 42.
The remainder of the tube, that is to say the lead-glass tube and the
bulb on the other side of the middle bulb, is completed in a similar
manner.
SUMMARY OF CONDITIONS NECESSARY FOR SUCCESS IN GLASS-BLOWING.
For the convenience of the student, it may be well to summarise the
chief essentials for success in glass-blowing, and at the same time to
add such brief notes on the various methods as may seem desirable.
_Adjustment of Blowpipe._--The air jet should be clean internally, and
so centered as to give a flame having a well-defined blue portion, the
tip of the flame should not be only slightly luminous but purple in
colour. In the case of a blowpipe burning oil or wax fuel the flame may
be a trifle more ragged without disadvantage.
_Bellows and Blowing._--The bellows should be adjusted to deliver air at
constant pressure, either by insertion of a tap or, better, by attention
to the wind reservoir if necessary. The movement of the foot in blowing
should be steady, not jerky.
_Heating Glass._--The tube or rod should be heated cautiously until it
has reached its softening point in its thickest part. Steady rotation of
the glass during the heating is almost essential.
_Blowing a Bulb or Expanding a Join._--Prolonged heating is necessary in
order that the thick parts may be heated completely through. Blowing
should take place by stages, in order that the thin parts, which tend
to expand first, have time to cool. The thick parts can then be expanded
by further blowing and thus a bulb or expansion of even thickness can be
obtained.
_Cutting Glass._--The most useful method for general use is by means of
the file or glass-blowers' knife. Either file or knife must be kept
sharp by grinding. Neither file nor knife should be used on hot glass.
The diamond and wheel cutter are useful for cutting sheet-glass, and
when the diamond is employed a singing noise is an indication of a
satisfactory cut.
_Leading a Crack._--A crack may be led in any desired direction by means
of a bead of hot glass or a small gas flame. The glass which it is
desired to crack should be heated at a point slightly in advance of the
crack, which will extend in the direction of the source of the heat.
_Turning Out the End of a Tube._--This is done by heating the end of the
tube and rotating it against an iron rod. The rod must be kept polished
and free from rust, and it must not be allowed to become too hot while
in use, otherwise the glass will stick to it.
_Joining Unlike Glasses._--Joints between unlike glasses are often
unstable. When such joints are made it is desirable to blow them as thin
as possible, and to avoid the junction of unlike glasses in any complex
joint, such as an internal seal. A transitional portion of tubing may be
built up by the successive addition and interfusion of beads of one of
the glasses to the end of a sealed tube consisting of the other glass.
_Joining a Tube to a Very Thin Bulb._--The bulb may be thickened at the
point of union by fusing on a bead of glass and expanding this slightly.
A small central portion of the expanded part may then be perforated by
bursting and the tube joined on.
_Insertion of One Bulb Within Another._--A bulb may be divided into two
halves by leading a crack round it and the inner bulb is then
introduced. The two halves of the outer bulb may be fitted together
(care being taken to avoid any damage to the edges), and the bulb may be
completed by rotating the contacting edges before the blowpipe until
they are soft, and then expanding slightly by means of air-pressure.
_Annealing._--For most purposes, in the case of thin, blowpipe-made or
lamp-blown glass apparatus, it is sufficient to cool slowly by rotating
the finished article over a smoky flame and setting it aside in a place
free from draughts, and where the hot glass will not come in contact
with anything.
Simple bulbs and joints do not even need this smoking; but thick
articles, and especially those that are to be subjected to the stress of
grinding, need more prolonged annealing in a special oven.
_Use of Lead-Glass._--When lead-glass is to be used, the blowpipe flame
should be in good adjustment and the glass should not be allowed to
approach so near to the blue cone as to be blackened. Slight blackening
may often be removed by heating the glass in the extreme end of the
flame.
Lead-glass articles tend to be rather more stable than similar articles
of soda-glass.
_Combustion-Glass._--This may be worked more easily if a small
percentage of oxygen is introduced into the air with which the blowpipe
flame is produced. If the air is replaced entirely by oxygen there is a
risk of damaging the blowpipe jet, unless a special blowpipe is
employed.
_Internal Seal._--There are two ways of making these, one, in which the
inner portion of the tube is fused on to the inside of the bulb or tube
through which it is to pass, an opening is made by bursting and the
outer tube is joined on. This is a quick and in some ways more
satisfactory method than the other, in which there is no separate inner
piece.
_Rubber Blowing Tube._--In complicated work it is often convenient to
use a thin rubber blowing-tube which is connected with the work either
by a cork and piece of glass tubing or by fitting over a drawn-out end.
The use of such a blowing-tube avoids the inconvenience of raising the
work to the mouth when internal air-pressure is required. One end of the
rubber tube is retained in the mouth during work.
_General Notes._--A large amount of glass-blowing is spoiled through
carelessness in arranging the work beforehand. The student should have
every detail of his manipulation clearly in mind before he commences the
work; he should not trust to evolving the method during the actual
manipulation.
Undue haste is another fruitful source of failure. Practically every
operation in glass-blowing can be carried out in a perfectly leisurely
manner, and it is better to err rather on the side of deliberation than
on the side of haste.
If, as will doubtless happen at times, a piece of work gives trouble and
it is necessary to pause and consider the whole question, or if for any
other reason it is necessary to stop during the construction of a
partially finished join or other operation, great care should be taken
not to allow the work to cool. A large, brush-like flame may be produced
by increasing the amount of gas admitted to the blowpipe, and the work
should be held just in front of the current of hot air produced by such
a flame.
It will then be possible to continue work on this without causing it to
crack when further heat is applied.
As time goes on, the student will find an increasing confidence in his
ability to manipulate the soft glass, and with increasing confidence
will come rapidly increasing power of manipulation. Perhaps the greatest
obstacle to success in glass-blowing is undue haste in manipulation.
INDEX
Absorption bulbs, 21, 23.
Airtube, flexible, 8, 102.
Alarm thermometer, 45.
Annealing, 7, 60.
Bellows, adjusting pressure of, 5, 6.
Bellows, foot, 5, 6.
Bending tubes, 23.
Blackening, 58, 101.
Branching, 18, 19.
Brushes of spun glass, 53.
Blowpipe flame, quality of, 3.
Blowpipe for mouth blast, 80, 82, 84.
Blowpipe, for paraffin wax, 82, 88.
Blowpipe, Herepath's, 2.
Blowpipe jet, centring, 3, 98.
Blowpipe jet, dirt in, 3.
Blowpipe jet, multiple, 4, 40.
Blowpipe, Letcher's, change, 4.
Blowpipe, simple form of, 80.
Bulb, medially on tube, 22.
Bulbs, 19, 20, 22, 38, 98.
Bulbs, absorption, (Liebig's), 21, 23.
Bulbs, dividing, 39, 95.
Bulbs from rod, 25.
Bulbs, internal, 38.
Bulbs, thick, 21.
Cages, from glass rod, 24, 25, 27.
Calibration, 72.
Carius tubes, 16.
Condenser, Liebig's, 37.
Condensers, various, 37, 38.
Cone, carbon, 8.
Crack, leading, 30, 99.
Cracking, subversive, 103.
Cutting glass with diamond, 30.
Cutting tubes, 11, 99.
Diamond (glazier's), use of, 30.
Dissimilar glass, joining of, 22, 94.
Drilling, 61.
Electrodes, sealing in, 42, 97.
Etching glass, 70.
Extemporised appliances, 80.
Examination of apparatus, 93.
Failure, Haste chief Source of, 103.
Failures, Notes as to, 97.
File, with oblique ground edge, 7.
Filing glass, 63.
Filter pumps, 35
Foot, 25
Fuels various, 82, 86, 87, 89.
Funnel, thistle, 23.
General principles and precautions, 1, 97
Glass, varieties of, 9, 55, 91-97.
Graduation, 72-76
Haste, Source of Failure, 103
Heat reflector, asbestos, 7.
Heating, intensive, 7, 57.
Heating precautions, 12, 98
Joining dissimilar glass, 22.
Joining glass to metal, 76.
Joining tubes, 16, 94, 100.
Knife, Glass blower's, 7, 99.
Lenses, grinding, 63.
Marking glass, 69.
Methods, analytic study of, 91, 93.
Oxygen for intensive heating, 57, 101.
Precautions and General Principles, 1, 97.
Pumps, Filter, 35.
Pumps, Sprengel, 49, 50.
Re-entering branch, 40.
Reflector of heat, asbestos, 7.
Rod, uses and articles from, 17, 25, 27, 28.
Rod, blowing to hollow, 17, 25, 26, 91.
Scrap glass, working, 90.
Sealing tubes, 12, 13, 14.
Sealed tubes for pressure, 15, 16.
Sealing in of Electrodes, 42, 97.
Seals, internal (airtraps), 32, 102.
Silvering glass, 77.
Soldering glass, 76.
Soxhlet-tube, 40.
Spirals, 23, 95.
Spray arrester, 34.
Spray producers, 36.
Sprengel pumps, 49, 50.
Spinning glass, 51.
Stopcocks, 60, 66.
Stoppering, 63.
Stirrers, 28, 29.
Summary as to precautions and failures, 97.
Taps, 60, 66.
Thermometers, Various, 44-49.
Thermo-regulator, 24.
Thistle Funnel, 23.
Tools, Various small, 7.
Turn-pins, 7, 8, 99.
Turning out open ends, 14, 99.
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