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Title: Farmers' Bulletin 1889 - Fireplaces and Chimneys
Author: Miller, Thomas A. H., Senner, Arthur H.
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

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                              and CHIMNEYS

                                NO. 1889

                             U.S. DEPARTMENT
                             of AGRICULTURE

THAT THE WORD "HEARTH" is synonymous with "home" in many languages is
not surprising since much of the enjoyment of home and camp life centers
about an open fire. In mild climates a properly built fireplace will heat
a single room, and when equipped with a convection heater will also heat
a second room on the same floor or an upper floor. In colder climates it
is a useful adjunct to other heating systems if provided with a damper.

This bulletin is intended to give the householder and prospective
builder, especially the farmer who might superintend the construction
of his home, a working knowledge of the principles to be observed in
planning and building fireplaces and chimneys. These principles, if
observed, will make the structures useful and satisfactory and insure
their safety.

Safe fireplaces and chimneys that function properly can be built by
applying the principles given in this bulletin, but a good chimney will
not last indefinitely without proper care and repair.

Fireplaces and chimneys, being conspicuous architectural features, should
be pleasing in appearance and conform with the general design of the
building and its surroundings.

This bulletin supersedes Farmers' Bulletin 1649, Construction of Chimneys
and Fireplaces.

Washington, D. C. Issued December, 1941


By Arthur H. Senner, _mechanical engineer, and_ Thomas A. H. Miller,
_agricultural engineer. Division of Farm Structure Research, Bureau of
Agricultural Chemistry and Engineering_



  Chimneys                                       2
    Design                                       2
    Construction                                 7
    Estimating brick                            18
    Smoke test                                  18
    Cleaning and repairing flues                19

  Fireplaces                                    22
    Characteristics                             22
    Modified fireplaces                         24
    Selecting a fireplace                       27
    Construction                                34
    Dimensions                                  35
    Cost estimate                               43
    Smoky fireplaces                            45

  Outdoor fireplaces                            46
    Types                                       47
    Obtaining plans                             48
    Construction                                48
    Operation                                   51

  Barbecue pits                                 51

  Dutch ovens                                   51

FIREPLACES AND CHIMNEYS should provide a safe place for
an open fire and a flue for draft to expel smoke from the fire
passage to the open air. They must be properly designed and constructed
(fig. 1) if good performance and protection against fire are
to be obtained.

[Illustration: Figure 1.--A properly designed and well-built chimney that
provides ample draft and protection against fire.]



Solid masonry is the most satisfactory and safest material to use
for chimneys and fireplaces. If a chimney fire occurs, the safety of
the building may be dependent on the soundness of the flue walls
(fig. 2). Cracked and leaky flues not only are inefficient, destroying
the draft as well as permitting smoke and gases to pass into adjacent
rooms, but are a dangerous fire hazard. The chimney as known today
was developed about 600 years ago. Experience has shown that the
satisfactory performance of a chimney flue is determined by its size,
direction, shape, height, tightness, and smoothness.


The draft of a chimney is the current of air created by the difference
in pressure resulting from variation in weight between the relatively
hot gases in the flue and the cooler outside air. The strength
or intensity of the draft depends, for the most part, on the height of
the chimney, and the temperature difference between the chimney
gases and the outside atmosphere. The draft is not so good in summer
as in winter because the difference in temperature between the
outside air and the gases in the flue is less.

A very common error in chimney design is failure to distinguish
between the size of the flue required for free passage of the volume of
smoke from a given amount of fuel and that which, with proper height,
will produce the required draft. A chimney may be high enough
(fig. 3), yet have an area too small to expel the volume of smoke; or
the size may be ample (fig. 4) but the height not great enough to
produce a strong draft. Either fault or a combination of the two will
result in unsatisfactory service.

Flue Sites

The dimensions of a flue for adequate draft depend principally on
the grate area and type of heating plant 1 and on the kind of fuel to be
burned, both of which should be determined before construction is
begun. If a chimney is found to be inadequate the only method of
improving it, short of reconstruction, is to increase its height. This is
not always effective and is often impracticable.

Table 1 gives the sizes of fire-clay flue linings ordinarily provided
for boilers, furnaces, stoves, or convection heaters burning soft coal.
These sizes have proved satisfactory for average flat-grate furnaces
under normal conditions. Manufacturers of heating equipment
usually specify certain requirements in chimney construction and will
not guarantee the performance of their heaters unless these requirements
are met. Therefore their recommendations should be followed
when differing materially from the dimensions given in this bulletin.

Height of Chimney

A chimney should extend at least 3 feet above flat roofs and 2 feet
above the ridge of peak roofs. Where chimneys cannot be built
high enough above the ridge to prevent trouble from eddies caused by
wind being deflected from the roof, a hood may be provided with the
open ends parallel to the ridge. Eddies which force air down the
flues may be caused by building the chimney too near trees (fig. 5, _B_)
or a higher structure (fig. 6).

[1] Farmers' Bulletin 1698, Heating the Farm Home, contains information
on estimating the size of the heating plant needed for houses of
different sizes and for determining grate areas.

[Illustration: Figure 2.--Heavy masonry chimneys of this type are still
being built in rural areas. The thick walls, with unlined flues, are in
good condition after 75 years of continuous use.]

Table 1.--_Sizes of flue linings and heights of chimneys recommended for
flat-grate furnaces burning soft coal_[A]

                Nominal size of flue lining
        ---------------------------------------------------  Height of
  Grate Round (inside                                        chimney top
  area  diameter)             Rectangular (outside           above grate
  (Sq.  at elevation          dimensions) at elevation       at elevation
  ft.)  indicated             indicated                      indicated
  - ---------------- --------------------------------------  --------------
     SL  2K  4K  6K  Sea Level 2,000 ft  4,000 ft  6,000 ft  SL  2K  4K  6K
  - ---------------- --------------------------------------  --------------
     In  In  In  In     In       In        In        In      Ft  Ft  Ft  Ft
  1   8   8   8  10  8½ by 8½  8½ by 8½  8½ by 8½  8½ by 13  22  26  32  36
  2  10  10  10  10  8½ by 13  8½ by 13  8½ by 13  8½ by 13  24  29  35  41
  3  10  10  12  12  8½ by 13  8½ by 13  13 by 13  13 by 13  26  33  41  49
  4  12  12  12  12  13 by 13  13 by 13  13 by 13  13 by 13  30  37  45  49
  5  12  12  15  15  13 by 13  13 by 13  13 by 18  18 by 18  32  37  43  52
  6  15  18  18  18  18 by 18  18 by 18  20 by 20  20 by 20  30  37  47  56
  7  18  18  18  18  20 by 20  20 by 20  20 by 20  20 by 20  32  41  49  64
  8  18  18  18  18  20 by 20  20 by 20  20 by 20  20 by 20  35  42  56  10

[A] If anthracite is to be burned the area of the flue cross section may
be reduced about 25 percent.

The ratings given are based on comparatively smooth lined flues with no
offsets greater than 30° with the vertical.

The smallest sizes of fuels require excessive drafts and may necessitate
taller chimneys.

Flue heights and sizes are based upon approximately the several altitudes
indicated; it is sufficiently accurate to use the column giving the
altitude nearest to that of the particular problem.

When 2 or 3 appliances are connected to the same flue their total grate
area may be reduced 15 percent. The method of determining the proper flue
size for an altitude of 2,000 feet, when 1 appliance with a grate area of
3 square feet and another with an area of 1.5 square feet are attached to
the same flue, is shown by the following example:

Add the 2 grate areas, 3 + 1.5 = 4.5 square feet. Reduce this total
area by 15 percent. Thus, 4.5 - 0.68 = 3.8 square feet is the required
area. Use the nearest whole number, 4. From the table it is seen that
for a grate area of 4 square feet at an elevation of 2,000 feet either
a 12-inch (inside diameter) round flue or a 13- by 13-inch (outside
dimensions) rectangular flue 37 feet high is required.

[Illustration: Figure 3.--This tall chimney produced good draft for
the kitchen range, but the flue was too small for a furnace. When the
house was remodeled, its appearance was greatly improved by building the
chimney inside.]

[Illustration: Figure 4.--Short chimneys are frequently provided for low
bungalows, for architectural reasons. This flue is ample in size but not
high enough for use with a stove. The stone masonry has been laid to
harmonize with the rustic surroundings.]

Frequently metal-pipe extensions are provided to increase the height of a
flue on account of the low cost and ease of installation, but these must
be securely anchored against wind and have the same area as the flue.
Metal extensions are likely to rust in a short time. They are available
with a metal cowl or top that turns with the wind to prevent air blowing
down the flue. Terra-cotta chimney pots or extensions are more durable
and attractive.

A chimney located entirely inside a building has better draft because the
masonry retains heat longer when protected from cold outside air.

[Illustration: Figure 5.--Two pleasingly designed fireplace chimneys that
fit into their surroundings. _A_, This chimney stands in the clear and
should provide a good draft; _B_, a chimney under overhanging trees is
likely to backdraft. Contrast the appearance of these two chimneys with
that of figure 3.]

[Illustration: Figure 6.--Several extensions were necessary before this
chimney would draw properly on account of the wind deflected from the
nearby wall.]

[Illustration: Figure 7.--_A_, A good foundation extending below the soil
affected by frost. This chimney is well protected from ground moisture
by the concrete carried above the surface; _B_, an insecure foundation.
Supporting a chimney in this manner is a dangerous practice.]


=Supporting the Chimney=

Stable foundations, preferably of concrete, should be provided, at least
6 inches wider all around than the chimney and 8 inches thick for
one-story and 12 inches thick for two-story houses. When there is no
basement or cellar (fig. 7, _A_), start the foundation of an exterior
chimney well below the frost line; otherwise, extend the base to the same
level as the bottom of the foundation of the building. Foundations for
tall, heavy chimneys require special consideration.

Where the wall of the house is of solid masonry 12 inches or more thick,
the chimney may be offset and carried on corbels or masonry brackets
instead of being carried down to the ground. The offset should not extend
more than 8 inches from the face of the wall, each course projecting not
more than 1 inch, and should not be less than 12 inches high. Often the
corbeling is started at the second- or third-floor level so that the
chimney is only one or two stories high.

[Illustration: Figure 8.--For structural safety the amount of offset must
be limited so that the center line, XY, of the upper flue will not fall
beyond the center of the wall of the lower flue. _A_, Offsetting of the
left wall of an unlined flue is started two brick courses higher than
on the right wall so that the area of the sloping section will not be
reduced after plastering; _B_, a lined flue showing the method of cutting
the tile.]

Chimneys in frame buildings should be built from the ground up or should
rest on the foundation or basement walls if of solid masonry 12 inches or
more thick.

A chimney resting on or carried by wooden floors, beams, or brackets or
hung from wooden rafters (fig. 7, _B_) is a fire hazard. Wood framing
shrinks, and beams supporting heavy loads deflect in time. Sagging beams
injure the walls and ceilings of the house and are apt to crack the
chimney, rendering it dangerous.

=Flue Linings=

Although, to save expense, chimneys are built without flue lining, those
with linings are more efficient. When the flue is not lined, the mortar
and bricks directly exposed to the action of fuel gases disintegrate.
This disintegration and that occurring from changes in temperature
frequently cause cracks in the masonry, thereby reducing the draft. An
unlined chimney is best if not plastered except at the sloped section
(fig. 8, _A_). However, the vertical and horizontal joints should be
filled with mortar and struck smooth and flush with the wall. Offsets or
bends in flues (fig. 8) should not be greater than 30° with the vertical.
This slope can be obtained by offsetting or corbeling each brick course
only 1 inch.

Flue lining must withstand rapid fluctuations in temperature and be
resistant to the action of ordinary flue gases. The shapes used as flue
lining should be of fire-clay, with shells not less than five-eighths
of an inch thick, and should be vitrified. As a safeguard against
over-burning and brittleness, the lining should be tested by submersion
in water at room temperature for 24 hours, during which a quantity of
water weighing more than 3 percent of the dry weight of the lining should
not be absorbed. Place each length of flue lining in position, setting
it in cement mortar with the joint struck smooth on the inside, and then
lay the brick around it. If the lining is slipped down after several
courses of brick have been laid, the joints cannot be filled and leakage
is almost sure to result. Fill any spaces between the lining and the
brickwork completely with mortar, especially if the round type of flue is

The lower section of flue lining, unless resting on solid masonry at the
bottom of the flue, should be supported on at least three sides by brick
courses projecting to the inside surface of the lining. When laying brick
and lining, it is advisable to draw up a tight-fitting bag of straw as
the work progresses so as to catch material that might fall and block the

Where offsets or bends are necessary in lined flues, tight joints can
be made by mitering or cutting equally the ends of abutting sections
(fig. 8, _B_). This can be done if a cement sack of damp sand is stuffed
firmly into the lining and a sharp chisel is tapped with a light hammer
along the line where the cut is desired. If the cutting is done after the
lining is built into the chimney, the lining may be broken and fall out
of place. The hole for the thimble can be cut the same way when a special
thimble section is not used.

The linings commonly used are rectangular or round. Rectangular linings
are better adapted to brick construction than round linings, but the
latter are considered more efficient. The sizes commonly used are
indicated in table 2.

=Wall Thickness=

Walls of chimneys not more than 30 feet high when lined should be 4
inches thick if of brick and reinforced concrete, 8 inches if of hollow
building units, and 12 inches if of stone. Linings may be omitted in
chimneys having walls of reinforced concrete at least 6 inches thick
or of unreinforced concrete or brick at least 8 inches thick, although
lining is desirable in the case of brick construction. Also the outside
wall of a chimney exposed to the weather is best made at least 8 inches

In chimneys containing three or more flues, building codes generally
require that each group of two flues be separated from the other
single flue or group of two flues by brick divisions or withes not
less than 3¾ inches wide (fig. 9) . Where two flues are grouped
without divisions, joints in the linings of adjacent flues are safer
if staggered at least 7 inches, and particular care should be taken to
have all joints filled with mortar. Individual flues are advisable for
fireplaces and heating furnaces or boilers.

Table 2.--_Dimensions of commonly used standard commercial flue lining_

            Rectangular linings[B]

   Outside      Cross-sectional area    Wall
  dimensions   --------------------- thickness
   (inches)      Inside     Outside
  -----------  ----------  --------  ----------
                 Square      Square
                 inches       feet     Inches

  4½ by 8½         23.6       0.26       5/8
  4½ by 13         38.2        .41       5/8
  7½ by 7½         39.1        .39       5/8
  8½ by 8½         52.6        .50       5/8
  8½ by 13         80.5        .78       3/4
  8½ by 18        109.7       1.10       7/8
  13 by 13        126.6       1.20       7/8
  13 by 18        182.8       1.70       7/8
  18 by 18        248.1       2.30       1-1/8
  20 by 20        297.6       2.60       1-3/8

                Round linings[C]

    Inside      Cross-sectional area    Wall
   diameter    --------------------- thickness
   (inches)      Inside     Outside
  -----------  ----------  --------  ----------

                 Square      Square
                 inches       feet     Inches

        6         28.3       0.29        5/8
        8         50.3        .49        3/4
       10         78.5        .75        7/8
       12        113.0       1.07        1
       15        176.7       1.62        1-1/8
       18        254.4       2.29        1-1/4
       20        314.1       2.82        1-3/8
       22        380.1       3.48        1-5/8
       24        452.3       4.05        1-5/8
       27        572.5       5.20        2

[B] All rectangular flue lining is 2 feet long.

[C] Round flue lining, 6 to 24 inches in diameter, is 2 feet long; that
27 to 36 inches in diameter is 2½ or 3 feet long.

[Illustration: Figure 9.--Cross section of chimney showing the proper
arrangement for three flues. The division wall should be well bonded with
the side walls by staggering the joints of successive courses. Note the
studs are kept 2 inches away from the brickwork for reasons explained on
page 14.]

When two or more flues are used in unlined chimneys, they must be
separated by well-bonded withes 8 niches thick. An attractive and
effective method of separating unlined flues in colonial times is shown
in figure 10.

Chimneys extending above the roof are exposed to the wind and may sway
enough during a gale to open up the mortar joints at the roof line.
Openings in a flue at this point are especially dangerous because sparks
from the flue may come in contact with the woodwork of the roof. It is
therefore good practice to make the upper walls 8 inches thick (fig. 11)
by starting to offset the bricks just below the intersection with the

The brickwork around all fireplaces and flues should be laid with cement
mortar, as it is more resistant than lime mortar to the action of heat
and flue gases. A good mortar to use in setting flue linings and all
chimney masonry, except firebrick, consists of 1 part portland cement,
1 part hydrated lime, and 6 parts clean sand, measured by volume.
Slacked-lime putty may be used in place of hydrated lime; firebrick is
best laid in fire-clay.

[Illustration: Figure 10.--This Williamsburg chimney shows the pains
taken to make the chimney attractive. The three flues are arranged as a T
with well-bonded withes between them. Often four flues were used in the
form of a cross.]

_Openings Into the Chimney_

No range, stove, fireplace, or ventilating register should be connected
with the flue being used for the heating apparatus because this is a
frequent cause of unsatisfactory operation. Fires may occur from sparks
passing into one flue opening and out through another where there are two
connections to the same flue. If an abandoned fireplace chimney is to be
used for a range or stove, close the fireplace flue tight about a foot
below the smoke pipe hole.

[Illustration: Figure 11.--Greater resistance to the weather is provided
by building the exposed upper section of a chimney with 8-inch walls.
Also the mortar joint, in which the counter-flashing is embedded, is not
so likely to fail as it is when the wall is only 4 inches thick.]

Gas-fired house heaters and built-in unit heaters, if not connected to a
masonry chimney, may be connected to flues of corrosion-resistant sheet
metal not lighter than 20-gage, properly insulated with asbestos or other
fireproofing material that will comply with the recommendations of the
Underwriter's Laboratories, Inc. Such flues should extend through the

A soot pocket[2] is desirable for each flue. Deep pockets permit the
accumulation of soot, which may take fire; therefore start them from a
point preferably not more than 8 inches below the center line of the
smoke pipe intake and fill the lower part of the chimney with solid
masonry instead of extending the pocket to the base of the chimney as is
often done. Clean-out doors are necessary at the bottom of deep pockets
and, if used, must fit snugly and be kept tightly closed so that air
cannot get in. Clean-outs should serve only one flue, for if two or
more flues are connected with the same clean-out, air drawn from one to
another affects the draft in all of them. Sometimes a door is placed just
below the smoke pipe, but one is not really necessary since the pipe, if
taken down each year for cleaning, allows removal of soot from shallow
pockets through the pipe hole.

[2] See soot pockets and clean-out doors in figures 12 and 35, pp. 13 and
38, respectively.

Close pipe holes, when temporarily not in use, with tight-fitting metal
flue stops; but, if a pipe hole is to be abandoned, fill it with bricks
laid in good mortar. This stopping can be readily removed. The practice
of closing a pipe hole with papered tin is dangerous, for if there is
another stove connected with the flue, the metal may become hot enough to
scorch the unprotected wallpaper or even set it afire.

Proper care in setting and looking after pipe at its connection with the
chimney will greatly lessen the number of fires chargeable to defective
construction. Fit the pipe so that no opening will be left around it,
and keep it from projecting into the flue. The connection can be made
airtight with a closely fitting collar and boiler putty, good cement
mortar, or stiff clay.

Smoke pipes should enter the chimney horizontally, and the hole through
the chimney wall to the flue should be lined with fire-clay, or metal
thimbles should be securely and tightly built in the masonry. Thimbles
or flue rings can be had of 6-, 7-, 8-, 10-, and 12-inch diameters and
6-, 9-, and 12-inch lengths. If the walls are furred (fig. 12), the space
between the thimbles and the wood furring should be covered with metal
lath and plaster.

[Illustration: Figure 12.--_A_, Connection to chimney where furring is
used. The brick are built out around the thimble as a protection against
its cracking. This is a fire hazard that is frequently overlooked. _B_,
Connection when plaster is applied directly to the masonry. Note that the
pipe extends too far into the flue. It should be as shown in _A_.]

When a smoke pipe is less than 18 inches from woodwork, the woodwork
requires protection against charring. A metal casing or asbestos board 2
inches from the upper half of the pipe is sometimes employed to protect
woodwork directly above it. A pipe, even so protected, should never be
closer than 9 inches to any woodwork or other combustible material.
Commercial fireproof pipe coverings can be purchased.

If a pipe must be carried through a wood partition, protection for the
woodwork can be provided by cutting an opening in the partition and
inserting a galvanized-iron double-wall ventilating shield at least 12
inches larger than the pipe (fig. 13) or by using at least 4 inches of
brickwork or other incombustible material. Smoke pipes should never pass
through floors, closets, or concealed spaces or enter a chimney in a

Gases formed by burning the sulfur contained in coal are the main cause
of corrosion of metal smoke pipes. Little corrosion occurs during the
heating season, when the pipe is kept hot and dry.

The life of metal pipes can be prolonged if each summer when they are not
in use they are taken down, cleaned, wrapped in paper, and stored in a
dry place. This is especially true of pipe to heaters in damp cellars.

[Illustration: Figure 13.--_A_, Elevation of protection around a
stovepipe passing through a frame partition; _B_, sectional view.]

[Illustration: Figure 14.--Method of insulating wood floor joists and
baseboard at a chimney with 4-inch walls. A single header is used as it
is less than 4 feet long.]


No wood should be in contact with a chimney. Leave a space of 2 inches
between the outside face of a chimney and all wooden beams or joists
except when 8 inches of masonry is used outside flue lining, in which
case the framing may be within one-half inch of the chimney masonry.
The space between the floor framing and the chimney may be filled with
porous, nonmetallic, incombustible material, such as loose cinders.
Brickwork, mortar, and concrete are not suitable. Place the filling
before the floor is laid, as it not only forms a fire stop but prevents
accumulation of shavings or other combustible material. Subflooring may
be laid within one-half inch of the masonry. Baseboards, when fastened to
plaster that is directly in contact with the wall of a chimney, can be
protected by a layer of fireproof material, such as asbestos, at least
one-eighth of an inch thick between the woodwork and the plaster (fig.

Wooden studding, furring, or lathing should not be placed against a
chimney but set back, as indicated in figure 9; or the plaster may be
applied directly to the masonry or to metal lath laid over the masonry.
The former is the better method, as settlement will not crack the
plaster. It is recommended that a coat of cement plaster be applied
directly upon the outside surfaces of masonry chimneys that are to be
incased by a wooden partition or other combustible construction. Metal
lath, lapped 6 inches on the masonry, at the intersection of chimneys
with partitions prevents corner cracks. (See plan in fig. 34.)

_Chimney and Roof Connection_

Where the chimney passes through the roof, provide a 2-inch clearance
between the wood framing and masonry for fire protection and for
expansion due to temperature changes, settlement, or slight movement of
the chimney during heavy winds.

[Illustration: Figure 15.--Method of flashing. Sheet metal, _h_, over
the cricket, extends under the shingles _k_, at least 4 inches and is
counter-flashed at _l_ in joint. Base flashings _b_, _c_, _d_, and _e_
and cap flashings _a_, _f_, and _g_ lap over the base flashings and
provide watertight construction. A full bed of mortar should be provided
where cap flashing is inserted in joints.]

A chimney must be flashed and counter-flashed (fig. 15),[3] to make
its junction with the roof watertight. When the chimney is not located
on the ridge but on a sloping roof, a cricket, _j_, is built, as
detailed in figure 16, high enough to shed water around the chimney.
Corrosion-resistant metal, such as copper, galvanized metal, zinc, or
lead, is best for the flashing and counter-flashing. When tin is used,
paint it well on both sides.

[3] See p. 26, Farmers' Bulletin 1751, Roof Coverings for Farm Buildings
and Their Repair, for method of installing flashing.

A feature, said to have originated in colonial Williamsburg as a
precaution against fire hazard, is to build the upper section of outside
chimneys 18 inches to 2 feet away from the gable ends of the house (fig.
17). This is not only a safety factor but a practical one because the
chimney can be more easily flashed, small windows can be used in the
walls of upper story rooms behind the chimney, and framing the roof is

_Capping the Chimney_

Various methods of terminating chimneys are shown in figures 11 and 18.
Whatever one is used should be architecturally acceptable, effective in
preventing disintegration, and so made as to keep water out of the flue.

[Illustration: Figure 16.--Cricket, _j_, as seen from the back of the
chimney shown in figure 15. A section through the cricket is also shown.
Note how counter-flashing is built into the mortar joint at _l_.]

It is advisable to project the flue lining 4 inches above the cap or top
course of brick and surround it with at least 2 inches of cement mortar
finished with a straight or concave slope to direct air currents upward
at the top of the flue; the sloped mortar also serves to drain water
from the top of the chimney. (See fig. 11.) Hoods are commonly used to
keep rain out of a chimney (fig. 18, _A_ and _B_). The area of the hood
openings should be at least equal to the area of the flue and each flue
should have a separate hood. Concrete and brick caps are usually made 4
inches thick, and it is advisable to project them an inch or two to form
a drip ledge.

Many of the chimneys built today are unsightly and frequently detract
from an otherwise well-designed house. Within the last 100 years the
size and attractiveness of chimneys ordinarily built has declined. The
large old chimneys of colonial days were proportioned to suit the house
and surroundings and at the same time provide for two or more large
fireplaces. With reduction in the size of fireplaces and the substitution
of several stoves and eventually one central heating plant, the chimney
has developed into a merely utilitarian shaft.

[Illustration: Figure 17.--A house in southern Maryland in which the
space between the chimney and the house wall shows clearly. The practice
of building the chimney in this way is common in the tidewater section of
the South.]

Spark Arresters

Spark arresters are desirable and, where chimneys are near combustible
roofs, lumber, forests, etc., they are sometimes required, depending on
the kind of fuel, waste materials, or refuse that may be burned and the
amount of deposits that may accumulate in the flues. While arresters
cannot be depended on to eliminate entirely the discharge of sparks under
all conditions; yet, when properly built and installed, they materially
reduce spark hazard.[4]

[4] See Standards for Construction and Installation of Spark Arresters
for Chimneys and Stacks, published by the National Fire Protection

In general all parts, whether of wire, expanded metal, or perforated
sheets, give longer service if they are of rust-resistant material.
Arresters for domestic purposes should have vertical sides extending
upward not less than 9 inches so as to provide a gross area of surface at
least twice the net flue area. They should be kept outside of the flue
area and be securely anchored to the chimney top.

Openings in the screen not larger than five-eights of an inch nor smaller
than five-sixteenths of an inch are advisable. Commercially made screens
can be purchased which generally last several years. Arresters must be
kept adjusted in position and renewed when the openings are worn larger
than the normal screen openings.

[Illustration: Figure 18.--_A_, A common type of arched hood; _B_, flat
stone hood; note the withe separating the two flues.]


The number of standard-size brick (8 by 3¾ by 2¼ inches) required
to build a straight chimney having only two or three flues can be
estimated by drawing the flue lining to scale and then drawing lines 4
inches to 8 inches outside of the lining depending on the thickness of
the brick walls. Lay out 4- by 8-inch rectangles in the space between the
lining and the outside lines to determine how many brick are needed per

For example, 15½ brick are needed for each course of the chimney in
figure 9. Assuming the height is 30 feet and one-half-inch mortar joints
are used, also that there are 4½ courses per foot, there would be 135
courses. Therefore, 135 multiplied by 15½ equals 2,092 brick; about
100 more will be needed to make the lower portion solid, or 2,200 brick
in all.

A more general method of estimating that is applicable to more complex
structures is given on page 43. Methods of determining the quantity of
mortar materials, labor, and cost are also given and can be applied to
this example.


Every flue should be subjected to the following smoke test before the
heater is connected with it and preferably before the chimney has been
furred and plastered or otherwise enclosed. Build a paper, straw, wood,
or tar-paper fire at the base of the flue. When the smoke is rising in a
dense column, tightly block the outlet at the top of the chimney with
a wet blanket. Smoke that escapes through the masonry indicates the
location of leaks. Frequently this test reveals bad leaks into adjoining
flues or directly through the walls or between the linings and the wall.
Remedy defects before the chimney is accepted for use. Such defects are
usually difficult to correct; hence it is wise to watch the construction
closely as it progresses.


Chimneys develop defects which if not promptly repaired cause trouble.
Most masonry requires replacement of worn or weathered material or
repointing of mortar joints, while flues become clogged and flashings

It is advisable to test a chimney every few years for tightness by the
smoke test just described; to examine the inside of the flues by lowering
a lantern or flashlight on a strong cord down from the top of the chimney
or by holding a hand mirror at the proper angle at a stovepipe hole; to
inspect the masonry for loose units, which are most likely to occur at
the top (fig. 3) where the action of the flue gases, especially when soft
coal is burned, disintegrates the mortar; to test mortar joints from the
outside by prodding with a knife or similar tool to determine if the
mortar is loose clear through the joint so as to leave a hole; and to
notice if the chimney is damp because of leaky flashings, absorption of
moisture from the ground, condensation, or excessive rain entering the


Bricks that fall from the top and lodge at offsets or contracted sections
can sometimes be reached and dislodged by a long pole or sections of
pipe screwed together. They can be caught on a shingle or piece of sheet
metal shoved into a stovepipe hole or removed through a clean-out door. A
weighted cement sack filled with straw and attached to the end of a rope
may be pulled up and down the flue to remove soot and loose material if
the offset is not too great.

Trouble with creosote and soot can be reduced when one understands how
they are formed. Smoke and soot are caused by imperfect combustion,
usually due to one or all of the following conditions: (1) Lack of
sufficient air to the fire; (2) improper mixture of air with furnace
gases; (3) low furnace temperature; (4) too small combustion space so
that the gases reach the comparatively cool furnace surface before they
are completely burned and, as a result, soot or tarry matter condenses
and then passes up the chimney in the form of smoke. Soft coal causes
more soot trouble than hard coal.

If soot accumulates fast or trouble is experienced with unusual smoke
when firing, it is probable that the heating equipment is not being
operated properly. The manufacturer or installer usually is able to
suggest proper adjustments.

Investigations by the United States Bureau of Mines[5] have shown that
various materials on being burned or volatilized form a vapor or smoke
which settles upon soot; causing it to ignite at a lower temperature and
burn more easily. For soot to burn, the gases in contact with it must
have a temperature high enough to ignite it and sufficient air to support
the combustion. The effectiveness of burning varies with the composition
of the remover, but it also depends upon conditions being favorable. It
will usually reduce somewhat the soot in a furnace and smoke pipe but not
in a chimney. It has no effect on the ash mixed with the soot. This ash
not only does not burn, but prevents complete burning of the soot mixed
with it.

[5] Nichols, P., and Staples, C. W. REMOVAL OR SOOT FROM FURNACES AND
pp., illus. 1932.

Soot removers cause soot to burn and are fire hazards. The correct and
most thorough method of cleaning a chimney is to do so manually or to
employ modern exhaust or vacuum methods used by furnace repairmen.
However, it is inconvenient to remove soot and ash accumulations
thoroughly more than once a year; hence a remover may help to keep
the passages of stoves and heaters clear between annual cleanings, if
deposits of soot accumulate quickly and reduce the draft.

Likelihood of success in cleaning is greater when the deposits of soot
are thick, provided they do not cut down the draft too much. If burning
is employed, there is less risk when it is done frequently enough to
prevent large accumulations, which cause intense fires. Also, freeing
the heater and pipe of soot permits better fuel burning and higher
temperatures in the chimney flue, thus reducing the amount of soot likely
to be deposited on the flue walls.

Common salt (rock or ice-cream salt) is not the most effective remover,
yet it is the most widely used because of its cheapness, ease of
handling, and general availability. Use two or three teacupfuls per
application. Metallic zinc in the form of dust or small granules is
often used; however, a mixture of salt and 10 percent zinc dust is more
effective than either salt or zinc alone.

One of the most effective mixtures of materials readily available
is 1 part dry red lead and 5 parts common salt, measured by weight.
Shake these together in a can with a tight-fitting lid. As lead is
poisonous, wash the hands after using. One or two teacupfuls are used per

Old dry-cell batteries contain suitable ingredients and when they are
thrown in a hot furnace the soot usually burns. Quicker action can be had
if they are chopped up.

Before a remover is used, the fire must be put in good condition with a
substantial body of hot fuel on top. Close the ash-pit door and the slots
in the firing door and scatter the remover on the hot coals. Close the
firing doors and at once reduce the draft by partially closing the pipe
dampers. The draft should not be closed so tight as to cause fumes to
escape into the cellar. Let the remover "stew" for 10 to 20 minutes or
until fumes stop rising from the coals; then make the fire burn fiercely
by opening the ash-pit door and the damper. Shaking ashes out will help.
The slots in the firing door can be opened or the door itself set ajar.
If soot in the furnace will not ignite, throw a little wood or paper on
the fire.

Instead of making a special job of cleaning at intervals, one or two cups
of salt may be thrown on the fire once a day with the expectation that
the furnace will produce a high enough temperature to ignite some of the
soot. This is most likely to succeed in cold weather when the furnace
temperatures are high.

Cause of Creosote

Creosote is the result of condensation in the chimney, and trouble from
this source is best avoided by preventing creosote formation. It is more
likely to form when wood is used for fuel than when coal is burned and
is more likely to form in cold than in mild climates. Green wood may
contain as high as 40 percent water, and dry wood 15 to 20 percent. When
wood is slowly burned, it gives off acetic and pyroligneous acid, which
in combination with water or moisture form creosote. When the draft is
strong and an active fire is maintained, much of the creosote is carried
off into the atmosphere. The trouble is aggravated when the fire does not
burn briskly and when an outside flue is subjected to chilling blasts.
The walls of the chimney, being comparatively cool, cause condensation
of the vapors contained in the smoke. Thus the creosote condenses and
runs down the flue, finding its way out of any joints that are not
perfectly tight. The formation of creosote is unusual in chimneys that
are surrounded by warm rooms. The outer walls of a chimney in an outside
wall should be at least two bricks thick and the chimney should have a
good flue lining.

Creosote is difficult to remove and when it ignites makes a very hot fire
that is likely to crack the masonry and char adjacent timbers. The only
safe method of removal is to chip it from the masonry with a blade or
straightened-out hoe attached to a pipe or handle. A heavy chain drawn up
and down the flue walls is sometimes effective. However, when creosote is
removed, care is necessary not to knock out mortar joints or to break the
flue lining.

Large quantities of salt thrown on the fire in the grate or fireplace
will extinguish a chimney fire. A fire in a fireplace flue can be checked
in its intensity and frequently extinguished by first quenching the fire
on the hearth and then holding a wet rug or blanket over the opening so
as to shut off the air. When this is done, the soot and creosote are
likely to slide from the flue walls and drop into the fireplace. Before
extinguishing a fire in a flue, cover openings into the rooms, so that
the soot will not spread over furnishings.

Repairing Chimneys

When a chimney is damp, examine the flashing at the junction with the
roof, especially if wet spots appear on the ceilings of rooms. Methods of
repairing flashing are given in Farmers' Bulletin 1751, Roof Coverings
for Farm Buildings and Their Repair. If the flashing is sound, possibly
water runs down the inside of the flue and through defective mortar
joints. Where these cannot be reached readily, the chimney may have to be
torn down and rebuilt. Sometimes a hood (fig. 18, _A_ and _B_) is built
on top of the chimney to keep out water or to prevent wind blowing down
it. To prevent dampness being drawn up from the ground, the mortar can
be raked from a joint at least 12 inches above the ground and a layer of
slate, asbestos shingles, or rust-resistant sheet metal and new mortar
worked into the joint. This work should be done by a mason. If bricks
are porous or eroded, raking out the mortar one-half of an inch deep and
applying three-fourths of an inch of cement plaster to the surfaces is
effective. Eroded joints in the rest of the masonry should be raked and
repointed. Where natural gas is burned, dampness due to condensation
is not unusual and a drain may be needed. Where such conditions exist,
advice should be sought from the manufacturers of the equipment as to the
proper remedy.

A chimney that becomes too hot to permit holding the hand against
it should be carefully inspected by a reliable mason and adequately
protected as suggested in the preceding pages.

If, after a chimney is cleaned, an examination discloses holes, unfilled
joints, or other unsound conditions out of reach for repair, it is
advisable to tear the masonry down and rebuild properly. Inside bricks
that are impregnated with creosote and soot should not be used in the
new work because they will stain plaster whenever dampness occurs. It
is almost impossible to remove creosote and soot stains on plaster and
wallpaper. Sometimes painting the plaster with aluminum-flake paint or
waterproof varnish hides the stains.

A hatchway cut through a roof is convenient when high chimneys are
repaired or cleaned, especially when access to the roof is difficult. The
hatchway should be located so that it will not be necessary to crawl over
the roof to reach the chimney and so that a ladder placed on the attic
floor will not be too steep for safe ascent. A watertight cover with
hooks to prevent its blowing off is essential. Such a hatchway is best
provided when the building is erected but can be readily built at any


A fireplace is ordinarily considered appropriate to a living room, dining
room, and bedroom; however, basement, porch, and outdoor fireplaces
are gaining in favor with the householder. Also public dining places,
offices, etc., frequently have fireplaces for the comfort and for the air
of informality they provide.

All fireplaces should be built in accordance with the few simple
essentials of correct design given herein if satisfactory performance is
to be realized. They should be of a size best suited to the room in which
they are used from the standpoint of appearance and operation. If too
small, they may function properly but do not throw out sufficient heat.
If they are too large, a fire that would fill the combustion chamber
would be entirely too hot for the room and would waste fuel.

The location of the chimney determines the location of the fireplace and
too often is governed by structural considerations only. A fireplace
suggests a fireside group and a reasonable degree of seclusion, and
therefore, especially in the living room, it should not be near doors to
passageways of the house.


The principal warming effect of a fireplace is produced by the radiant
heat from the fire and from the hot back, sides, and hearth. In the
ordinary fireplace practically no heating effect is produced by
convection, that is, by air current. Air passes through the fire and up
the chimney, carrying with it the heat absorbed from the fire; at the
same time outside air of a lower temperature is drawn into the room.
The effect of the cold air thus brought into the room is particularly
noticeable farthest from the fire. Heat radiation, like light, travels
in straight lines, and unless one is within range of such radiation,
little heat is felt. Tests made by the Bureau of Agricultural Chemistry
and Engineering showed that about five times the amount of air required
for even liberal ventilation may be drawn into a living room by the
operation of a fireplace. Such excessive ventilation may cause chilling
drafts. Persons located at advantageous points in the room will be
comfortable under such conditions, but those out of the radiation zone
will not.

[Illustration: Figure 19.--In 1744 Franklin promoted a metal fireplace of
this type to be set out into a room. These are known as Franklin stoves
and sometimes are equipped with andirons for burning wood or a grate for
burning coal. The metal blower, shown in front of the opening was used
with grates and set in place when starting the draft and then removed so
that the cheery heat of glowing coals could be enjoyed. At one time this
type of stove was highly thought of because it threw out more heat than
the built-in fireplace. A few manufacturers specialize in Franklin stoves
because of the present-day demand.]

Tests conducted by this Bureau indicate that, as ordinarily constructed,
a fireplace is only about one-third as efficient as a good stove or
circulator heater. Nevertheless, they have a place as an auxiliary to the
heating plant and for their cheerfulness and charm. In milder climates,
fireplaces may suffice as the sole source of heat; also certain materials
often wasted may be utilized for fuel. The disadvantages of the ordinary
fireplace are lessened by "modified" fireplaces.


The Franklin stove (fig. 19) is a type of modified fireplace.

The modified fireplaces of today are of several types, as shown in
figures 20 and 21.

Both the last two types of modified fireplaces are manufactured as units
of heavy metal, designed to be set into place and concealed by the usual
brickwork, or other construction, so that no practical change in mantel
design is required by their use. The modifications are built-in standard
parts of the fireplace--only the grilles show (fig. 22).

[Illustration: Figure 20.--In this modified fireplace air enters the
inlet, _a_, from outside and is heated as it rises by natural circulation
through the back chamber, _c_, and the tubes, _t_, being discharged into
the room from the register, _b_. Air for supporting combustion is drawn
into the fire at _d_ and passes between the tubes up the flue A damper is
also provided to close the air inlet.]

One advantage claimed for modified fireplace units is that the correctly
designed and proportioned firebox, manufactured with throat, damper,
smoke shelf, and chamber, provides a form for the masonry, thus reducing
the risk of failure and assuring a smokeless fireplace! However, there is
no excuse for using incorrect proportions; and the desirability of using
a foolproof form, as provided by the modified unit, merely to obtain good
proportions should be considered from the standpoint of cost. Even though
the unit is well designed, it will not operate properly if the chimney is
inadequate; therefore the rules for correct chimney construction must be
adhered to with the modified unit as well as with the ordinary fireplace.

Manufacturers claim labor and materials saved tend to offset the purchase
price of the unit; also that the saving in fuel justifies any net
increase in first cost. A minimum life of 20 years is claimed for the
type and thickness of metal commonly used today in these units.

Field tests made by this Bureau have proved that, when properly
installed, the better designs of modified-fireplace units circulate heat
into the cold corners of rooms and will deliver heated air through ducts
to adjoining or upper rooms. For example, heat could be diverted to a
bathroom from a living-room fireplace.

[Illustration: Figure 21.--In this fireplace the air is not drawn in
directly from outdoors but through the inlet, _a_, from the room that is
being heated. The air is heated by contact with the metal sides and back
of the fireplace, rises by natural circulation, and is discharged back
into the room from the outlet, _b_, or to another room on the same floor
or in the second story. The inlets and outlets are connected to registers
which may be located at the front of the fireplace, as shown in figure
22. The registers may be located on the ends of the fireplace or on the
wall of an adjacent room.]

The quantity and temperature of the heated air discharged from the
grilles in figures 20 and 21 were measured to determine the merits of
the convection features. These measurements showed that very appreciable
amounts of convected heat are produced by the modified unit when
properly installed and operated. Discharge-air temperatures in excess
of 200° F. were attained from some of the units tested. The heated air
delivered from the discharge grilles of some of the medium-sized units
represented a heating effect equivalent to that from nearly 40 square
feet of cast-iron radiation of the ordinary hot-water heating system, or
sufficient to heat a 15- by 18-foot room built with average tightness to
70° F. when the outside temperature is 40° F. Additional convected heat
can be produced with some models by the use of forced-circulation fans.

[Illustration: Figure 22.--Except for the registers and metallic sides
and back, the appearance of modified fireplaces is like that of ordinary
ones. An interesting effect is secured by the mirror--the reflection of
the opposite wall appears like a recess over the mantel.]

However, the nature of operation, with the unavoidably large quantity of
heated air passing up the stack, makes the inherent over-all efficiency
of any fireplace relatively low. Therefore, claims for an increased
efficiency of modified fireplaces should be understood merely as
constituting an improvement over the ordinary fireplace and not over
stoves or central heating plants.

When a fireplace is being selected the kind of fuel to be burned should
be considered; also, the design should harmonize with the room in
proportion and detail (figs. 23 and 24).

[Illustration: Figure 23.--A well-designed commercial mantel that suits
the room. Since it is painted the same color as the walls, it does not
focus attention, as the handsomely carved formal mantel or mahogany shown
in figure 37 is intended to do.]

In colonial days, when cordwood was plentiful, fireplaces 7 feet wide and
5 feet high were common, especially when used in kitchens for cooking
(fig. 25). They required large amounts of fuel and too frequently were

Where cordwood (4 feet long) is cut in half, a 30-inch width is desirable
for a fireplace; but, where coal is burned, the opening can be narrower
(fig. 26). Thirty inches is a practical height for the convenient tending
of a fire where the width is less than 6 feet; openings about 30 inches
wide (fig. 27) are generally made with square corners. The higher the
opening, the greater the chance of a smoky fireplace.

[Illustration: Figure 24.--Another good design is this revival of early
New England architecture, which is frequently used for remodeling public
dining rooms. The random-width pine planks were selected especially for
variety in the pattern of the knots. Note the use of otherwise wasted
space for bookshelves and closet.]

[Illustration: Figure 25.--_A_, A fireplace at Mount Vernon, Washington's
home, typical of those used before cooking stoves were introduced.
This type of fireplace, if not too large, is often retained (_B_) when
a kitchen is remodeled into a living room. Note the Dutch oven at the
right, formerly used for baking.]

[Illustration: Figure 26.--Fireplaces originally intended for wood were
frequently bricked up, and small cast-iron units of this type were built
in, since the large openings required for wood were wasteful when coal
was used. This was a very popular type of grate for hotel and private
bedrooms about 1860 and can still be seen in old houses in coal regions.
Note the plain and neat mantel of wide plank.]

In general, the wider the opening the greater should be the depth. A
shallow opening throws out relatively more heat than a deep one of the
same width but accommodates smaller pieces of wood; thus it becomes a
question of preference between a greater depth which permits the use of
large logs that burn longer and a shallower depth (fig. 28, _A_ and _B_)
which takes smaller-sized wood but throws out more heat.

In small fireplaces a depth of 12 inches will permit good draft if the
throat is constructed as explained above, but a minimum depth of 16 to 18
inches is advised to lessen the danger of brands falling out on the floor.

As a rule, fireplaces on the second floor are smaller than those on the
first floor and it is well to follow this practice because the flue
height is less for second floor fireplaces (fig. 29).

Unless a fireplace 6 feet wide is fully 28 inches deep, the logs will
have to be split, and some advantage of the wide opening will be lost.

Screens of suitable design should be placed in front of all fireplaces
(fig. 30).

[Illustration: Figure 27.--This inexpensive fireplace 32 inches square
shows how a plain brick front can be used in a small room.]

A fireplace 30 to 36 inches wide is generally suitable for a room having
300 square feet of floor (fig. 31). The width should be increased for
larger rooms, but all other dimensions should be taken from table 3 for
the width selected.

The corner of a room often is the favorite location for a fireplace (fig.
32). Fireplaces of the type shown in figure 28 are also built in corners.

[Illustration: Figure 28.--A, A shallow fireplace, with a copper hood,
built as shown in B, throws out considerable heat after the hood gets
hot. The wall should be of fire-resistant masonry.]

[Illustration: Figure 29.--This shallow fireplace with a sloping back is
a type that was frequently built in bedrooms before the general use of
stoves. Note the neat and well-proportioned mantel.]

[Illustration: Figure 30.--Screens are almost essential to protect the
upholstery of nearby furniture from sparks. This fireplace shows artistic
use of small stones and makes a pleasing contrast with the log walls.]

Units providing for burning gas are often built in to resemble fireplaces
(fig. 33).

Pleasing designs result from exercising good taste in use of materials
and mantels that suit the room. The photographs in this bulletin have
been selected to illustrate various architectural effects that can be
developed and should help in the choice of a type suitable for houses of
different designs. The essentials for safety and utility, however, should
not be sacrificed for style.

[Illustration: Figure 31.--This 36-inch-wide fireplace does not seem too
large for the small room, but its size would have been accentuated by the
use of a mantel.]


The ordinary fireplace is constructed generally as shown in figure 34. It
is essential (1) that the flue have the proper area, (2) that the throat
be correctly constructed and have suitable damper, (3) that the chimney
be high enough for a good draft, (4) that the shape of the fireplace be
such as to direct a maximum amount of radiated heat into the room, and
(5) that a properly constructed smoke chamber be provided.

[Illustration: Figure 32.--An adobe fireplace of the Mexican-Indian type
commonly built in the Southwestern States, especially when the house
walls are of adobe. The logs are stood up, leaning against the back of
the grate, in order to secure a high-licking flame.]


Table 3 gives recommended dimensions for fireplaces of various widths and

If a damper is installed, the width of the opening j, figure 34, will
depend on the width of the damper frame, the size of which is fixed by
the width and depth of the fireplace and the slope of the back wall. The
width of the throat proper is determined by the opening of the hinged
damper cover. The full damper opening should never be less than the flue
area. Responsible manufacturers of fireplace equipment give valuable
assistance in the selection of a suitable damper for a given fireplace. A
well-designed and well-installed damper should be regarded as essential
in cold climates.

When no damper is used, the throat opening j should be 4 inches for
fireplaces not exceeding 4 feet in height.

Table 3.--_Recommended dimensions for finished fireplaces_

[Letters at heads of columns refer to figure 34]

  Table Key

  _w_  Opening Width
  _h_    "  Height
  _d_  Depth
  _c_  Minimum back (horizontal)
  _a_  Vertical back wall
  _b_  Inclined  "    "
  _o_  Outside dimensions of standard flue lining
  _i_  Inside diameter of standard round flue lining

   _w_     _h_     _d_      _c_     _a_     _b_     _o_     _i_
  Inches  Inches  Inches   Inches  Inches  Inches  Inches   Inches

    24      24     16-18    14      14      16    8½ by 8½    10
    28      24     16-18    14      14      16    8½ by 8½    10
    24      28     16-18    14      14      20    8½ by 8½    10
    30      28     16-18    16      14      20    8½ by 13    10
    36      28     16-18    22      14      20    8½ by 13    12
    42      28     16-18    28      14      20    8½ by 18    12
    36      32     18-20    20      14      24    8½ by 18    12
    42      32     18-20    26      14      24    13 by 11    12
    48      32     18-20    32      11      24    13 by 13    15
    42      36     18-20    26      11      28    13 by 13    15
    48      36     18-20    32      14      28    13 by 18    15
    54      36     18-20    38      14      28    13 by 18    15
    60      36     18-20    44      14      28    13 by 18    15
    42      40     20-22    24      17      29    13 by 13    15
    48      40     20-22    30      17      29    13 by 18    15
    54      40     20-22    36      17      29    13 by 18    15
    60      40     20-22    42      17      29    18 by 18    18
    66      40     20-22    48      17      29    18 by 13    18
    72      40     22-28    51      17      29    18 by 18    18

[Illustration: Figure 33.--In regions where natural gas is plentiful and
in cities, fireplaces of this type, burning gas with a flickering flame,
are frequently used as an auxiliary to the main heating plant. Some types
have imitation logs of metal perforated for gas jets.]


Footings for chimneys with fireplaces should be provided as described
on page 7; for chimneys without fireplaces, the footings should rest
on good firm soil.

[Illustration: Figure 34.--A typical fireplace, illustrating practical
details of construction. An alternate method of supporting the hearth
is shown in the lower right-hand corner. The various letters refer to
specific features discussed in the text.]


The hearth should be about flush with the floor, for sweepings may then
be brushed into the fireplace. When there is a basement, an ash dump
located in the hearth near the back of the fireplace is convenient. The
dump consists of a metal frame about 5 by 8 inches in size, with a plate,
generally pivoted, through which ashes can be dropped into a pit below
(fig. 35).

[Illustration: Figure 35.--The ash-pit should be of tight masonry and
should be provided with a tightly fitting iron clean-out door and frame
about 10 by 12 inches in size. A clean-out for the furnace flue as shown
is sometimes provided.]

In buildings with wooden floors the hearth in front of the fireplace
should be supported by masonry trimmer arches (fig. 34) or other
fire-resistant construction. Hearths should project at least 16 inches
from the chimney breast and should be of brick, stone, terra cotta, or
reinforced concrete not less than 4 inches thick. The length of the
hearth should be not less than the width of the fireplace opening plus 16
inches. Wooden centering under trimmer arches may be removed after the
mortar has set, though it is more frequently left in place. Figure 36
shows a recommended method of floor framing around a fireplace.

Wall Thickness

The walls of fireplaces should never be less than 8 inches thick, and
if of stone they should be at least 12 inches thick. When built of
stone or hard-burned brick, the back and sides are often not lined with
firebrick, but it is better to use firebrick laid in fire-clay. When
firebricks are laid fiat with the long sides exposed there is less danger
of their falling out. They are generally placed on edge, however,
forming a 2-inch protection, in which case metal ties should be built
into the main brickwork to hold the 2-inch firebrick veneer in place.
Thick metal backs and sides are sometimes used as lining. When a
grate for burning coal or coke is built in, firebrick at least 2 inches
thick should be added to the fireplace back unless the grate has a
solid iron back and is only set in with an air space behind it (fig. 37).


The jambs should be wide enough to give stability and a pleasing
appearance; they are frequently faced with ornamental brick or tile.
For an opening 3 feet wide or less, a 12- or 16-inch width is generally
sufficient, depending on whether a wood mantel is used or the jambs
are of exposed masonry. The edges of a wood mantel should be kept
at least 8 inches from the fireplace opening. For wider openings and
large rooms, similar proportions should be kept.

[Illustration: Figure 36.--Where a header is more than 4 feet in length,
it should be doubled, as shown. Headers supporting more than four tail
beams should have ends supported in metal joist hangers. The framing may
be placed one-half inch from the chimney because the masonry is 8 inches


Lintels of ½- by 3-inch flat iron bars. 3½- by 3¼- by ¼-inch
angle irons, or damper frames are used to support the masonry over the
opening of ordinary fireplaces. Heavier lintel irons are required for
wider openings.

Where a masonry arch (fig. 38) is used over the opening, the jambs
should be heavy enough to resist the thrust of the arch. Arches over
openings less than 4 feet wide seldom sag, but sagging is not uncommon
in wider fireplaces, especially where massive masonry is used.


The sides of the fireplace should be vertical up to the throat, or
damper opening (_ff_ fig. 34). The throat should be 6 to 8 inches or
more above the bottom of the lintel and have an area not less than
that of the flue and a length equal to the width of the fireplace opening.
Starting 5 inches above the throat, _ee_, the sides should be drawn in at
_tt_ to equal the flue area.

Proper throat construction is so necessary to a successful fireplace
that the work should be inspected several times a day during construction
to make certain that the side walls are carried up perpendicularly until
the throat is passed and that the full length of opening is provided.

Smoke Shelf and Chamber

The smoke shelf is made by setting the brickwork back at the top
of the throat to the line of the flue wall for the full length of the
throat. Its depth may vary from 6 to 12 inches or more, depending on
the depth, d, of the fireplace.

[Illustration: Figure 37.--Grates of this type are commonly used in
fireplaces for burning coal or coke. This one has a metal back and ends
and is only set in to permit proper circulation of air around it.]

The smoke chamber is the space extending from the top of the
throat, _ee_, up to the bottom of the flue proper, tt 9 and between the
side walls. The walls should be drawn inward 30° to the vertical after
the top of the throat, _ee_, is passed and smoothly plastered with
cement mortar not less than one-half inch thick.


A properly designed damper, as shown in figure 34, affords a means
of regulating the draft and prevents excessive loss of heat from the
room when the fire is out. A damper consists of a cast-iron frame
with a lid hinged so that the width of the throat opening may be
varied from a closed to a wide-open position. Various patterns are
on the market, some designed to support the masonry over the opening,
others requiring lintel irons.

[Illustration: Figure 38.--This well-designed small stone fireplace was
built in accordance with the principles given in this bulletin. It is a
good heater and does not smoke. The jambs are wide enough to resist the
thrust of the arch.]

A roaring pine fire may require a full-throat opening, but slow-burning
hardwood logs may need only 1 or 2 inches of opening. Regulating the
opening according to the kind of fire prevents waste of heat up the
chimney. Closing the damper in summer keeps flies, mosquitoes, and other
insects from entering the house down the chimney.

In houses heated by furnaces or other modern systems, lack of a
damper in the fireplace flue may interfere with uniform heating,
particularly in very cold windy weather, whether or not there is a fire
on the hearth. When air heated by the furnace is carried up the chimney
there is a waste of the furnace fuel, but a damper partially open serves
a slow fire of hardwood without smoking the room or wasting heated
air from the main heating system.

[Illustration: Figure 39.--Diagram showing front view and cross section
of an entire chimney such as is commonly built to serve a furnace,
fireplace, and kitchen stove. Two sets of dimensions are given, those in
rectangles refer to the approximate sizes of the voids or openings; the
others refer to the outside dimensions of the brickwork. These are used
in estimating the number of bricks in a chimney. The letters _A_-_F_
indicate sections used in estimating the quantities of brick required
(See p. 44.)]


The area of lined flues should be a twelfth or more of the fireplace
opening, provided the chimney is at least 22 feet in height, measured
from the hearth. If the flue is shorter than 22 feet or if it is unlined,
its area should be made a tenth or more of the fireplace opening. The
fireplace shown in figure 34 has an opening of 7.5 square feet, or
approximately 1,080 square inches, and needs a flue area of approximately
90 square inches; a rectangular flue, 8% by 18 inches, outside dimensions,
or a round flue with a 12-inch inside diameter might be used, as
these are the nearest commercial sizes of lining (table 2). It is seldom
possible to obtain lining having exactly the required area, but the inside
area should never be less than that prescribed above. A 13- by
13-inch flue was selected for convenience when combining with the
other flues. If the flue is built of brick and is unlined, its area should
be approximately one-tenth of the fireplace opening, or 108 square
inches. It would probably be made 8 by 16 inches (128 square inches)
because brickwork can be laid to better advantage when the dimensions
of the flue are multiples of 4 inches. The principles of construction
given under Chimneys (p. 7) apply to fireplace flues.

Table 4 is convenient in selecting the proper size of flue or for
determining the size of fireplace opening for an existing flue. The area
of the fireplace opening in square inches is obtained by multiplying the
width, _w_, by the height, _h_, (fig. 34), both measured in inches.


A convenient method for estimating the number of bricks in a chimney
is to calculate the volume of the various sections which differ in
outside dimensions and then subtract the voids or cavities resulting
from ash-pits, fireplace, and flues. This will be the total cubic feet of
brickwork which, when multiplied by 22.5, is converted to number of
bricks. For convenience, inches as indicated in figure 39 have been
converted to decimals of a foot.[6]

[6] Inches and fractions of an inch are converted to feet and decimals by
multiplying by 0.0833; thus 2 X / inches × O.0833 equals 0.208 feet.

Table 4.--_Sizes of fireplace flue linings_[D]

             dimensions      Inside
   Area of   of standard   diameter of
  fireplace  rectangular  standard round
   opening   flue lining   flue lining
  ---------  -----------  --------------
   inches_     _Inches_      _Inches_

      600      8½ by 8½        10
      800      8½ by 13        10
    1,000      8½ by 18        12
    1,200      8½ by 18        12
    1,400      13 by 13        12
    1,600      13 by 13        15
    1,800      13 by 18        15
    2,000      13 by 18        15
    2,200      13 by 18        15
    2,400      18 by 18        18
    2,600      18 by 18        18
    2,800      18 by 18        18
    3,000      18 by 18        18

[D] Based on a flue area equal to one-twelfth the fireplace opening. Sec
table 2 for areas of flue lining.

Number of Bricks

(1) Estimate the total volume of masonry by multiplying together the
length, width, and height of the various sections (fig. 39).

        _Length    _Width    _Height      _Volume
 _Section_ Feet_     Feet_     Feet_      Cubic feet_
   _AB_     6.0  by  2.75 by  12.66  =   209.0
   _BC_     4.25 by  2.5  by   1.66  =    17.6
   _CD_     3.5  by  2.0  by   2.0   =    14.0
   _DE_     3.5  by  1.75 by  10.16  =    62.2
   _EF_     4.33 by  2.5  by   6.0   =    65.0
        Total volume including voids  367. 8

(2) Estimate the total volume of voids by multiplying together their
length, width, and height.

                        _Length  _Width            _Height  _Volume
      _Item_             Feet_    Feet_             Feet_    Cubic feet_

  Ash-pit                 2.33 by 1.5              by  7.0  = 24 46
  Fireplace               3.0  by 1.5              by  3.5  = 15.75
  Smoke chamber           2.0  by 1.16             by  2.0  =  4.64
  8½- by 13-inch flue[E]          0.78 square feet by 28 5  = 22.23
  13- by 13-inch flue[E]          1.20 square feet by 18.75 = 22.50
  8½- by 8½-inch flue[E]           .50 square feet by 18.75 =  9.37
                                       Total volume of voids  98.95

[E] See table 2 for outside areas of flues in square feet.

(3) Subtract volume of voids from volume of masonry.

_Cubic feet_ Total volume, including voids 368 Total volume of voids 99
--- Total volume of masonry 269

(4) Multiply net volume of masonry by the number of brick per cubic foot.

  269 by 22.5 = 6,053 brick, or 6.1 thousand bricks.


To estimate the mortar needed, multiply the mortar material given below
for 1,000 brick by 6.1 to determine how much will be needed to build the
chimney, using 1:1:6 mixture recommended on page 10.

  Bags of hydrated lime       2.6 by 6.1 = 16 bags.
  Sacks of portland cement    3.5 by 6.1 = 22 sacks.
  Cubic feet of sand         18.0 by 6.1 = 110 cubic feet = 4 cubic yards.


Concrete needed for foundation can be estimated as follows: Concrete for
foundation should be 1:2½:5 and for the top 1:2½. The foundation
is 7 by 3.75 by 1, or 26.25 cubic feet, or 1 cubic yard, and will
require 5 sacks of cement, 0.46 cubic yard of sand, and 92 cubic yard
of gravel. The cap is 4.5 by 2.66 by 0.5 = 5.9 cubic feet The area of
the three flues above must be deducted: 5.9 minus 2.48 = 3.42 cubic
feet, or one-ninth of a cubic yard. As 1 cubic yard was assumed for the
foundation, extra cement and sand are not needed. Other material needed:

  1 8-inch thimble, 9 inches long.
  1 6-inch thimble, 9 inches long.
  28 feet of 8½- by 13-inch flue lining.
  20 feet of 13- by 13-inch flue lining.
  20 feet of 8½- by 8½-inch flue lining.
  Damper, 36- by 10-inch throat opening.
  2 clean-out doors and 1 ash dump.
  Mantel as selected.

If firebrick is to be used or the exposed breast is to be of face or
special brick (or ceramic tile) the number should be counted or estimated
and deducted from the number of common brick as estimated above.


The labor required to build a chimney depends on the thickness of the
walls, the height, and the amount of cutting to build in specialties,
provide offsets, etc. In general, a mason will take 16 hours with 8 hours
of laborer's help to lay 1,000 brick. On this basis, 16 by 6.1 = 97.6
hours of mason's time and 48.8 hours of laborer's time will be required.


The approximate cost of the chimney can be determined by using actual
local cost of materials and wages as follows:[F]

  6,100 brick at $15.00 per thousand                          $91.50
  27 sacks of cement[G] at $0.70 per sack                      18.90
  16 bags of lime at $0.50 per bag                              8.00
  5 cubic yards of sand[G] at $2.25 per cubic yard             11.25
  1 cubic yard of gravel at $2.00 per cubic yard                2.00
  98 hours, mason's time, at $1.00 per hour                    98.00
  49 hours, laborer's time,[H] at $0.30 per hour               14.70
  28 linear feet of 8½- by 13-inch flue at $1.00 per foot      28.00
  20 linear feet of 13- by 13-inch flue at $1.15 per foot      23.00
  20 linear feet of 8½- by 8½-inch flue at $0.40 per foot       8.00
  1 8-inch thimble                                               .60
  1 6-inch thimble                                               .40
  2 clean-out doors              }
  Damper, lintel mantel, ash dump}                             65.00
                                      Total net cost     [I] 369.35

[F] The prices used in this example are merely illustrative.

[G] Includes material for footing and cap.

[H] Includes labor for footing and cap.

[I] Where the chimney is built by contract, 10 to 15 percent should be
added for profit and overhead.


When a fireplace smokes, it should be examined to make certain that the
essential requirements of construction as outlined in this bulletin
have been fulfilled. If the chimney is not stopped up with fallen brick
and the mortar joints are not loose, note whether nearby trees or tall
structures cause eddies down the flue. To determine whether the fireplace
opening is in correct proportion to the flue area, hold a piece of sheet
metal across the top of the fireplace opening and then gradually lower
it, making the opening smaller until smoke does not come into the room.
Mark at the lower edge of the metal on the sides of the fireplace. The
opening may then be reduced by building in a metal shield or hood across
the top so that its lower edge is at the marks made during the test; or
the trouble can generally be remedied by increasing the height of the


Outdoor fireplaces range from simple makeshifts to elaborately equipped
structures harmonizing with the architecture of the house.

No one type will meet all conditions, but all types should be practical
to use and yet not be fire hazards or eyesores.

[Illustration: Figure 40.--A, A fireplace built for 30 cents, cash. One
hundred and twenty bricks and six concrete blocks were picked up a few
at a time along the road. One sack of cement was purchased, one-half
of which was used for another job Sand was available on the site. B f
Detailed drawings show dimensions of this fireplace. As the fireplace is
ordinarily built, the material would cost about $5 and the labor from $5
to $10, depending on local conditions.]


The tendency is to build too large an outdoor fireplace. Where only
a little cooking is to be done occasionally in a small yard or at a
picnic, several concrete blocks or stones set on the ground about 12
to 16 inches apart will serve. The shelf of an old refrigerator may be
used for a grille. If permanence is desired, the walls should be laid in
cement mortar and the fireplace should have a suitable foundation and a
permanent grille. An end wall is recommended to prevent embers from being
scattered by drafts blowing between the side walls. Smoke annoyance while
cooking is lessened by making the fireplace long enough to permit a short
chimney (fig. 40).

[Illustration: Figure 41.--An outdoor fireplace built back of an inside
fireplace and opening onto a paved terrace provides comfort in early

A circle of stones laid loosely on the surface, larger stones set partly
into the ground, or carefully laid masonry walls on a stable foundation
may be used for campfires and small barbecue parties. A cast-iron pot
with a lid can be buried in the ashes for baking. Pipe supports for pots
and pans built into the masonry are a convenience; they can be homemade
or purchased. Spits for roasting can be improvised or bought.

Fireplaces opening onto an enclosed porch or paved terrace, are often
built as an integral part of the house chimney (fig. 41). The corner of
boundary walls permits effective treatment. Such fireplaces should meet
the regulations of local fire authorities and be built with the same care
and be subject to the same rules as inside fireplaces.


Plans for outdoor fireplaces are available from various publishing
houses; several magazines feature illustrations that can be adapted to
the material at hand. If a structure is to be built with local labor and
material, simple designs are advisable. The size of stones, joints, and
proportions have a direct influence upon appearance, and good personal
taste frequently results in more pleasing structures than blind adherence
to conventional designs. The various combinations of ovens, cranes,
grilles, storage compartments, benches, lights, sinks, etc., to be used
as built-in features affect the design. Before planning a structure with
these features, catalogs of dealers in outdoor fireplace equipment should
be consulted for sizes of the available accessories so that ample space
and proper details can be provided in the masonry for building them in.
Skilled labor should be employed for elaborate designs (fig. 42) when
much equipment is built in or when the fireplace, as in figure 41, is an
integral part of a permanent building.

[Illustration: Figure 42.--This fireplace, set at a focal point in the
garden, enhances the landscape. It was built by a skilled mason.]


Ordinarily the fire is built on the hearth, no grate being used. Fire
regulations in hazardous localities may require firing doors, dampers,
spark screens, and a solid-plate cooking surface; otherwise these
features are not essential. Two and a half square feet of cooking surface
is desirable, while access to both sides and the end permit several
people to cook at the same time. The side walls should have fairly level
tops for pots and pans. Side walls are made 2 to 6 inches higher than the
cooking level to permit anchoring the grille; if too high, they interfere
with cooking. Commercial grilles are available, but satisfactory ones can
be made of ½-inch to ¾-inch pipe or 5/8-inch reinforcing rods. The
pipes should be 6 to 10 inches longer than the width of the firebox; they
should be spaced not more than 1¼ inches apart and have their tops
exactly level to prevent pots and pans from wobbling. Two or three pipes
can be used for a lintel over the opening into the flue if regular iron
lintels are not available. Where a solid top is desired, it should be of
boiler plate at least ¼-inch thick. Such plates must be stiffened to
prevent buckling by alternate heating and cooling; for ordinary purposes
they are merely set on top of the grid though they may be hinged at the
rear so they can be tipped back against the chimney.

The best draft is secured when the fireplace faces the direction of
prevailing breezes and is protected from strong winds which might scatter
sparks. If the fireplace is built too near shrubbery or under trees,
the heat and smoke may damage or burn the foliage. A slight rise or a
gentle slope that affords good drainage should be selected. Paving the
ground around the fireplace, with flagstones or covering it with a layer
of gravel or sand will prevent the area from becoming a mudhole or an
unsightly bare spot; also, danger of starting brush fires by sparks
falling from the firebox is lessened.

Fireboxes 12 to 16 inches wide, 16 to 24 inches long, and 6 to 8 inches
deep with the hearth at 9 to 16 inches above the ground are sufficient
for most purposes. Large fireboxes are wasteful of fuel; while, if the
grille is too high above the hearth, much of the best cooking heat from
glowing coals is lost. Most grilles are set 15 to 24 inches above the
ground, though 30 inches may be desirable to avoid the necessity of
stooping when cooking. The hearth should slope 1 to 2 inches toward the
front so that rain water will drain away.

The area of the chimney flue should be at least one-eighth the vertical
cross-sectional area of the firebox.

Fire-clay linings for the firebox and flue are not absolutely necessary
except when required by fire regulations or where hot fires are
maintained for long periods. They, or common brick linings, are advisable
for the more permanent and expensive structures or where it is necessary
to use porous stone, such as sandstone and most stratified rocks, which
absorb water and flake or chip upon exposure to fire.

Most rocks or stone that can be worked up without special tools or skill,
brick, and concrete are adaptable for the average fireplace (fig. 43).
The size of the stones determines the thickness of the walls; no wall
should be less than 8 inches thick.

Where suitable stone is difficult to get in sufficient quantity, the
exposed surface may consist of a shell the thickness of the stone and the
inner portion of the wall be made of concrete or large stone bedded in
concrete. When flue lining is necessary, it will serve as a form for the
flue: otherwise a metal stovepipe makes a practical form, or the flue can
be formed of brick laid on edge. The stones can be laid and the concrete
deposited with the least trouble by building the veneer and flue only 6
to 8 inches high at one time. All masonry should be laid in mortar, as
described on page 10. Concrete made in the proportions 1:2½:4 will serve
most purposes where wall sections are about 8 inches thick. For heavy
foundations and thick walls not subject to direct fire, 1:3:6 concrete is
strong enough.

A concrete slab 4 to 8 inches thick with the bottom 4 to 6 inches below
the surface provides a sufficient foundation for medium-sized structures
where frost is negligible and the soil is well drained and firm. If
the soil is not well drained a 6- to 8-inch layer of stone, cinders, or
sand should be provided under the slab and the surface of the immediate
vicinity graded or otherwise protected from water. It is advisable to use
¼-inch or ½-inch reinforcing rods, 6 inches apart in both directions,
one-third the distance from the top in slabs for all but the smallest
fireplaces or where frost is not severe.

[Illustration: Figure 43.--_A_, Ground plan of a stone fireplace that can
be built in different sizes to suit the landscape; _B_, vertical-section
sketch; _C_, the completed fireplace. The stones for this fireplace
were picked up at "the swimming hole." About 8 bushels were used over a
backing of concrete; 16 bushels would have been needed if the concrete
had not been used. Gravel and sand were dug from the excavation. The
chimney is battered 4 inches in the 24-inch height.]

Heavy and expensive structures, especially those having tall chimneys,
should have foundations below the surface affected by frost or erosion
and strong enough to prevent settlement or cracks. Such a foundation can
be made of concrete, with a liberal use of large stones for economy,
extending under the whole structure, or be continuous walls with a
footing similar to those used for houses. The advice and help of builders
may save money in the construction of foundations of large expensive
fireplaces, especially where climatic conditions are severe and the
bearing power of soil is not known.


Cooking should be done over glowing coals, as flames and smoke smudge
the utensils or even the food. When the wood is nearly charred, most of
the smoke has been driven off and the chimney is hot enough to draw the
smoke up the flue. All fires should be banked before they are left, to
avoid setting fires. Banking can be safely and effectively done by raking
the coals and unconsumed fuel into a pile on the hearth and covering the
pile with a few inches of earth. Water thrown on a hot fire may result in
scalds from the steam and may crack the fireplace.


Barbecue ovens are rather expensive unless for community use. Outdoor
fireplaces, without or with spits for roasting, are frequently called
barbecues in some sections of the country. For occasional barbecuing
parties, a hole in the ground will serve. Dig a hole several feet deep
and several feet larger each way than the size of the carcass to be
roasted; then place stones in the bottom to retain the heat. A trench
30 inches deep, 36 inches wide, and about 10 feet long will accommodate
about 400 pounds of beef. A fire should be built sufficiently ahead of
time, about 3 hours, to heat the stones and bottom and accumulate ashes
for proper banking. Have someone with previous experience operate the
pit, because improper wrapping of the meat and handling of the coals
results in poor cooking.


Dutch ovens (fig. 25) are often built in connection with both indoor and
outdoor fireplaces to copy early kitchen fireplaces or for actual baking.

When used as an ornament, the oven is fitted with a cast-iron door, and
the space thus formed may be used for wood storage. An open firebox or
compartment below may be similarly used. Spaces used for wood storage
should be separated from the fireplace by a brick or stone partition at
least 8 inches thick, all joints being completely filled with mortar.

If the oven is intended for baking, it is advisable to line it with
firebrick, and the masonry should be at least 8 inches thick. A greater
heat-storing capacity is secured by using thick walls. An ash drop of
standard cast-iron unit type is provided for modern ovens and may lead
either to the side of the fireplace or to an ash-pit in the chimney
base. The throat or dome should be carefully formed with brick molded or
ground to an arch and preferably should be fitted with a damper. If the
top of the oven is flat, several lintel irons will be needed to support
the brick. A separate flue with a damper is recommended. For ovens of
ordinary size an 8½- by 8½-inch flue is ample. The oven is preheated by
fire or hot coals. Before food is placed in the oven, the coals and ashes
are removed through the ash drop.

Figure 44 shows a Dutch oven made of tapered adobe bricks and plastered
outside with adobe mud. A hole in the top permits the escape of smoke,
while the orño is being heated by the fire built inside on the floor.
This hole is closed with an adobe block after the coals are raked out.
When loaves of bread have been placed on the floor with a wooden paddle,
the door hole is stopped with adobe brick.

[Illustration: Figure 44.--This Dutch oven of adobe bricks is a type very
common in the States along the Mexican border. It is called an orño and
is usually built a short distance from the kitchen door.]

Boy Scouts and campers frequently improvise Dutch ovens by packing damp
sandy clay, 8 to 12 inches thick, around a wooden barrel, a tin wash
boiler, or slabs of rock to form a vault. After the earth has been
gradually dried and baked with a slow fire, the oven is ready for use.
The hole in the top and the door can be closed as in the orño.

       *       *       *       *       *


For sale by the Superintendent of Documents, U. S. Government Printing
Office Washington 25, D. C. - Price 15 cents

       *       *       *       *       *

Transcriber Note

The footnotes for the tables were changed from arabic numerals to
alphabetic characters to distinguish them from the text footnotes.

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