Are usually built of red brick, or, rarely, of brick and stone together.
Occasionally, refractory brick is used, but it is not necessary. The
foundations are usually made of stone. There are several precautions
necessary in constructing the walls. The brick should be sufficiently
hard to resist the fire, and should therefore be tested before using. It
is an unnecessary expense to use either second or third quality
fire-brick. As the pyroligneous acid which results from the distillation
of the wood attacks lime mortar, it is best to lay up the brick with
fire-clay mortar, to which a little salt has been added; sometimes loam
mixed with coal-tar, to which a little salt is also added, is used. As
the principal office of this mortar is to fill the joints, special care
must be taken in laying the bricks that every joint is broken, and
frequent headers put in to tie the bricks together. It is especially
necessary that all the joints should be carefully filled, as any small
open spaces would admit air, and would materially decrease the yield of
the kiln. The floor of the kiln was formerly made of two rows of brick
set edgewise and carefully laid, but latterly it is found to be best
made of clay. Any material, however, that will pack hard may be used. It
must be well beaten down with paving mauls. The center must be about six
inches higher than the sides, which are brought up to the bottom of the
lower vents. Most kilns are carefully pointed, and are then painted on
the outside with a wash of clay suspended in water, and covered with a
coating of coal-tar, which makes them waterproof, and does not require
to be renewed for several years.
[Illustration: RECTANGULAR KILNS FOR THE MANUFACTURE OF CHARCOAL.]
The kilns were formerly roofed over with rough boards to protect the
masonry from the weather, but as no special advantage was found to
result from so doing, since of late years they have been made
water-proof, the practice has been discontinued.
The wood used is cut about one and a fifth meters long. The diameter is
not considered of much importance, except in so far as it is desirable
to have it as nearly uniform as possible. When most of the wood is
small, and only a small part of it is large, the large pieces are
usually split, to make it pack well. It has been found most satisfactory
to have three rows of vents around the kiln, which should be provided
with a cast-iron frame reaching to the inside of the furnace. The vents
near the ground are generally five inches high--the size of two
bricks--and four inches wide--the width of one--and the holes are closed
by inserting one or two bricks in them. They are usually the size of one
brick, and larger on the outside than on the inside. These holes are
usually from 0.45 m. to 0.60 m. apart vertically, and from 0.80 m. to
0.90 m. apart horizontally. The lower vents start on the second row of
the brickwork above the foundation, and are placed on the level with the
floor, so that the fire can draw to the bottom. There is sometimes an
additional opening near the top to allow of the rapid escape of the
smoke and gas at the time of firing, which is then closed, and kept
closed until the kiln is discharged. This applies mostly to the best
types of conical kilns. In the circular and conical ones the top
charging door is sometimes used for this purpose. Hard and soft woods
are burned indifferently in the kilns. Hard-wood coal weighs more than
soft, and the hard variety of charcoal is usually preferred for blast
furnaces, and for such purposes there is an advantage of fully 33-1/3
per cent. or even more in using hard woods. For the direct process in
the bloomaries, soft-wood charcoal is preferred. It is found that it is
not usually advantageous to build kilns of over 160 to 180 cubic meters
in capacity. Larger furnaces have been used, and give as good a yield,
but they are much more cumbersome to manage. The largest yield got from
kilns is from 50 to 60 bushels for hard wood to 50 for soft wood. The
average yield, however, is about 45 bushels. In meilers, two and a half
to three cords of wood are required for 100 bushels, or 30 to 40 bushels
to the cord. The kiln charcoal is very large, so that the loss in fine
coal is very much diminished. The pieces usually come out the whole
size, and sometimes the whole length of the wood.
The rectangular kilns were those which were formerly exclusively in use.
They are generally built to contain from 30 to 90 cords of wood. The
usual sizes are given in the table below:
1 2 3 4
Length 50 40 40 48
Width 12 15 14 17
Height 12 15 18 18
Capacity, in cords 55 70 75 90
1 and 2. Used in New England. 3. Type of those used in Mexico. 4. Kiln
at Lauton, Mich.
The arch is usually an arc of a circle. A kiln of the size of No. 4, as
constructed at the Michigan Central Iron Works, with a good burn, will
yield 4,000 bushels of charcoal.
The vertical walls in the best constructions are 12 to 13 feet high, and
1-½ brick thick, containing from 20 to 52 bricks to the cubic foot of
wall. To insure sufficient strength to resist the expansion and
contraction due to the heating and cooling, they should be provided with
buttresses which are 1 brick thick and 2 wide, as at Wassaic, New York;
but many of them are built without them, as at Lauton, Michigan, as
shown in the engraving. In both cases they are supported with strong
braces, from 3 to 4 feet apart, made of round or hewn wood, or of cast
iron, which are buried in the ground below, and are tied above and below
with iron rods, as in the engraving, and the lower end passing beneath
the floor of the kiln. When made of wood they are usually 8 inches
square or round, or sometimes by 8 inches placed edgewise. They are
sometimes tied at the top with wooden braces of the same size, which are
securely fastened by iron rods running through the corners, as shown.
When a number of kilns are built together, as at the Michigan Central
Iron Works, at Lauton, Michigan, shown in the plan view, only the end
kilns are braced in this way. The intermediate ones are supported below
by wooden braces, securely fastened at the bottom. The roof is always
arched, is one brick, or eight inches, thick, and is laid in headers,
fourteen being used in each superficial foot. Many of the kilns have in
the center a round hole, from sixteen to eighteen inches in diameter,
which is closed by a cast iron plate. It requires from 35 M. to 40 M.
brick for a kiln of 45 cords, and 60 M. to 65 M. for one of 90 cords.
* * * * *
The belief that population in the West Indies is stationary is so far
from accurate that, as Sir Anthony Musgrave points out, it is increasing
more rapidly than the population of the United Kingdom. The statistics
of population show an increase of 16 per cent. on the last decennial
period, while the increase in the United Kingdom in the ten years
preceding the last census was under 11 per cent. This increase appears
to be general, and is only slightly influenced by immigration. "The
population of the West Indies," adds Sir A. Musgrave, "is now greater
than that of any of the larger Australian colonies, and three times that
of New Zealand."
* * * * *