al is
an important question.

Kilns for the manufacture of charcoal are made of almost any shape and
size, determined in most cases by the fancy of the builder or by the
necessities of the shape of the ground selected. They do not differ from
each other in any principle of manufacture, nor does there seem to be
any appreciable difference in the quality of the fuel they produce, when
the process is conducted with equal care in the different varieties; but
there is a considerable difference in the yield and in the cost of the
process in favor of small over large kilns. The different varieties have
come into and gone out of use mainly on account of the cost of
construction and of repairs. The object of a kiln is to replace the
cover of a meiler by a permanent structure. Intermediate between the
meiler and the kiln is the Foucauld system, the object of which is to
replace the cover by a structure more or less permanent, which has all
the disadvantages of both systems, with no advantages peculiar to
itself.

The kilns which are used may be divided into the rectangular, the round,
and the conical, but the first two seem to be disappearing before the
last, which is as readily built and much more easily managed.

ALL VARIETIES OF KILNS

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.