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, so