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.