Boards of practically the same width, if less than 7" wide, are sometimes stacked in double courses, as shown in Fig. 24, the outside of the boards, or the sides which grew nearer the out side of the tree being placed together, thus allowing the inside of the boards, or the sides which grew toward the center of the tree, to receive more air than the sides which are placed together, and therefore to dry out faster, which will reduce the warping to a minimum. After a pile is completed, it should be covered with old boards to protect the top courses from the weather.

(F.) A pile of valuable lumber should be restacked every six or eight months, as the boards are apt to become discolored where the lumber sticks are placed; in this rehandling, the warped boards should be placed with the concave side underneath.

(G.) If lumber is cut in winter or midsummer, and properly cared for, it is not apt to be injured by any rain which may drive into the pile, if there is free circulation of air; nor is it so liable to decay as lumber which is cut at other times of the year.

17. Weather-dried lumber. - Lumber which has been dried in the stack out of doors is not dry enough for use in the manufacture of inside finish or furniture, as it has dried out only to the degree of moisture in the outside air. If it is then worked up and placed in an artificially heated house, the heat will cause more moisture to evaporate, the wood to shrink, and the joints to open. For material to be used in the frames of buildings, in wagons, or in other places where the greatest possible strength is re quired, not less than two years weather-drying is pre ferred, as the material retains its full strength.

FIG 24. - Lumber piled in Double Courses.

18. Kiln-dried lumber. - Lumber for furniture or for inside finish should be seasoned by the process known as "kiln-drying." This means that lumber is exposed to a temperature of from 120° to 200° F. by which the moisture is extracted and evaporated. Lumber thus treated is apt to be more or less weakened by the action of the heat upon the fibers of the wood, which causes thousands of minute fractures, and in many cases the life and the elas ticity of the lumber is destroyed. The results of kiln drying depend largely upon the kiln, and upon the skill with which the lumber is piled, the heat applied, and the rapidity of evaporation of the moisture regulated.

For these reasons, much kiln-dried lumber is suitable for use where but little strength is required and where the color and the grain are the important points to consider.

19. Moist air kilns. - (A.) There are two types of dry kilns in common use: the natural draft, or moist air, kilns, and the induced draft kilns. These two types are made by different manufacturers, nearly all of whom use certain devices of which they control the patents, and which constitute the chief difference between their kiln and those made by other manufacturers.

(B.) The moist air kilns are so constructed as to allow the freest possible circulation of the heated air, and to provide opportunities for the moisture to be expelled in accordance with certain natural laws, which results are obtained by a carefully planned and managed system of ventilation. These kilns operate upon the principle that heated air circulating naturally through lumber will become charged to a much greater degree with moisture than if it were forced through rapidly, as in the induced draft kilns. Thus, heated air by passing slowly through a pile of lumber may become charged with mois ture nearly to the dew point.

If the humidity of the heated air is maintained at that point, by allowing the moisture to pass out as it accu mulates, with a small amount of heated air, which is replaced with fresh air from the outside, it is claimed that the boards will dry out from their centers. (C.) As the warm, moist air which circulates through the pile will keep the outsides of the boards moist, it will prevent case hardening, or the hardening of the outsides of the boards. This is caused by very warm dry air, which "cooks," or closes the pores of the surface of the boards, and this prevents the outsides from shrinking, while the insides will be so badly checked and discolored as to destroy the boards.

After the moisture is all out of the lumber, that held in suspension will gradually pass out of the kiln, and the air inside will become perfectly dry.

(D.) It is claimed that all kinds of lumber in com mon use may be put into this type of kiln perfectly green, except oak and other very hard woods, which should have at least thirty days' drying under good dry ing conditions for each inch in thickness. It is also claimed that the moist air kiln is simply weather drying accel erated, - the moisture being thoroughly extracted from the lumber, the result being the same as though it were stacked out of doors for several years, - and that the lumber has lost none of its strength, elasticity, or characteristic color.

(E.) This method sometimes is applied by steam pipes extending between each course of boards, and in this way the lumber is dried out very rapidly. Lumber used in this sort of kiln should be thoroughly weather-dried, or otherwise the high temperature will cause it to check badly. In certain forms of these kilns, the lumber is saturated with live steam after it is piled in the kiln, before the heat is turned on.

20. Induced draft kilns. - (A.) This system of kiln drying consists of a power-driven fan, which forces the heated air at a high rate of speed through the spaces be tween and around the lumber piled in the chamber.

(B). Manufacturers have different devices for extract ing the moisture from the air after it has passed through the lumber piles. It may be passed over condensing plates, or through coils of pipes in which cold water is continually circulating, both of these devices being for the purpose of extracting the moisture from the heated air. If the moisture is separated from the air by condensation, it runs away, but if not, a certain per cent of the heated air is expelled out of doors, being replaced by fresh air. The air in the kiln, somewhat cooled from contact with these cooled surfaces, is returned to the heater, reheated, and again forced through the kiln, which operation is repeated continuously and automatically. Thus the heated air becomes charged with a small percentage of moisture each time it passes through the kiln chamber; this moisture is extracted and the air is again heated before beginning another circuit, instead of slow circulation which allows the heated air to become saturated with moisture be fore it is discharged, as in the moist air kiln.