Fig. 24.—Lumber piled in Double
Courses.
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 outside 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 required, not less than two years weather-drying is preferred, as the material retains its full strength.
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 elasticity 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 moisture nearly to the dew point.