162 FOREST UTILIZATION WOOD USES AND ECONOMICS 



have wide bands of large pores in the springwood. The diffuse porous 

 woods are more homogeneous and continuous in their structure and 

 texture. Approximately 79% of the annual lumber production from 

 our forests is of coniferous growth which satisfies the large demand 

 for lumber for construction and general building purposes. The hard- 

 woods are more specialized in their applications, being used chiefly 

 for furniture, cabinet work, implement and vehicle stock, interior 

 finish, flooring, etc. 



A knowledge of wood structure is very important for understand- 

 ing how best to season lumber and other forest products. About 50% 

 of the total weight of green wood consists of water. Seasoned by 

 usual methods, lumber generally contains from about 15 to 20% mois- 

 ture, and thoroughly kiln-dried lumber only 6 to 12% moisture. The 

 purposes of seasoning are to decrease the danger from decay and there- 

 fore increase the durability of wood, to prevent warping, twisting, 

 checking, and shrinkage after the wood is placed in service, to in- 

 crease the strength and stiffness and decrease its weight and there- 

 fore economize on transportation charges. When green or freshly 

 sawn, lumber is exceedingly susceptible to fungus and insect attacks. 

 Lumber is ordinarily seasoned by exposing it to the air with adequate 

 facilities for the passage of air currents to assist in the elimination of 

 moisture. Much more lumber is being kiln-dried to hasten the process 

 of seasoning than formerly. Adequate temperature and moisture con- 

 trol must be maintained in order to secure the best results. The char- 

 acter of wood structure must be thoroughly known in order to regu- 

 late the methods of artificially seasoning wood by drying it in kilns. 

 The rapidity of seasoning varies with the structure of the wood, the 

 size and shape of the lumber or piece to be seasoned, as well as the 

 method of seasoning and piling. For example, hardwoods are much 

 more difficult to season than softwoods; 4" stock seasons much more 

 slowly and with more difficulty than V lumber. White pine dries 

 more quickly than oak because of differences in structure and density. 

 Softwoods generally require from two to five months for air seasoning, 

 whereas hardwoods may require from six to ten months. In kiln- 

 drying processes, lumber may be seasoned in a few days. Usually 

 temperatures of 150 to 180 F. are maintained to dry lumber prop- 

 erly in kiln. The humidity in the air of the kiln must also be care- 

 fully regulated. 



Weight or density of wood also varies very widely. Fuel value 

 is in almost direct relationship to weight. Weight influences trans- 

 portation charges to the market, and it has already been explained 

 that both logs and lumber may be shipped long distances to market. 



