ECONOMIC WOODS OF THE UNITED STATES 55 



all that is necessary to render even the most perishable wood 

 indefinitely immune from decay is to keep it dry. Wood con- 

 taining not more than 10 per cent of moisture will not decay. 



Rate of seasoning differs with the kind of wood and with 

 its shape. A thin piece dries more rapidly than a thicker one; 

 sap wood more rapidly than heartwood; a light, open wood more 

 readily than one that is dense and heavy. Large beams or logs 

 are exceedingly slow in drying, requiring from two to several 

 years' seasoning in the open air before reaching an air-dry condi- 

 tion in the interior. Ties require from three months to a year 

 to season, depending on the kind of timber and the climate. 

 Much depends upon the method of piling, since boards in open piles 

 often dry twice as fast as those in solid piles. 



As a result of numerous experiments by the U. S. Forest 

 Service upon large beams of Pinus palustris and P. tceda, the 

 following conclusions were reached (Bui. 70, p. 123): 



" (1) The drying-out process takes place almost wholly through 

 the faces of the beam and not longitudinally, except near the ends. 



" (2) The ratio of evaporation through a surface is proportional 

 to the rate of growth or density of the wood near the surface, 

 being most rapid in the case of sapwood. 



" (3) If the whole stick is made up of heartwood or the pro- 

 portion of sapwood is uniform throughout, the longitudinal dis- 

 tribution of moisture is very regular. If the proportion of sap- 

 wood is not uniform, on the other hand, the portion containing 

 the most sap is the most susceptible to moisture influences; i.e., 

 it will dry or will absorb moisture the most rapidly. 



"The average of two cross-sections of longleaf pine sticks, 

 12 by 12 inches and 8 by 16 inches and 16 feet long, which were 

 air-dried for two years, , showed an average moisture content in 

 the outer portion, cut halfway from surface to centre, of 17.7 

 per cent, while the inner part contained 25.7 per cent. 



"From this it is quite evident that where timber of structural 

 sizes is used, the strength ordinarily reckoned upon should not 

 be greater than that of the green condition." 



References 



ROTH, F.: Timber, Bui. 10, U. S. Div. Forestry, pp. 29-31. 

 TIEMANN, H. D. : The Effect of Moisture upon the Strength and Stiffness of 

 Wood, Bui. 70, U. S. Forest Service, 1906, p. 144. 



