TIMBER PHYSICS SOUTHERN PINE. 



Injlurnce of moisture OH fln-ni/tlt. 



353 



iier cent green, 20 per cent hulf dry, 15 per cent yard dry, 10 per t-eiit room dry. 



S ff 7 8 



FIG. 93. Variation of compreseion strength with moisture. 



It will be observed that the strength increases by about 50 per cent in ordinary good yard seasoning, and that 

 it can be increased by about 30 per cent more by complete seasoning in kiln or house. 



Large timbers require several years before even the yard-season condition is attained, but 2-inch and lighter 

 material is generally not used with more than 15 per cent moisture. 



WEIGHT AND MOISTURE. 



So far the weight of only the kiln-dry wood has been considered. In fresh us well as all yard and air-dried 

 material there is contained a variable amount of water. The amount of water contained in fresh wood of these 

 pines forms more than half the. weight of the fresh sapwood, and about one-fifth to one-fourth of the heartwood ; in 

 yard-dry wood it falls to about 12 to 18 per cent, while in wood kept in well-ventilated and especially in heated 

 rooms it is about 5 to 10 per cent, varying with size of piece, part of tree, species, temperature, and humidity of air. 

 Heated to 150 F. (65 ('.) the wood loses all but about 1J to 2 per cent of its moisture, and if the temperature is 

 raised to 175 F. there remains less than 1 per cent, the wood dried at 212 F. being assumed to be (though it is not 

 really) perfectly dry. Of course large pieces are in practice never left long enough exposed to become truly kiln -dry, 

 though in factories this state is often approached. 



As long as the water in the wood amounts to about 30 per cent or more of the dry weight of the wood there is 

 no shrinkage 1 (the water coming from the cell lumen) and the density or specific gravity changes simply in direct 



'In ordinary lumber and all large si/e material the exterior parts commonly dry so much sooner than the bulk 

 of the stick that checking often occurs, though the moigture per cent of the whole stick is still far above 30. 



H. Doc. 181 23 



