20 



TIMBER PINES OF THE SOUTHERN UNITED STATES. 



WEIGHT AND MOISTURE. 



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

 and air dried material there is contained a variable amount of water. Tin- 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 1- to is per 

 cent, while in wood kept in well-ventilated, and especially in heated rooms it is about "> to 10 

 per cent, varying with size of piece, part of tree, species, temperature, and humidity of air. 

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

 temperature is raised to 175 P. there remains less than 1 per cent, the wood dried at I'll' F. 

 being assumed to be (though it is not really) perfectly dry. Of course, large pieces arc 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 ' (the water coming from the cell lumen >, and the density or specific 

 gravity changes simply in direct proportion to the loss of water. When the moisture per cent 

 falls below about 30, the water comes from the cell wall, and the loss of water and weight is accom- 

 panied by a loss of volume, so 

 that both factors of the frac- 

 tion 



Specific gravity = xM 

 volume 



are affected, and the change 

 in the specific gravity no lon- 

 ger is simply proportional to 

 the loss of water or weight. 

 The loss of weight and vol- 

 ume, however, being unequal 

 and disproportionate, a marked 

 reduction of the specific grav- 

 ity takes place, amounting in 

 these pines to about 8 to 10 per 

 cent of the specific weight of 

 the dry wood. 



SHRINKAGE. 



The behavior of the wood of 

 the Southern pines in shrink- 

 age does not differ materially. 

 Generally the heavier wood 

 shrinks the most, and sapwood 

 shrinks about one- fourth more 

 than heartwood of the same specific weight. Very resinous pieces ("light wood") shrink much less 

 than other wood. In keeping with these general facts, the shrinkage of the wood of the upper 

 logs is usually 15 to 20 per cent less than that of the butt pieces and the shrinkage of the heavy 

 heartwood of old trees is greater than that of the lighter peripheral parts of the same, while tin- 

 shrinkage of the heavy wood of saplings is greatest of all. On the whole, the wood of these pines 

 shrinks about 10 per cent in its volume 3 to 4 per cent along the radius, and 6 to 7 per cent along 

 the tangent or along the yearly rings. 



After leaving the kiln the wood at once begins to absorb moisture and to swell. In an 

 experiment with short pieces of loblolly and shortleaf, representing ordinary flooring or siding 



'In ordinary lumber and all large size material the exterior parts commonly dry so much sooner than the liulk 

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



AIIO/TV 



FIG. 4.- 



KIlftORr 'O 2O 7/ 334 



-Diagram showing loss of water in kiln drying and reabsorption in air, shrinking anil 



swelling. 



