20 



TIMliKU PINES OK THE SOUTHERN UNITED STATES. 



Spt'cific gravity = 



■WEIGHT AND MOISTURE. 



So far the woiplit of only the kiln-dry wood lias been eonsidered. In fresh as well as all yard 

 and air dried material there is contained a variable amount of water. The amonnt of water 

 contained in fresh wood of these pines forms more than lialf the weight of tlic fresh sapwood, and 

 about ont'-lifth to one fourth of the lioartwood. In yard-dry wood it falls to about lli to l.S per 

 cent, wiiile in wood kept in well-ventilated, and esi)eeially in heated rooms it is about 5 to 10 

 per cent, varying with .size of ])ieee, part of tree, .species, temperature, and humidity of air. 

 Ueated to loO^ F. (6^>" C), the wood loses all but about U to 2 per cent of its moisture, and if the 

 tem|)erature is rai.sed 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) jierfectly dry. Of course, large itieces are in ])ractice 

 never left long enougli ex|)0sed 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 slirinkage ' (the water coming from the cell lumen), and the density or specific 

 gravity change.^ simply in direct [iroportion 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- 



])auied by a loss of vohiuu!, so 

 that both factfjrs of the frac- 

 tion 



weight 

 volume 



are affected, and the change 

 in the specific gravity no lon- 

 ger is simply i)roportional to 

 the loss of water or weight. 

 The lo.ss of weight and vol- 

 ume, however, lieiiig uneijual 

 and disproportionate, a marked 

 reduction of the si)ecifl(! grav- 

 ity takes place, amounting in 

 these pines to about 8 to in 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 difler materially. 

 **^*^ ^u^o^r 'o eo 7, 33* Genei'ally tlie heavier wood 



Fio. 4.— Dl.iirram nlmwirc Ina" "f w.itcr in kiln drvinc and rcabBorption in air, ulirinkinc and sliriuks the mOSt, aud sapWOod 



'"'■""'"'•' shrinks about one-fourth more 



than heartwood of the same siiccific weight. Very resinous ])ieces (''light wooil'') shrink much less 

 than other wood. In keeping with these general facts, the shrinkage of the wood of the ui)per 

 logs is usually l.'i to 20 i)er 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 i)arts of the same, while tin; 

 shriidiage of the heavy wood of saplings is greatest of all. On the whole, the wood of these jjines 

 shrinks about 10 per cent in its volume — 3 Xa 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 onlin.irv liiiiil)er and all large size material tliu oxtorior jiarts coninionly dry so uiucli HciDnrr than tlic Imlk 

 of ihe stick that chucking often occnra thongh the nioisttirc jicr cent of the whole stick is still far above 30. 



