20 



TIMBElt PINES OF THE SOUTHERN UNITED STATES. 



Si>c'(;ifl(' gravity = 



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 tliero 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-lifth to one fourth of the lieartwood. In yard-dry wood it falls to about 12 to IS i>er 

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

 per cent, varying with size of ))iece, part of tree, species, temperature, and lnunidity of air. 

 Heated to 150^ F. (Ct't^ C), the wood loses all but about U to 2 per cent of its moisture, and if the 

 temperature is raised to 175o F. there remains less than 1 per cent, the W(jo(l 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 ' (the water coming from the cell lumen), and the density or specific 

 gravity cliaugco simply in direct i)roportion to the loss of water. When the moisture i)er 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 



weight 

 volume 

 are all'ected. and the change 

 in the specific giavity no lon- 

 ger is simply projjortional to 

 the loss of water or weight. 

 The loss of weight and vol- 

 ume, however, being unequal 

 and disproportionate, a marked 

 reduction of the sj)ecilic grav- 

 ity takes place, anu)unting in 

 these pines to about S to 10 i)er 

 cent of the specific weight of 

 the dry wood. 



SHRINKAGE. 



Tlie behavior of the wood of 

 the Southern pines in shrink- 

 age does not differ nuvterially. 

 Generally the licavicr wood 

 shrinks the most, and sapwood 

 shrinks about one-fourth more 

 than lu^artwood of the same si)ecific weight. Very resinous pieces (••light wood'') shrink uuich less 

 than other wood. In kcei)ing with tlu^se general facts, the shrinkage of the wood of the uiii)er 

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

 lieartwood of ohl trees is greater than that of tiie lighter periidieral jtarts of the same, while the 

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

 .shrinks about 10 percent in its volume — 3 to 4 jier cent along the radius, and ti to 7 percent 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 sliortleaf, representing ordinary flooring or siding 



'In oriliuarv liiml)«r iiiiil all liiigc size iiiatorial the oxtorior parts coiiiiiKHily dry »» iniuli sikiihi- than thr Imlk 

 of the stick that cliccking ol'tou occnre though tlu' inoistiiri' ]>i'r rent of the whole stick is still far aliove 30. 



/*iRo/fr ff/iAfOfff fO ao 7/ ■334- 



Fig. 4.— Bla^ram showinp loss of water in kiln drying and rcabsorptiiin in air, shrinking and 



8 well in jr. 



