36 Drying of Wood

Transition zone0.55Relative density at breast height0.500.450.400.350.300.25Douglas-firWestern larchWestern hemlockYellow - cypressLodgepole pineSitka spruceInterior spruceSubalpine firWestern red cedar0.200Juvenile wood10 20 30Age (y)Mature wood40 50 60FIGURE 36.43 Trends in basic density at breast height for commercial second-growth softwoods in British Columbia.(Adapted from Josza, L.A. and Middleton, G.R., A discussion of wood quality attributes and their practical implications,Forintek Canada Special Publ. SP-34, 1994.)In practice, however, it is not easy to distinguish theeffect of density with that of moisture content.Within-ring density variations can cause problemsbecause of the differential shrinkage at the ringboundary. Subsequently, severe drying stresses maycause deformation and internal checking (Booker,1994). Further, a low absolute density in earlywoodcan result in collapse on drying, particularly underhigh-temperature conditions (Booker, 1996).Within-tree variations in density can be highlysignificant. Cown and McConchie (1983) show thatthe density in a 24-year-old radiata pine tree can varyfrom 300 kg m 3 in the top log to greater than 450 kgm 3 in the outer wood of the butt log. Consequently,the drying kinetics of boards taken from the same logmay be markedly different (Davis, 2001). Trends inbasic density for a number of second-growth softwoodsare illustrated in Figure 36.43.The situation is more complex with hardwoods.The growth rate has little effect on the wood propertiesof diffuse-porous hardwoods, but has a markedimpact on the density of ring-porous hardwoods. Unlikesoftwoods, these produce denser wood when fastgrown.Regardless of the species or where the forests areestablished, the variation in wood properties betweentrees is very great and can be great even in boardssawn from the same tree. In particular, the socialstatus of the tree in the stand (whether dominatedor dominant trees) has a great effect on the growthrate in diameter and the occurrence of reactionwood. Table 36.10 lists some of the characteristics ofTABLE 36.10Characteristics and Properties of Reaction Wood Compared with Corresponding Normal WoodFeatures Compression Wood Tension WoodPhysical characteristics Darker in color and very hard Darker in color and silvery sheen in mosttemperate hardwoodsDensity 10–100% greater 10–30% greaterLongitudinal shrinkage Order of magnitude greater (up to several fold) About fivefold greaterWarp on drying Liable to warp badly Can warp and is liable to collapseStrength Comparable strength, does not reflect higher density Superior strengthSource: Keey, R.B., Langrish, T.A.L., and Walker, J.C.F., The Kiln-Drying of Lumber, Springer, Berlin, 2000.ß 2006 by Taylor & Francis Group, LLC.