Prefreezing Reduces Shrinkage and Alters Sorption in Black Walnut

Black walnut (Juglans nigra L.) heartwood and sapwood was prefrozen green at 6 temperatures from ?F to -320?F to test the effect of prefreezing on hygroscopicity, shrinkage, and extractives availability. Inferences on hygroscopicity came from adsorption-desorption at 80?F established by equilibration of the wood over 9 saturated salt solutions producing relative vapor, pressures (rvp’s) from 0.03 to 0.97. Volumetric shrinkage was measured on thin wafers and on blocks large enough to develop drying stresses. The availability of hot water and hot alcohol-benzene soluble extractives was determined for the 6 prefreezing temperatures and for prefreezing-to-extraction time intervals of 1 to 56 days. Extractives availability did not differ with prefreezing temperature, but increasing time between prefreezing and extraction decreased alcohol-benzene solubles availability. Matched material kiln-dried after the same time intervals did not shrink differently. From this it was concluded that extractives affected by time are not the extractives most effective in bulking, and prefreezing-induced bulking differences are not evidenced by changes in extractives avail ability. Block volumetric shrinkage was different among prefreezing treatments. At the end of desorption the unfrozen blocks had shrunk the most, and blocks prefrozen at -110?F had shrunk the least. The following conclusions resulted from this research: 1) Shrinkage reduction in prefrozen black, walnut is the combination of the separate effects of prefreezing on increased extractives bulking and altered rheology. 2) The optimum prefreezing temperature for 2- by 4-inch black walnut lies near -110?F. It is proposed that at lower temperatures the freeze-concentrated extractives become less available for bulking cell walls.

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