Drying Behavior of Prefrozen Black Walnut

Shrinkage, strain, and set development, moisture gradients and drying rate, drying defects, and changes in extractive availability were measured in 2- by 4- by 36-inch surfaced dimension parts of black walnut (Juglans nigra L.) prefrozen at -10?, -100?, and -320?F. Prefrozen boards showed less shrinkage after kiln-drying than boards not frozen before drying. Prefreezing at -100?F was the most effective in reducing shrinkage and collapse, whereas prefreezing at -320?F was the least effective. Prefreezing at -100?F reduced shrinkage in set-free slices, but it reduced shrinkage in boards even more. Set-free slices prefrozen to -100?F shrunk 5.53 percent, while control set-free slices shrunk 6.26 percent. Boards prefrozen at -100?F shrunk 4.08 percent, while untreated boards shrunk 5.93 percent. Pre-freezing appeared to have an effect on wood fiber hygroscopicity and shrinkage, and an accompanying effect on board set development due to altered rheological properties. The exact nature of these freezing-induced charges was not evident, but early in drying they resulted in greater compression set in core slices and reduced tension set in surface slices for the prefrozen boards. However, after stress reversal there was a greater relaxation of core compression set in prefrozen boards than in control boards. Consequently, the amount of core compression set at the end of kiln-drying was significantly less for boards prefrozen at 10?F and 100?F than for control boards. Drying behavior indicated that much of the effect of prefreezing was dependent on the free water fraction, i.e., the portion of the water in the wood that froze. Freezing temperature had a pronounced effect on, the availability of extractives soluble in hot alcohol-benzene and hot water. Prefreezing at -10?F caused the extraction yield to drop from 15.27 to 10.62 percent of the ovendry sample weight. Prefreezing at -100?F caused a drop from 11.45 to 10.00 percent of the ovendry weight. Samples frozen in liquid nitrogen at -320?F yielded more extractives (11.67 percent) than the control samples (10.62 percent).

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