Effect of Prefreezing on the Drying of Redwood

The drying behavior of clear, all-heart, and construction- heart grades of California redwood ranging in moisture content from 70 to more than 200 percent were studied by conventional and prefreezing techniques. Samples lost essentially no moisture during freezing. Most of the research was carried out on redwood; some work was done on madrone, tanoak, incense-cedar, and black walnut. Prefreezing was shown to improve the drying behavior significantly. Collapse and honeycomb were greatly reduced or eliminated and shrinkage was reduced irrespective of collapse. Prefreezing increased the drying rate at a dry-bulb temperature of 190?F and low relative humidity. At dry-bulb temperatures of 150?F and approximately 65 percent relative humidity, there was no significant difference in drying rate between prefrozen and control boards. Collapse was eliminated and shrinkage reduced, however, in the prefrozen material dried at 150?F. It is believed that prefreezing has an effect upon shrinkage that is independent of its influence upon drying rate. It is suggested that this effect is one of reducing the tensile strength of the free water, thereby causing a reduction in hydrostatic tension stresses. Complete freezing of the free water appears to be a requirement. Some breakage occurred during freezing treatments due apparently to high moisture content as well as freezing duration and type of grain structure. Control of this effect is suggested but requires additional research.

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