Drying of Prefrozen Redwood . . . Fundamental and Applied Considerations

Freezing significantly improved the subsequent accelerated drying behavior of redwood heartwood. Defects were greatly reduced or eliminated and shrinkage was reduced irrespective of collapse development. Improved drying behavior was obtained in the absence of increased drying rates or modified strain patterns, indicating that the effect is not attributable to reductions in gross drying stresses. Data analysis shows that collapse is due to stress that develops in the free water. It is believed, however, that this stress is a temperature dependent phenomenon directly attributable to the nature of the free water-extractive system. Its development, therefore, does not require cell saturation or that pit pores be discrete openings with a difinite maximum size. An almost perfect correlation was found for control boards between percentage of thickness shrinkage and percentage of water soluble extractives — the higher the extractives, the higher the shrinkage. After freezing there was no association between the two variables. Since the total amount of soluble extractives or the migration pattern was not altered by freezing, it is believed that the denaturation consists of a modification of the physical chemistry of the free water colloidal system. Drying results obtained in the laboratory were confirmed in a 2 thousand board foot experimental kiln run conducted at Scotia, California. Prefrozen light and medium segregation lumber, l-inch thick, was dried defect free to an average moisture content of 8 percent in less than 8 days. Results showed that heavy segregation should be given preliminary air-drying of from 2 to 3 months prior to freezing and then kiln-dried.

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