Prefreezing Alone and Combined with Presteaming in the Drying of Redwood Dimension

The effect of prefreezing, alone and combined with presteaming, was evaluated in the drying of redwood heartwood dimension. Several different drying experiments were conducted. It was determined that the combination of prefreezing and presteaming is not as effective as prefreezing by itself in reducing shrinkage. This was true regardless of the order in which the two pretreatments were applied. Freezing after steaming reduced shrinkage more than the reverse order, indicating that the kiln drying behavior was dependent not only upon the type of pretreatment but also the order in which they were applied. In the series of experiments the decreasing order of effectiveness was as follows: prefreezing, presteaming followed by prefreezing, prefreezing followed by presteaming, and then either presteaming or controls. In two of the experiments we employed samples that differed significantly in color. The light colored samples contained approximately 10 percent extractives by weight, whereas the darker colored samples contained closer to 20 percent. The dark colored samples, for treatments not involving prefreezing, shrunk significantly more than the light colored ones. Among samples prefrozen only, however, there was essentially no difference in the shrinkage of light and dark, indicating that prefreezing eliminated some normal contribution of extractives to the shrinkage of redwood samples dried at the temperature employed in these tests, 145?F. Shrinkage values for presteamed only and controls correlated with percent extractive content but not with the other factors evaluated, initial moisture content and bulk specific gravity. Prefreezing significantly reduced the shrinkage of redwood dimension that had been predried down to an average moisture content of from 50 to 120 percent. This. supports the feasibility of short-term, low-temperature predrying of heavy segregation prior to freezing. Three days of drying at. 110?F and 90?F are believed adequate for preventing freezer damage to heavy segregation. The heavy segregation could then be dried under high temperature, greatly accelerated kiln conditions.

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