ADVANTAGES OF PREFREEZING FOR REDUCING SHRINKAGE-RELATED DEGRADE IN EUCALYPTS - GENERAL-CONSIDERATIONS AND REVIEW OF THE LITERATURE

Difficulties associated with the drying of ash eucalypts including col lapse and internal checking, are discussed briefly. Prefreezing is one method that has been used successfully as a pretreatment for the dryi ng of both hardwoods and softwoods from temperate and tropical regions . Prefreezing has produced marked reductions in shrinkage, collapse an d drying degrade of the heartwood in the following species: California redwood, black walnut, black cherry, tanoak, toon, bamboo, and eucaly pts. Little or no collapse reduction has been observed in New Zealand red beech, Pacific madrone, white birth, sitka spruce, and white ash. Limited response has been observed for numerous other species notably red oak and white oak. Reduced drying time in response to prefreezing has been observed in jarrah, karri, black walnut, Asian oak, toon, and California redwood; in Pacific madrone and tanoak the drying time inc reased. Not all species which respond with a reduction in shrinkage sh ow reduced drying rates. Prefreezing wood at -20 degrees C appears to be the most practicable temperature, although some species respond bet ter at lower temperatures. However, in all cases, it is critical to en sure that the wood freezes and remains frozen for a number of hours. I ndications are that the effect is retained for days to weeks and that the length of time of freezing need not exceed 12-24 hours. A number o f explanations have been put forward to explain the behaviour of prefr ozen wood. It is suggested that the main mechanism responsible for red uced shrinkage is due to the migration of moisture from the cell wall onto frozen lumen water. The moisture loss from the cell wall produces a 'cold shrinkage'; water to ice transformation leads to an expansion of liquid water in the lumen, thus imparting a compressive stress to the cell wall, which together with the moisture loss, make the cell mo re rigid, and therefore likely to shrink less. There is some evidence that certain types of wood extractives migrate into the cell wall duri ng freezing and may play a role in the reinforcement of the wall. Redu ced shrinkage after prefreezing has also been attributed to a reductio n of the plasticising effect of wood extractives in wood dried at high er temperatures and low humidities; this effect does not occur at low temperatures.