Advantages of prefreezing for reducing shrinkage-related degrade in eucalyptus: general considerations and review of the literature.
Difficulties associated with the drying of ash eucalypts (Eucalyptus spp.), including collapse and internal checking, are discussed briefly. Prefreezing is one method that has been used successfully as a pretreatment for the drying of both hardwoods and softwoods from temperate and tropical regions. Prefreezing has produced marked reductions in shrinkage, collapse and drying degrade of the heartwood in the following species: California redwood (Sequoia sempervirens), black walnut (Juglans nigra), black cherry (Prunus serotina), tanoak (Lithocarpus densiflorus), toon (Toona ciliata), bamboo (Bambusa gigantica [Dendrocalamus giganteus]), and eucalypts (Eucalyptus regnans and Eucalyptus delegatensis). Little or no collapse reduction has been observed in New Zealand red beech (Nothofagus fusca), Pacific madrone (Arbutus menziesii), white birch (Betula alba), Sitka spruce (Picea sitchensis), and white ash [no data]. Limited response has been observed for numerous other species notably red oak (Quercus borealis [Q. rubra]) and white oak (Quercus alba). Reduced drying time in response to prefreezing has been observed in jarrah (Eucalyptus marginata), karri (Eucalyptus diversicolor), black walnut, Asian oak (Quercus mongolica), toon, and California redwood; in Pacific madrone and tanoak the drying time increased. Not all species which respond with a reduction in shrinkage showed reduced drying rates. Prefreezing wood at -20°C appears to be the most practicable temperature, although some species respond better at lower temperatures. However, in all cases, it is critical to ensure that the wood freezes and remains frozen for a number of hours. Indications 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 of explanations have been put forward to explain the behaviour of prefrozen wood. It is suggested that the main mechanism responsible for reduced 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 more rigid, and therefore likely to shrink less. There is some evidence that certain types of wood extractives migrate into the cell wall during freezing and may play a role in the reinforcement of the wall. Reduced shrinkage after prefreezing has also been attributed to a reduction of the plasticizing effect of wood extractives in wood dried at higher temperatures and low humidities; this effect does not occur at low temperatures. Although the hygroscopicity of the wood is altered in some instances, the permanency of this is not known.