dickm":pgc3pck5 said:
Anyone know the precise time-temp relationship for killing these beasties by freezing?
Freeze treatments to destroy insect pests, including eggs, grubs and larvae work best when the temperature drops rapidly. The target temperature should be reached within twenty four hours throughout the object to be most effective, not just the surrounding air temperature or the outer portions of the piece. Insects can adapt to slow changes in temperature much better than they can cope with relatively quick changes. The freezer must be capable of taking temperatures down to at least -18ºC (-0.4ºF) and then once the object has reached this temperature it has to be maintained for at least two weeks. If the freezer can maintain temperatures as low as –30ºC (-22ºF) the treatment period, once the necessary temperature is reached, can be as short as three days.
A difficulty that has to be overcome using freeze treatments is maintaining the moisture content of the object being treated. Water migrates out of wood as atmospheric relative humidity drops, and cold air has very low relative humidity so water moves out of wood and it may become too dry-- freeze drying is one method of seasoning timber. Small objects must be wrapped and sealed in plastic bags to help maintain moisture content levels, and if air can be evacuated out of the bag prior to freezing this helps to maintain the object’s moisture content. After treatment the wrapping needs to stay in place until the object properly reaches normal temperatures to prevent condensation onto it as this may cause damage to finishes and so on.
Even rather large furniture items such as tables and chairs can be treated successfully if they are bagged and frozen in a walk-in freezer. Relative humidity needs maintaining at a suitable level, typically approximately 50%, in order to maintain wood moisture content at roughly 10%. If the piece has a lot of decorative veneer or inlay there is a high risk of irretrievable damage to these parts so freeze treatment is very likely to be inappropriate. There may also be a risk of damage to the applied finish that could negate the possibility of using this form of treatment.
Freezing might also succeed if there is an infestation in unworked but seasoned wood at about 10% MC or below where the boards can be close or dead stacked, similarly plastic wrapped as a pack, and frozen. However, wood is a good insulator and a large stack of wood might take considerable time to reach the target temperature. In this case individual wrapping and stickering as for seasoning would significantly reduce the time required to freeze the wood. Slainte.
