The Technique of FURNITURE MAKING

Kiln drying

The use of hot air in drying wood requires specialized knowledge, and therefore the process is described in broad outline only.

The principles are simple enough, but expert handling is required at every stage. In essence, wood is stacked as for air drying in enclosed compartments or buildings through which hot air is passed, either by natural draught or by power-driven fans. As the heated air would induce very rapid moisture evaporation from the wood surfaces, with consequent severe shrinkage and distortion, live steam is injected into the compartment, so that the rate of evaporation is always maintained at a safe level. Constant control is necessary, as the degree of heat and level of humidification must be adjusted so that there is a progressive fall in the relative humidity, until the wood has reached the required moisture content. As woods differ widely in their reactions, specific schedules have been laid down for practically all those in common use; but other factors, i. e. wood quality, intended use, thickness, whether quartered or plain sawn, etc., permissible degrade (defects), must also be taken into account, so that much depends on the skill and judgement of the operator.

The advantages of kiln drying are obvious, for green wood can be satisfactorily dried down to any given moisture content in a matter of weeks only, moreover much lower values are possible than with air drying. While it is true that any tendency to warp or buckle is increased in kiln drying, this can be mitigated by keeping the initial temperature low and thus extending the drying time, or by air drying the wood down to 20 per cent and then kiln drying, and this latter method is probably ideal. As a rough guide, green hardwoods can be kiln dried to a moisture content of 10 per cent in from three to 12 weeks, according to the species and the schedules adopted, and air-dried hardwoods with 20 per cent moisture content down to the same value in one to four weeks. It must not be forgotten, however, that as soon as the timber leaves the kiln it will begin to take up moisture again (moisture pick-up), and will gradually revert to the normal equilibrium content of 15 to 22 per cent. It should, therefore, be placed in a warm store, or otherwise close piled and covered with tarpaulins until used.