The Technique of FURNITURE MAKING

Relative humidity

Even in the driest of weather the atmosphere will always contain a certain amount of invisible "•vater vapour which can vary considerably from hour to hour. The actual degree of moisture vapour present at any one time (relative humidity) is measured with a wet and dry Hygrometer, and is expressed as the percentage of moisture in the air at a given temperature, compared with the maximum amount saturation point) the air would hold at the same temperature without actual precipitation. The humidity rate can fluctuate rapidly, but fortunately wood is a slow conductor, and therefore rapid fluctuations either side of the seasonal average can usually be ignored.

Equilibrium moisture content

As wood is an organic and hydroscopic material susceptible to moisture changes it will always endeavour to reach a state of equilibrium with the surrounding atmosphere, giving up or taking in water like a sponge until the balance is achieved. The term 'equilibrium moisture content' is therefore an evaluation of the prevailing atmospheric humidity in terms of the corresponding actual water content the wood has achieved in that humidity; it is expressed as a percentage of the dry weight of wood substance (see Measurement of moisture, p. 13). The important point to remember is that no matter how old the timber happens to be it will always respond in precisely the same way, and that as its environment changes so will its moisture content.

Methods of seasoning

The whole object of seasoning is to bring the moisture • content of the wood down to acceptable levels, according to the use to which it will be put. as speedily but as gently as possible. The drying out of the actual fibres after the pores are emptied is accomplished by surface evaporation and consequent capillary attraction from the moist interior of the wood. If the evaporation rate is too fast then partial vacuums will be created in the wood substance and the cell walls will collapse, causing widespread splitting and distortion. If, however, the drying-rate is controlled, then the water loss will be replaced by air and the natural strength and elasticity of the wood will always strive to accommodate the strains imposed. In the literal sense, therefore, 'seasoning' means controlled drying, and any wood which has been satisfactorily dried down to an acceptable level is usually regarded as seasoned timber/lumber. There is no doubt, however, that additional storage time over the prescribed minimum does

bring certain advantages not measurable in any scientific way. In effect it will have matured like wine—it will be no drier but the moisture will be more evenly distributed, and it will be kinder, more mellow, and more mature, and therefore not so liable to distort. These advantages may be fractional but they do exist and the old craftsmen who matured their oak for 14 years under the shade of apple trees, and surrounded by chest-high nettles, did so not through any mystique but for definite practical reasons.

Two methods of seasoning currently practised are air drying and kiln drying. Generally speaking air drying is slower and cheaper, kiln drying is simpler but more expensive. However, as air drying can never reduce the moisture content to much below 15 per cent, kiln drying is always necessary for lower levels; but as certain timbers do not respond satisfactorily to kiln drying in the green state, preliminary air drying is advisable. Any lingering prejudice against kiln drying—and it still exists—has no basis in actual fact, for both methods are complementary.