The Technique of FURNITURE MAKING

Air drying

Ideally, hardwoods should be sawn and piled for drying when humidity values are high and initial drying relatively slow; but in practice most merchants or dealers cut and pile timber throughout the year, stacking the boards fairly close so that the drying-rate is not too rapid. On no account should green wood be close piled without air spaces between each plank, as this will inevitably invite mould and fungi attack (the same applies to seasoned wood unless it can be adequately protected from the rain). The normal procedure, therefore, is to sweep the sawdust from each plank as it is cut and stack as soon as possible on firm, level foundations in such a way that an uninterruped flow of air is maintained over every surface. The stack itself should be built up on brick or concrete piers or creosoted timber/lumber sleepers, with wood bearers across the piers not less than 9 in (23 cm) above ground level, and the planks or boards piled exactly over each other and separated by stickers of neutral wood—fir, poplar, horse - chestnut, etc. — placed from 18 in (45 cm) to 24 in (60 cm) apart according to the thickness (Figure 9). These stickers can be 1 in square section (25 mm) for thick planks and 3/4 in (19 mm) by 1/2 in (12.5 mm) for thinner boards, the 3/4 in (19 mm) dimension laid flat so that the stickers do not roll over as the boards are positioned. The line of the stickers must correspond with the position of the lower bearers, and they must be exactly over each other throughout the height of the stack, with the ends of the planks or boards well supported and not overhanging each other in random lengths, for the function of the sticker is not only to permit the free passage of air throughout the stack but also to restrain as much as possible any tendency to warping.

When completed the top of the stack should be weighted down with odd wood slabbing, and well protected from heavy rain and hot sun by a suitable lean-to roof with a generous overhang all round. Rain driving into the sides of the stack is not harmful, always provided the timber has a chance to dry out, and therefore the stack should be kept as narrow as possible and never over 6 ft (1.82 m) wide. Shielding from very hot sun, however, particularly at the ends, is advisable in the early stages. Some form of
degrade (splitting, checking, etc.) must be expected on the ends where the moisture is drawn from the cut pores fairly rapidly, but this can be kept within reasonable limits by covering the ends with special paint. Unfortunately, merchants or dealers usually cannot afford these extra precautions for the common run of wood, but some square edge woods imported from tropical countries are end coated in this manner.

As a general rule most hardwoods are air dried out of doors and can remain there indefinitely, although at a later stage the better logs are usually moved into open-sided storage - sheds where they can be fully protected from the rain.

Drying times The usual time allowed for the air drying of timber to the normal equilibrium moisture content of from 15 to 22 per cent, according to the season, is one year for every inch in thickness; but given favourable conditions 1 in (25 mm) hardwoods piled in autumn will in all probability be down to 20 per cent by the following summer, and 2 in (50 mm) planks to the same moisture content a few months later. (Softwoods will dry to about 20 per cent moisture content in three months.) Further drying to 15 per cent is only possible in very dry weather, and in exceptional heatwaves 12 per cent might be reached, but under normal conditions the moisture content will swing backwards and forwards within the higher range, and any further drying time will make no difference beyond distributing the remaining moisture more evenly, and helping to equalize the built-in stains. For lower moisture contents other methods must, therefore, be adopted.