Seasoning

Also called: air drying, kiln drying, conditioning timber

Seasoning is the process of drying timber down to a moisture level fit for use, whether that happens slowly in the open air or quickly in a heated kiln. As the wood's moisture evaporates, its fibres shrink, and the timber becomes smaller in bulk and lighter in weight.

How wood shrinks

Shrinkage during seasoning is not even. Timber shrinks mainly along the curve of its growth rings rather than evenly from the outer face toward the centre, so how a plank moves depends on where in the log it was cut. Overall, the width and thickness of a piece of timber can shrink by as much as eight per cent during seasoning, while its length is barely affected. Species dry at different rates, too: Dutch elm, for instance, dries out quickly once felled, and while it is still green its sapwood shows a colour clearly different from its heartwood.

Air drying

Air drying's chief aim is to evaporate as much moisture as possible before the wood is used, or before it goes on to a kiln. It suits a shed open at the sides and ends, with the roof carried on pillars, which shelters the stacked timber while still letting air move through it. A sawmill's own schedule sometimes cuts air drying short, sending the wood on to the kiln at a higher moisture content; some mills now use faster pre-drying technology that also gives more control over drying green timber, freshly felled wood that has not yet lost its moisture. Once a first period of drying was done, logs in the timber yard were often re-cut into smaller joists, planks or boards, then stacked again to keep drying. Nineteenth-century practice set out clear minimum times. General carpentry timber was not meant to be used less than two years after the tree was felled. As Tredgold wrote in 1871: "For joiners' work it requires four years, unless other methods be used; but for carpentry natural seasoning should have the preference, unless the pressure of the air be removed." By removing the pressure of the air he meant drying the wood in a vacuum, as an alternative to seasoning it naturally.

Kiln and other faster methods

Industrial kiln drying uses heated air inside a large, sealable room; trolleys wheel the stacked, green timber inside for controlled drying, and doing it properly calls for specialised knowledge. Where air drying alone can take many months, a kiln can bring timber fit for building work in three to six days. One further process on record took five to eight hours, after which about a fortnight of good-weather seasoning finished the timber for use; the process itself is not identified. Two older methods worked by different means again. Soaking timber in water seasons it, though slowly, and suits wood destined for use in water or other damp settings. Boiling timber in oil seasons and strengthens it effectively, though the method is costly; it was used for the wooden teeth of mortise gears, gear wheels with teeth fitted in as separate pieces rather than cut from the solid.

Natural against artificial: a disagreement

Not everyone in the trade agreed that speed came free. Ellis (1908) held that natural seasoning kept timber stronger and more durable than any artificially accelerated method. Hasluck (1907) took a similar view of the trade-off: artificial processes shorten the time between felling and use, he wrote, but the strength and toughness of the timber suffers for it. Because seasoning by natural means was costly and slow, some experimenters tried treatments meant to solidify the sap and stop the wood shrinking without weakening it; whether these treatments actually worked was not established.