Wood changes its size when its moisture content changes. The size change is different along each of the planes of wood growth - radial (R at right), tangential (T) and lengthwise (X). So, it’s important that woodworkers know when the moisture content of their wood has reached equilibrium with the workshop, so that its dimensions have settled down.

What is the moisture content (MC) of wood compared to its dry weight? Typically 60% for green hardwoods, up to double that for softwoods. The moisture is in two forms - chemically bound and free. The first aim of seasoning lumber is to degrade the chemical bonds to water, so that all that remains is free to move. This happens faster at higher temperatures, which is why kiln drying saves so much time. However, too-high kiln temperatures degrade wood, and most kiln drying is done as quickly as possible to produce cheap construction grade softwood. That’s why kiln drying has a bad name among instrument makers.

Skilled kiln drying can actually produce better wood than air drying. When wood is air dried, the outside dries faster than the inside, and this results in residual stresses in the wood. If the kiln humidity is controlled to match that of the wood throughout the process, and the high temperature is used just to speed up the rate at which the chemical bonds are degraded and moisture equilibrium is reached, residual stresses can be significantly reduced below those of air dried wood. But, there is no way to know, just looking at a piece of wood.