Why Does New Timber Split? It Is Still Full of Water

Water in timber sits in two distinct places and behaves quite differently. Free water fills the hollow cavities of the cells and can be removed without the wood changing dimension at all, since the cavities simply empty. Bound water is held within the cell walls themselves, chemically associated with the wood substance, and removing it makes the walls shrink, which is what causes timber to change size and shape as it dries. All the free water leaves before any bound water does, so drying produces no movement at all until a threshold is passed, and everything that happens afterwards is shrinkage.

Drying the outside faster than the inside creates the classic defects:

• •The surface shrinks while the wet core does not, putting it in tension
• •That splits the surface, producing checks along the grain
• •End grain dries fastest of all, so ends split first
• •The core later shrinks against a set surface, tearing internally
• •Uneven drying between faces makes the board cup, bow or twist
• •Case hardening leaves stresses that release when the board is sawn

A drying kiln does not simply apply heat, and the humidity control is the important part. Air in the chamber is held at a temperature and a humidity that together set how fast moisture can leave the surface, and at the start the humidity is kept high deliberately so that the surface dries slowly enough for the interior to keep up. As the average moisture content falls, the humidity is reduced in stages according to a published schedule specific to the species and the thickness, since dense hardwoods must be dried far more slowly than softwoods. A conditioning phase at the end raises the humidity again to relieve the stresses that drying has left.

Air drying was the only method for most of history and it survives alongside the kiln for specific purposes. Boards are stacked with thin spacers between every layer so that air passes on all sides, the stack is covered against rain and weighted on top to resist distortion, and it is left for a period traditionally reckoned at a year per inch of thickness. That is slow and ties up capital and space, and it cannot reach the low moisture content a heated interior requires, so air-dried timber is frequently finished in a kiln. Its advantages are lower cost, no energy use, and gentler drying that some makers consider leaves better timber.

Dried timber is not permanently dry, which is the point most people miss. Wood continuously exchanges moisture with the air around it and settles at a content determined by the humidity of its surroundings, so a board dried to the right figure for a heated interior will take up moisture again if stored in a damp shed and will move when it is finally installed. That is why timber is dried to a target matching its intended environment rather than to a single standard, why it is acclimatised in the room before fitting, and why furniture built in a damp workshop opens at the joints once it reaches a centrally heated house.

Water filling cell cavities leaves without any change in size, and only water held in the cell walls causes shrinkage, so nothing moves until the cavities are empty. Drying the surface faster than the core splits it and leaves internal stresses, so kilns control humidity as much as heat and follow schedules by species and thickness. Wood then re-equilibrates with whatever air surrounds it afterwards.