Wood movement in furniture should not be underestimated. All the things in your home made of real wood are expanding and contracting to achieve the same moisture content of the air around it. Interestingly wood hardly moves along its length. Nearly all movement is across its width. A 36″ (900mm) wide table top can move by as much as half an inch (12mm) with enough strength to split the wood. This is a challenge when using wood to make furniture.
Wood begins life wet. As a tree it is saturated with moisture. You can see that fresh cut or “green” wood oozes sap. Sap is predominantly water. It’s pretty obvious that most of this water must go before the wood is suitable for making things. There is a long tradition of stacking newly cut timber boards, with spacers, and allowing it to slowly give up its moisture to the air. But standard practice today is to dry it in a special kiln, using heat to drive down the wood’s moisture level.
The moisture content of wood will always change until it reaches equilibrium with its environment. Relative humidity is directly proportionate to moisture content. In the UK the average outdoor humidity is 65% which equates to a moisture content of 12%. Near the coast the average is nearer 16%. The average relative humidity of UK homes is 50% (= 9% moisture content) in summer and central heating drives it down to 30% (= 6% moisture content) in winter. So air drying wood will bring moisture content down to its average environment, around 12% whilst kiln drying brings down moisture content to nearer 7%. The wood’s moisture content will then change to equal that of its new environment, your home.
So what does all this mean for the woodworker? Here are a few pointers…
- Acclimatise the wood where you intent to use the finished article for a couple of weeks.
- Wait until the wood has acclimatised before cutting it to its final size, it may have changed shape (see drawing).
- For wide pieces out of sight use materials which move less, such as plywood and fibre board, e.g. cabinet back boards.
- Use finishing products that reduce the wood’s ability to transfer moisture, such as wax and varnish, but make sure you apply it on all sides, even the unseen ones, to avoid uneven moisture transfer and warping.
- Use techniques to accommodate movement such as expansion gaps under the skirting when laying wooden floors, “floating” panels in cabinet frames and slotted pilot holes across the grain.
Wood is such a beautiful material to work with it is better to accommodate its natural tendency to move rather than to try to prevent it or, worse, live without wood.