For several years I have been buying in kiln-dried timber for my furniture making business, but I was unhappy with the stability of the timber due to its moisture content.

A tree in Britain starts to take up water in the spring within the sapwood, and does so throughout the summer, hence the terms, the ‘sap is rising’, and ‘summer growth’. The new cells are formed just under the bark, and this is why the tree gains size in its girth. The rings seen when a tree is cross cut shows the annular rings, each representing one years’ growth.

There are two distinctive areas of growth in our climate, those being summer and winter growth (easily seen in English Oak, Ash or Douglas Fir for example) As kids start counting the rings, they will probably be counting the winter growth which shows as the darker, more tightly packed cells, with the faster summer growth being shown as more open grained in between. Tropical hardwoods, such as West African mahogany shows a far more even growth throughout the year when seen in cross section, and the annular rings are far more difficult to count.

Timber merchants were buying in tree trunks, planking them into boards and selling them ready for use within the furniture industry after kiln drying them, but I my opinion, not to a satisfactory level. The British Standard moisture content for kiln dried timber is 12% or 2% either up or down. This is for any thickness that has been dried in a kiln. We were turning the wood into furniture, and our customer would complain that the wood was cracking, or joints were opening up after a few months in their homes.

I cut into a piece of 4” thick oak once with a rip saw, and wished that I had worn a raincoat, I was soaked. This led to a string of events:

1. I bought a moisture meter.
2. I measured the moisture content (which was 35%).
3. I called the timber supplier who informed me that I shouldn’t have accepted the timber from them as I unloaded it if it was unsuitable.  (I was young and nieve.)
4. I decided not to use them again, and take control of the situation myself.
5. Start to learn how to dry it myself.

I have now been drying the wood for about 15 years without many dramas.
So far, we have only used dehumidifying kilns, as we are furniture makers, not timber merchants, and we cannot justify the expense of more sophisticated kilns. In my opinion, the best way to dry oak is to air dry it for at least six months before putting it into the kiln, as the wood will have lost most of its moisture and become more stable.

At this stage, the moisture content will be in the mid 20’s, and my aim is to take it down to about 9 or 10%. This is slightly less than the British Standard but it will allow for some moisture take up when it leaves the kiln (about 1% per month). The workshop is quite dry, so there is not a dramatic rise over several months. I have shipped almost two container loads of oak furniture to the south of Spain without any problems, and the moisture content of the beams in the 16th Century castle was 10%.

Centrally heated houses in the U.K. seem to be about 10%, with about 2% either way. We have noticed that it makes a huge difference whether there are plants in the rooms, as they give off moisture during transpiration through the pores of their leaves. The moisture meter will read down to about 7% - 8%, and we have had furniture in a room where we could not get a reading.

Since we have been drying our timber to this level, we have had very few problems. One thing that does however occur occasionally, is that drawers will tighten in place within the carcase of the furniture, as the thin wood of the drawer side takes up moisture more readily once it is out of the stack and planed quite thin. (This is soon solved by planing a little off the edge of the drawer, as long as you can get it out!) This only tends to happen if the furniture is hanging around for a week or two before it is polished after it has been made. This problem has now been minimised as the polishing shop is drier now that the extraction fan exhaust hole through the roof can be closed when the fan is not being used, stopping the moist air from outside blowing in and raising the moisture content of the room.

As a result of a delivery of wet wood that the supplier refused to take back, we have taken to drying it ourselves, to a much more consistent and satisfactory level, without too many problems. We can buy in quality timber in tree form, mill it into boards, air and kiln dry it, turn it into furniture, and by doing so we have cut out a middle man, saved on our overheads, and been able to pass on this saving to our customers. We are now confident in the furniture being in good condition in twelve months time, and not expecting a phone call regarding the wood moving, (tables have almost been known to walk around the room on their own).