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:
- I bought a moisture meter.
- I measured the moisture content (which was 35%).
- 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.)
- I decided not to use them again, and take control of
the situation myself.
- Start to learn how to dry it myself.
I have now been drying the wood for about 15 years without
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).