Kiln Drying

Timber dries out under the influence of warmth and low ambient moisture levels
in certain ways. As moisture migrates out of the log shrinkage takes place in
a manner which in general ruins it for furniture-making, except where green-wood
construction techniques are employed. Even so timber to be used for exterior
purposes usually needs milling early after felling and to be allowed to dry
reasonably slowly "in stick". Strictly speaking there is no such thing as
"naturally dried timber". Timber (trees) have a natural cycle of growth, death
and decay and at no time does it dry in a manner where it can be used and cut
into boards for standard furniture production.

Milled air dried timber in the Northern UK will dry to about 16% moisture content
and interior furniture with central heating and the sort of comfort levels we have
come to expect require a MC of 6 ~10%.

The answer is to kiln dry it by various means. The cheapest method is probably
the dehumidification kiln which is basically a fridge in reverse where slightly
warmed air is circulated over and through the stack. The warmth allows a higher
saturation moisture level and therefore more moisture to the coils on which it
condenses and is drained away.

The process is reasonably controlled and uses little electricity, but it is slow.
The process is also "reactive" in that should the surface moisture level reduce
quicker than expected a higher moisutre gradient within the timber may result
and damage the timber.
The gradient is the all-important aspect and the rule of thumb we work to is
to avoid anything more than 4% between surface and centre. If a higher gradient
is detected the dehumidification cycle can be switched off as the moisture in
the timber is given the chance to migrate to the surface.

Stacking Kiln - Our first Oak

More control can be employed
with Heat-Vent kilns and the heat
introduced can come from various
sources which might offer efficiencies
otherwise lost. The process is quicker
than with dehumidification and
more control can be exercised.

For example, well air dried stock
can be conditioned by raising the
temperature slightly but not
exhausting the moisture which allows
some migration from the centre and
reduces stresses.