"JD" wrote in message
oups.com...
First, I don't own a moisture meter (haven't justified the purchase
yet). I was wandering, if my house is mainatained at 40% humidity, and
my garage is at from 50-60% humidity, how do I know when my bowls are
dry enough to finish? If I finish the bowls that have reached
equilibrium in my garage, they warp when I take them into the house. I
recently converted a refrigerator into a kiln and I'm measuring the
humidity in the refrigerator to try to establish a point at which I
think the bowls are dry. If I dry them to 40% humidity, shouldn't that
stop most of the movement I get when I take them into the house?
The kiln appears to be working well. No cracks. Humidity has dropped
from 85% to 70% since March 10 (this might be a little fast). I'm
using light bulbs on a thermostat to control the heat. Humidity is
being controlled via vents in the refrigerator (still learning how to
adjust these). I've tried to search the web for some information about
how long, how hot, how much ventilation, etc. I'm finding a mixed bag
of information (most are dealing with large commercial type kilns).
Any information you might provide would be appreciated.


Wood is dry relatively - depending on relative humidity of the environment.
Wood loses and gains bulk with moisture loss and gain, so it'll always move.
The wood can take a "set" if you overdry it, so if you're up to the risk,
you can do that, and minimize subsequent range of movement.

If you have a hygrometer, and they're cheap, you can monitor the RH and use
the tables in
http://www.fpl.fs.fed.us/documnts/fp.../fplgtr113.htm chapter
three to see what the moisture content by weight really is. Equilibrium is
indicated by no weight change, but also depends on no change in the RH.
While you're reading, take a look at the average shrinkage values for the
various species of wood. It's a good indicator of when you need to take the
first weight. You can also use them to cut your time to equilibrium by
gaming the grain and contour and how thin you cut, as long as you're pretty
satisfied with the shape you've given to the rough piece. One piece of
information to keep in mind is that the tables are based on shrinkage from
the FSP of about 30% moisture to oven dry, or zero. You'll get about two
thirds the average figures on most configurations, because you're only going
to around 10%.

Note another reality, which is that wood contracts on itself, so cutting
with a broad bottom and nearly vertical sides will give you more shrink, and
more shrink stress than cutting with a narrower bottom and sloped sides.

Adding heat in a closed, or nearly closed environment will effectively lower
the relative humidity, because warm air can carry more volume of water. It
will also energize the process of unbonding, the release of water held by
hydrogen bonds to the wood itself. If you're going to use heat and
enclosure, I suggest that you spin things up to eject as much unbound water
as possible centrifugally, and even blow compressed air through to eject
more. This will get you to and through the FSP more rapidly, because the
minimum ventilation won't have to dilute and carry away so much water.