"Mike Marlow" wrote in message
...
This is where all of the discussion here about wood swelling with moisture
etc., goes a little astray. First off - yes it does swell with moisture,
I
am aware.

The rates that we
see published for wood expansion and moisture absorbtion are for raw,
unsecured wood. They do not apply to sealed wood and they do not apply to
wood that is secured as in a mortice and tenon.

You are incorrect. The fibers will expand and contract when the moisture
gets to them, and at the same rate, less compression set. You may slow the
arrival with occlusive finishes, but wood loves water and will find a way.

The reason M/T joints work loose is related to this reality. What went
together "spit tight" at 12% MC is less so at 8 or 4. The joint may then be
subject to racking strain, compressing some contact areas which will make
the joint sloppy even when the MC returns to 12%. That's why the glue and
pegs - to deny motion even when the tenon tries to become smaller in the
mortise.

Works the other way, too, though more slowly. Joints made at 4%, if the
don't split the wood getting to 12, begin to develop some compression set
which remains after the cycle returns to 4, progressively loosening the
joint.

Glue or surface fiber will eventually succumb, but the M/T will still bear
load in design direction, and if pegged, should not withdraw.