On Mon, 08 Dec 2003 13:52:32 GMT, "js"
wrote:

True, but doesn't slowing down the rate at which it drys out reduce the risk
of splitting?

No, because you're dealing with rings.

Drying generates a strain across the board, which leads to shrinkage.

If you dry a board evenly and at moderate speed, it shrinks evenly. If
you dry it rapidly, or unevenly, then it will develop cracks
(typically the honeycomb checking of poor seasoning). This creates a
false impression that "all cracking is caused by poor drying
technique".

If your board is flat and radial-sawn, then this stress causes linear
shrinkage. You get a smaller, flat board. You may notice that radial
shrinkage is less (typically half) the tangential shrinkage.

If your board isn't radial, and it's large relative to the curvature
of the rings within it, then this difference in shrinkage causes the
board to cup.

If the "board" is a ring, containing the centre, then the "ring"
constrains it from cupping. The rings try to shrink more along their
circumference than they do on their radius, so the board is stressed
by circumferential tensions that can't go anywhere. It can't shrink,
as the centre is in the way, and it doesn't shrink so much radially.
If you cut a hoop with a central hole, or if the centre of the log is
soft, then the ring _can_ shrink, by compressing the area of the
centre. In a solid log though, it can only build up a tension.

In any solid disk of more than about 4" diameter, then drying it to
typical "dry" levels generates enough strain to crack it. The strength
of timber varies widely with species, but the strength/stiffness
ratios less so, and so all species show this same problem.

If the disk is off-centre, then some of this compressive stress may be
dissipated instead by warping instead (like the non-radial board). But
any solid section that includes the heart is almost guaranteed to
develop a radial crack.

You _may_ find a dry disk that's uncracked. But there's a huge stress
in there, and any disturbance is likely to cause it to fail
dramatically.

By using PEG-1000 to displace water, you may reduce shrinkage, and
avoid cracking that way. If the disk is thin, then it may warp out of
flat instead. But this radial cracking problem won't just go away, no
matter how carefully you dry it.

--
Die Gotterspammerung - Junkmail of the Gods