On 11 May 2005 19:58:09 -0700, "
wrote:

When it dries, why do you think wood splits from the center to the
perimiter?

If you care about this stuff, find yourself a copy of Hoadley's
"Understanding Wood". It's not light reading, but it's well-written and
fascinating. You can't be a really skilled woodworker unless you've
learned this stuff, and this is perhaps the best book on it (The US
Forest Products handbook isn't bad either, and can be read on-line for
free).


The reason why it splits isn't obvious. It depends on the fact that
tangential shrinkage (around the rings) is about twice radial shrinkage.
This ratio varies with species, but "twice" is OK as a rule of thumb.
This just isn't obvious without making a scientific study of drying
timber, making measurements and taking notes of them (or else reading
someone else's notes, like Hoadleys).

If the ratio was 1, then timber would just get smaller as it dried out -
no splits. As it is though, the timber turns into a series of rings,
each of which is now in tension. Do it on a halved log and the tendency
is for the rings to pull themselves straighter, hence cupping of sawn
boards. If you do this to a very ring porous timber like sweet chestnut
(and probably black ash too) then you might see "ring shakes"
developing, where the weak porous timber splits under the shearing
force, leaving each ring separate.

Now consider a disk from a log - we've all picked these things up after
some chainsaw felling, most of us have thought about making rustic
stools or tables from them. Yet it _never_ works. No matter what the
species, or how strong it is, you can't get one to dry without
splitting. As Hoadley himself admits, he's never dried one bigger than
4" without it going.

The reason why the cracking is so inevitable is related to Hooke's law
(the simple law of springs and suchlike). Strain (percentage change in
length) is proportional to stress (applied force), by a factor we call
the Young's modulus. This modulus varies between species, some much
stiffer than others.

Despite our natural tendencies, don't think about forces here. Think
instead about strains - the length changes. Timber varies between
species in the amount of shrinkage with moisture changes, the initial
moisture content and also the strength. However if we look at the total
strain from green tree to bone-dry then it's much more consistent and
also the strain to break timber in radial tension is consistent, at
about 4%. So _any_ strain greater than 4% will cause a crack, no matter
whether this is a weak timber where a small force caused this strain, or
a strong timber where it took a larger force, but that's also the same
size of force to cause that much change in size. Now as "drying" timber
(from vaguely green to vaguely dry) can be relied on to generate a 10%
strain, we can see that _any_ drying of disks in _any_ timber will cause
the cracks.

There are a couple of ways to avoid this.

PEG - a non-volatile glycol used by woodturners to displace water in
green timber, rather than evaporating it to dry. This way the timber
doesn't shrink.

Cutting a hole in the centre. This allows the disk to shrink as smaller
hoops. As the radial force is removed (the centre hole merely shrinks)
then the hoops happily shrink without generating the cracking force.

Allowing it to distort. If you saw a _thin_ disk and dry it, it won't
crack, but it will twist and buckle into a potato crisp shape (for much
the same reason potato crisps do). As a variant of this, a hollow
hemispherical bowl turned in green wood can also shrink without cracking
after turning, but you'll get distortion instead.

--
Cats have nine lives, which is why they rarely post to Usenet.