When it dries, why do you think wood splits from the center to the
perimiter? I think I have finally worked it out, just wanted to know
what you all think first

Dean

In article .com,
" wrote:

When it dries, why do you think wood splits from the center to the
perimiter? I think I have finally worked it out, just wanted to know
what you all think first

Because the wood gnomes are tall and thin. They live in the center of
the logs and when the tree is cut they need to get out to find a living
tree. They can't move lengthwise in the log for physiological reasons,
so they force their way out from the center to the perimiter.

Geez. What do *YOU* think? Probably some of that differential drying
rate crap...

--
~ Stay Calm... Be Brave... Wait for the Signs ~
------------------------------------------------------
One site: http://www.balderstone.ca
The other site, with ww linkshttp://www.woodenwabbits.com

On Wed, 11 May 2005 21:36:45 -0600, Dave Balderstone
wrote:

Geez. What do *YOU* think? Probably some of that differential drying
rate crap...

That's well out of line Dave, it's _not_ obvious why timber splits
radially.

wrote in message
oups.com...
When it dries, why do you think wood splits from the center to the
perimiter? I think I have finally worked it out, just wanted to know
what you all think first

Dean


It doesn't split radially from center to perimeter, rather perimeter to
center.

These folks have it pretty well worked out.
http://www.fpl.fs.fed.us/documnts/fp.../fplgtr113.htm

Enjoy.

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.

Angy! Wow great reply!

Ok so why does it strain that way, that is the real question. Why does
it shrink on the outside more than the inside?

The reason I think is this. When I look at a round disk of wood, as its
drying, you can see the darkness change as it dries. It dries faster on
the outside of the disk, than the inside. That's why it splits.

Why does it dry faster on the outside of the disk? Because it has more
surfaces through which to evaporate (i.e. the outer surface, where the
bark may be). The inside part of the disk can only evaporate through
the front of back of the disk. Anyway, the disk does look like its
darker on the inside and dry on the edge, when I look at one, its
fairly obvious.

Well, anyway, that's my hypothesis anyway.

Comments?

Dean

" wrote:

Angy! Wow great reply!

Ok so why does it strain that way, that is the real question. Why does
it shrink on the outside more than the inside?

Because it reduces in direct proportion to the initial size as Andy
notes--ergo, larger diameter reduces more than smaller.

The reason I think is this. When I look at a round disk of wood, as its
drying, you can see the darkness change as it dries. It dries faster on
the outside of the disk, than the inside. That's why it splits.

Why does it dry faster on the outside of the disk? Because it has more
surfaces through which to evaporate (i.e. the outer surface, where the
bark may be). The inside part of the disk can only evaporate through
the front of back of the disk. Anyway, the disk does look like its
darker on the inside and dry on the edge, when I look at one, its
fairly obvious.

The moisture has to diffuse to a surface before it can evaporate and
diffusion is driven by concentration differential and controlled by the
material properties. In a uniformly porous material, diffusion is more
nearly homogenous in all directions where as in wood it is constrained
much more to go in the direction parallel to the growth rings. The more
porous-diffuse the wood, the more extreme the differential.

Dean,

As you probably see wood drying/shrinkage is not a simple subject.

There are lots of resources on the web, check the link below and look at the
diagram on the 4th page. I find it helpful to understand how a piece of
wood will change shape as it dries.

http://www.uvm.edu/extension/publica...mberdrying.pdf

Drying the outside of a full round rapidly will contribute to the spliting
but that is not the root of the problem. If you dry a full round very
slowly in controlled conditions it will still split.

as mentioned the "ratio" of tangential to radial shinkage is the key.

As to your question of WHY is it different, again not a simple answer. The
theory that I find easiest to accept and remember is that the wood rays
restrict shrinkage in the radial direction.

(it does not shrink more on the outside than inside, it is the direction)

Rays are fibers running radially from the pith to the surface.

Hope this doesn't create more confusion.

Glen
wrote in message
ups.com...
Angy! Wow great reply!

Ok so why does it strain that way, that is the real question. Why does
it shrink on the outside more than the inside?

The reason I think is this. When I look at a round disk of wood, as its
drying, you can see the darkness change as it dries. It dries faster on
the outside of the disk, than the inside. That's why it splits.

Why does it dry faster on the outside of the disk? Because it has more
surfaces through which to evaporate (i.e. the outer surface, where the
bark may be). The inside part of the disk can only evaporate through
the front of back of the disk. Anyway, the disk does look like its
darker on the inside and dry on the edge, when I look at one, its
fairly obvious.

Well, anyway, that's my hypothesis anyway.

Comments?

Dean

wrote in message
ups.com...
Angy! Wow great reply!

Ok so why does it strain that way, that is the real question. Why does
it shrink on the outside more than the inside?

The reason I think is this. When I look at a round disk of wood, as its
drying, you can see the darkness change as it dries. It dries faster on
the outside of the disk, than the inside. That's why it splits.

Why does it dry faster on the outside of the disk? Because it has more
surfaces through which to evaporate (i.e. the outer surface, where the
bark may be). The inside part of the disk can only evaporate through
the front of back of the disk. Anyway, the disk does look like its
darker on the inside and dry on the edge, when I look at one, its
fairly obvious.

Well, anyway, that's my hypothesis anyway.

Comments?

Had you read the information in the FPL post, you'd have the answer without
having to hypothesize. And it's not greater surface. The side of a straw
is greater in surface than the walls, but the water runs end to end, anyway.

"With respect to shrinkage characteristics, wood is an anisotropic
material. It shrinks most in the direction of the annual
growth rings (tangentially), about half as much across the
rings (radially), and only slightly along the grain (longitudinally).
The combined effects of radial and tangential
shrinkage can distort the shape of wood pieces because of the
difference in shrinkage and the curvature of annual rings."

In article , Andy Dingley
wrote:

That's well out of line Dave, it's _not_ obvious why timber splits
radially.

Apparently attempts at humour aren't obvious, either.

--
~ Stay Calm... Be Brave... Wait for the Signs ~
------------------------------------------------------
One site: http://www.balderstone.ca
The other site, with ww linkshttp://www.woodenwabbits.com

"George" george@least wrote in message
...
Had you read the information in the FPL post, you'd have the answer
without
having to hypothesize. And it's not greater surface. The side of a straw
is greater in surface than the walls, but the water runs end to end,
anyway.


Read before posting, dummy! Sides of straw greater area than the _ends_ is
what I meant to say. Structure of wood is designed to move fluid up and
down. That's why end grain loses 10-15 times faster than face, which loses
faster than quarter.

wrote in message
oups.com...
When it dries, why do you think wood splits from the center to the
perimiter? I think I have finally worked it out, just wanted to know
what you all think first

Dean


I think the easiest way to understand it is this - think of a flat sawn
board. As a woodworker, you know the board will shrink most in width, a
little less in thickness, and almost not at all in length. It is the
difference in shrinkage between width and thickness that matter. Then think
of that board within a round log. Because the log is shrinking radially
greatly, but in diameter only slightly, the fibers are pulled apart radially
around the log.

Not a scientific explanation, but an easy one to grasp.

--
********
Bill Pounds
http://www.billpounds.com

Yes, you are describing the effect, but not the cause.

I have heard that wood disks can be dried without splitting, if its
done very very slowly over a couple of years. So this to me says
something about the rate of drying being important to understand, not
just the anisotropic behaviour or wood radially and tangentially.

Which is why I was saying it seems that the drying process is faster on
the outer rings of the disk, since its more exposed with additional
surfaces (or other properties of those layers of the wood are more
prone to drying for some reason).

Dean

"Pounds on Wood" wrote in message
...

wrote in message
oups.com...
When it dries, why do you think wood splits from the center to the
perimiter? I think I have finally worked it out, just wanted to know
what you all think first

Dean


I think the easiest way to understand it is this - think of a flat sawn
board. As a woodworker, you know the board will shrink most in width, a
little less in thickness, and almost not at all in length. It is the
difference in shrinkage between width and thickness that matter. Then
think
of that board within a round log. Because the log is shrinking radially
greatly, but in diameter only slightly, the fibers are pulled apart
radially
around the log.

Not a scientific explanation, but an easy one to grasp.


Also a bit backward. As the FPL says, the shrinkage tangentially is
greatest. Thus the radial checks - perpendicular to tangential force.

Take a log and measure a chord which does not cross latewood near the
center. Let's say it's an inch at best at 20 years. Now go out near the
outside and measure a chord of say 5 inches at seventy. With a 10% rate
of shrinkage, it means the interior wants to lose 1/10 of an inch, the
farther, 5/10. The difference is made up in open air as the radial split
opens.

On 12 May 2005 07:28:39 -0700, "
wrote:

Comments?

Sometimes I wonder why I bother posting.

On 12 May 2005 13:08:13 -0700, "
wrote:

I have heard that wood disks can be dried without splitting, if its
done very very slowly over a couple of years.

I've heard that Elvis is still alive and serving chips, but that doesn't
make it true.

You have two choices here. One is to make some disks and spend a few
years drying them very carefully indeed. Come back in about 10 years and
tell us how you got on. The second is to listen to the explanations, or
if you don't believe the peanut gallery here (not entirely unwise!) go
look it up for yourself in Hoadley or the Forest Products guide.


There's no _need_ for you to listen to or believe anything you read
here. But equally there's no need for you to ever learn anything. That's
not our problem.

Andy Dingley wrote in
:

snipped

There's no _need_ for you to listen to or believe anything you read
here. But equally there's no need for you to ever learn anything. That's
not our problem.

It may not have been the motive, but if it had been a troll, I doubt it
could have been more successful. Some of the more knowledgable and usually
helpful folks responded.

Patriarch

On Thu, 12 May 2005 12:41:36 -0700, "Pounds on Wood"
wrote:

I think the easiest way to understand it is this - think of a flat sawn
board. As a woodworker, you know the board will shrink most in width, a
little less in thickness, and almost not at all in length. It is the
difference in shrinkage between width and thickness that matter. Then think
of that board within a round log. Because the log is shrinking radially
greatly, but in diameter only slightly, the fibers are pulled apart radially
around the log.

Not a scientific explanation, but an easy one to grasp.

That's an excellent explanation. It also accounts for why the splits
are wider, further from the center.

"Andy Dingley" wrote in message
...
On 12 May 2005 07:28:39 -0700, "
wrote:

Comments?

Sometimes I wonder why I bother posting.

For the rest of us that did read the interesting information you had?

On Thu, 12 May 2005 11:59:19 +0100, Andy Dingley
wrote:

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).

Snip

Thanks for the post, Andy- particularly useful, as I've been scavaging
deadfall for the lathe lately!


Aut inveniam viam aut faciam

On 12 May 2005 13:08:13 -0700, "
wrote:

Yes, you are describing the effect, but not the cause.

The cause is that the wood cells are very much longer in the vertical
[alive and well] direction as compared to the horizontal. Think of a
lot of thin tubes lightly glued [bonded] together. Suppose they all
shrink equally in all directions. Now think of the structure of the
tree, and simplify it. One cell in the center. More around that,
more around that, more around that ...ever increasing circles of
cells. Now let all shrink in all directions equally, so losing the
same percentage per cell. The inner ones will each shrink by the same
absolute amount as those individual cells around the outside, but
since there are more around the outside, the total absolute amount
will be greater there.

If this is correct, then [near as dammit is to swearing], thinking in
term of a sequence of circles, the percentage of crack-space in an
inner circle should be the same as the percentage of space in the
outer edge, and in fact throughout the material: More material, more
space, same percentage.

If still confused, think of expansion and contraction of a steel rod
due to heating/cooling. A longer rod will expand and contract further
than a small one. The measure of wood around the outside of a log is
greater than around the center, and the tree grows in definite rings.

Why does it crack in the first place? Bonding. Even steel cracks
under pressure since it is uneven throughout, even though it looks
uniform outside of a microscope. I'll quit now. My brain is
exploding.

On Fri, 13 May 2005 09:01:53 -0400, Guess who
wrote:

If still confused, think of expansion and contraction of a steel rod
due to heating/cooling.

That's a bad analogy. Steel is isotropic - it has the same behaviour in
all directions. Timber is anisotropic, the shrinkage is different on all
three axes. A "steel tree" wouldn't crack as it dried out.

Most carpenters know that timber shrinks "crosswise" but not
"lengthwise". That's still not enough to cause cracking. What many
carpenters don't realise is that the tangential shrinkage is twice the
radial shrinkage. It's this, coupled with the fact that trees are made
of nested cylinders, which causes the radial cracking.

The outer _may_ dry out faster than the inner (although this is much
overshadowed by the effects of the ends, and the much greater water
transport lengthwise). If you bake the outside of a log to dry it you
could even "case harden" the timber and cause checking - however this
would be the usual honeycomb checking of bad seasoning, not the
inevitable radial cracking of drying logs in the round.

A consideration of shrinkage and the geometry will show us that a dry
log cracks, no matter how slowly or carefully you do it. Thinking about
the strains rather than the stresses will show that all timbers will do
this, not just "weak" ones.

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

In article .com,
wrote:
Yes, you are describing the effect, but not the cause.

I have heard that wood disks can be dried without splitting, if its
done very very slowly over a couple of years. So this to me says
something about the rate of drying being important to understand, not
just the anisotropic behaviour or wood radially and tangentially.

Which is why I was saying it seems that the drying process is faster on
the outer rings of the disk, since its more exposed with additional
surfaces (or other properties of those layers of the wood are more
prone to drying for some reason).

Dean


I imagine this would work, say you very carefully controlled the
drying of a tree trunk cross section so that it took 100 years, then
it would probably not split until after the first fifty years or so
had passed.


--

Larry Wasserman Baltimore, Maryland