I've read several posts that warn against working with stuff
that has The Pith in it. Seems like cracks and splits will start
there. And it'll be a hole in the bottom of your piece - making
it impossible to hold water - though why that's important, since
most turned vessels / hollow forms will never actually hold
any liquid, is an issue is a mystery to me.

So what is it about pith that makes turning anything with
pith in it a No No!?

charlie b

"charlie b" wrote in message
...

So what is it about pith that makes turning anything with
pith in it a No No!?


If you don't start right, with no cracks near the pith, it'll open up a bit
before closing under compression as the section dries on endgrain
orientation. On face grain orientation, the cracks can run right into the
work, being unrestrained above the pith.

I think what people usually mean when they say don't leave the pith is that
if you don't allow room on endgrain for the outside to move as it contracts,
trying to make smaller circles, it will split from the edge to the center.
Nothing to do with the pith on that, of course, they stop there, rather than
originate.

Charlie
I am not sure as to the "why" of not leaving in the pith, but I do have a
what. This is the place where the cracks start. If you do not mind the
cracks, leave it in. On hollow forms I often leave in the pith and turn the
sides thin to accomodate movement as they dry. Usually the pith does not
crack, but when it does it is a beaut.
______
God bless and safe turning
Darrell Feltmate
Truro, NS, Canada
www.aroundthewoods.com
"charlie b" wrote in message
...
I've read several posts that warn against working with stuff
that has The Pith in it. Seems like cracks and splits will start
there. And it'll be a hole in the bottom of your piece - making
it impossible to hold water - though why that's important, since
most turned vessels / hollow forms will never actually hold
any liquid, is an issue is a mystery to me.

So what is it about pith that makes turning anything with
pith in it a No No!?

charlie b

charlie b writes:

I've read several posts that warn against working with stuff
that has The Pith in it. Seems like cracks and splits will start
there. And it'll be a hole in the bottom of your piece - making
it impossible to hold water - though why that's important, since
most turned vessels / hollow forms will never actually hold
any liquid, is an issue is a mystery to me.

So what is it about pith that makes turning anything with
pith in it a No No!?


Wood expands and contracts differently in different dirrections of the
grain. Bruce Hoadley in his book Understanding Wood has a great
picture. He shows a round slice of a tree trunk, with a slit made
from the edge to the middle of the pith.

After drying, this round piece of wood looked like a pie with a slice
missing. If you kept the pith in a piece of wood, then the stresses
from the pith to the edge during the drying process would cause large
cracks.

--
Sending unsolicited commercial e-mail to this account incurs a fee of
$500 per message, and acknowledges the legality of this contract.

So what is it about pith that makes turning anything with
pith in it a No No!?

Not all tree have pith. What they are saying is that a green piece of wood that
still contains the center of the log is more likely to check due to internal
stresses caused by drying shrinkage. The forces are greater at the periphery,
and are nil at the center. These differential forces are the culprit. Such a
log will check at the outer edge and it may continue inward towards the center.
An engineer would say the center is the 'neutral axis'. Remove the neutral axis
and the shrinkage forces have less to work on. Sorta like removing the fulcrum
from under a lever.

Whenever I process a tree, I have my lumberjack cut down the middle of the log
with his portable sawmill. On a large log, he'll take out a 2" slab from the
center.

Dan

On Thu, 07 Sep 2006 23:55:31 -0700, charlie b wrote:

I've read several posts that warn against working with stuff
that has The Pith in it. Seems like cracks and splits will start
there. And it'll be a hole in the bottom of your piece - making
it impossible to hold water - though why that's important, since
most turned vessels / hollow forms will never actually hold
any liquid, is an issue is a mystery to me.

So what is it about pith that makes turning anything with
pith in it a No No!?

charlie b

Pretty pithy question,Charlie....

I turn a lot of bowls with the pith in, either because the log wouldn't be big
enough to work with if it was cut out, or in some cases the pith is where the
color and feature is...

Yeah, they crack sometimes, but that's the chance you always take with green
wood, right?

The link below (which I'll get the text done on some day) is an ash bowl where
all the color is in the pith and the bowl would be kind of dull without it...

https://home.comcast.net/~mac.davis/ash_bowl.htm

Mac

https://home.comcast.net/~mac.davis
https://home.comcast.net/~mac.davis/wood_stuff.htm

Hi Charlie,
This is the scurge of the turner.

Imagine making a cylinder out of wet clay. Now, surround that cylinder
of clay with some wet topsoil.

Let the cylinder sit and dry.

As it does, you will see the topsoil around the clay begin to split and
crack open, but the clay is relatively the same. The more it dries the
more the topsoil cracks completely apart and away from the clay
cylinder that is shrinking in size but not nearly as much or as fast as
the topsoil.

This is what happens to wood that is drying when it contains pith. Pith
wood is much more dense than the surrounding growth wood that envelopes
it. The non pith heart wood cell's are spread out from each other more
than that compared to the tightly packed pith cells.

So as both dry, the cells of the heart wood travel more, and the mass
of it shrinks more, as the water occupying those spaces is eliminated.
The pith though, has less water, and since the cells are more compact,
dont travel much to cling to each other. So it shrinks very little.

All that heart wood is shrinking and getting smaller, and the pith is
in the direct path of where the heart wood needs to go. So since the
pith is blocking the way, the heart has no choice but to split apart
from the shrinking it is doing.

Basically, heart/sap wood shrinks more than the pith as they both dry.

I wouldnt want liquids in my vessels and bowls either. Although I have
made some stunning vessels that would be nice to hold live flower
arrangements in. I opt to find a glass container that fits inside of my
vessels to hold the water, and not expose my wood to it. Liquid will
alter the shape of the wood once it starts entering the dry cells
again.

If you need info on cutting logs to produce bowl blanks, I have a
pictorial guide to donig this at my site. You can jump there by
clicking this link:

http://handturnedbowls.biz/data/how_to_saw_log.pdf

James "cad" Holland

Handturnedbowls.biz
charlie b wrote:
I've read several posts that warn against working with stuff
that has The Pith in it. Seems like cracks and splits will start
there. And it'll be a hole in the bottom of your piece - making
it impossible to hold water - though why that's important, since
most turned vessels / hollow forms will never actually hold
any liquid, is an issue is a mystery to me.

So what is it about pith that makes turning anything with
pith in it a No No!?

charlie b

For my 2 cents worth, I don't know if I have ever gotten a log without
it having some cracking coming off the center of the growth rings,
which is seldom in the center of the log. I have found that any cracks
that are left in a turned green piece will continue to get bigger as
the piece drys, no matter what you do to prevent it. Usually there is
one crack, roughly in a line through the center, and I will cut the log
on or through this line. You can wait until the piece is dry, and then
use all sorts of fillers to repair the crack, or just leave it as is
and call it art. I found that most of the time I was spending a lot
more time repairing things than it was worth, and now for my utility
bowls, I turn out all of the cracks, or junk the piece. I actually take
a box of 'unsanded, unfinished factory rejects to one show, and sell or
trade for very cheap. They do sell, they just aint pretty.
robo hippy
cad wrote:
Hi Charlie,
This is the scurge of the turner.

Imagine making a cylinder out of wet clay. Now, surround that cylinder
of clay with some wet topsoil.

Let the cylinder sit and dry.

As it does, you will see the topsoil around the clay begin to split and
crack open, but the clay is relatively the same. The more it dries the
more the topsoil cracks completely apart and away from the clay
cylinder that is shrinking in size but not nearly as much or as fast as
the topsoil.

This is what happens to wood that is drying when it contains pith. Pith
wood is much more dense than the surrounding growth wood that envelopes
it. The non pith heart wood cell's are spread out from each other more
than that compared to the tightly packed pith cells.

So as both dry, the cells of the heart wood travel more, and the mass
of it shrinks more, as the water occupying those spaces is eliminated.
The pith though, has less water, and since the cells are more compact,
dont travel much to cling to each other. So it shrinks very little.

All that heart wood is shrinking and getting smaller, and the pith is
in the direct path of where the heart wood needs to go. So since the
pith is blocking the way, the heart has no choice but to split apart
from the shrinking it is doing.

Basically, heart/sap wood shrinks more than the pith as they both dry.

I wouldnt want liquids in my vessels and bowls either. Although I have
made some stunning vessels that would be nice to hold live flower
arrangements in. I opt to find a glass container that fits inside of my
vessels to hold the water, and not expose my wood to it. Liquid will
alter the shape of the wood once it starts entering the dry cells
again.

If you need info on cutting logs to produce bowl blanks, I have a
pictorial guide to donig this at my site. You can jump there by
clicking this link:

http://handturnedbowls.biz/data/how_to_saw_log.pdf

James "cad" Holland

Handturnedbowls.biz
charlie b wrote:
I've read several posts that warn against working with stuff
that has The Pith in it. Seems like cracks and splits will start
there. And it'll be a hole in the bottom of your piece - making
it impossible to hold water - though why that's important, since
most turned vessels / hollow forms will never actually hold
any liquid, is an issue is a mystery to me.

So what is it about pith that makes turning anything with
pith in it a No No!?

charlie b

cad wrote:

Hi Charlie,
This is the scurge of the turner.

snip

This is what happens to wood that is drying when it contains pith. Pith
wood is much more dense than the surrounding growth wood that envelopes
it. The non pith heart wood cell's are spread out from each other more
than that compared to the tightly packed pith cells.

So as both dry, the cells of the heart wood travel more, and the mass
of it shrinks more, as the water occupying those spaces is eliminated.
The pith though, has less water, and since the cells are more compact,
dont travel much to cling to each other. So it shrinks very little.

All that heart wood is shrinking and getting smaller, and the pith is
in the direct path of where the heart wood needs to go. So since the
pith is blocking the way, the heart has no choice but to split apart
from the shrinking it is doing.

Basically, heart/sap wood shrinks more than the pith as they both dry.

I thought pith was softer than the surrounding wood. Trees often
rot from the pith outward. And having poked an awl into both
the pith as well as the non-pith surrounding wood. The pith felt
softer. And in the 3" diameter pieces of fruitwood trunk, with
the pith almost centered, the pith is very very small - in some
cases you really have to look closely to see it.

So why not drill out the pith in the bottom of a hollow vessel
and plug it with heartwood or sapwood?

charlie b

Charlie
Some do. Whatever works is the general credo of the wood turner.
______
God bless and safe turning
Darrell Feltmate
Truro, NS, Canada
www.aroundthewoods.com
"charlie b" wrote in message
...

So why not drill out the pith in the bottom of a hollow vessel
and plug it with heartwood or sapwood?

charlie b

Well I may be incorrect on it being harder, but as another person said,
it is smaller. This makes it more dense and harder, to compress. I use
the wrong words sometimes and am not as eloquent as some here.

From a practical view, there is no reason you cant drill it out and
plug it.

From a purist cult view though, the idea is to turn away everything you
don't want and end up with your bowl with only the turning tools to do
it, just like the wood turners of our past.

I have done some bowls with the pith in various angles of my bowls and
those bowls always get the most attention it seems.

It does make for very unusual bowls.

Whether you have pith or not, I always rough out my object, then let it
dry. Wood does some weird things drying and a green bowl will contort
if you finish turing it before it is dry.

cad
From
charlie b wrote:
cad wrote:

Hi Charlie,
This is the scurge of the turner.

snip

This is what happens to wood that is drying when it contains pith. Pith
wood is much more dense than the surrounding growth wood that envelopes
it. The non pith heart wood cell's are spread out from each other more
than that compared to the tightly packed pith cells.

So as both dry, the cells of the heart wood travel more, and the mass
of it shrinks more, as the water occupying those spaces is eliminated.
The pith though, has less water, and since the cells are more compact,
dont travel much to cling to each other. So it shrinks very little.

All that heart wood is shrinking and getting smaller, and the pith is
in the direct path of where the heart wood needs to go. So since the
pith is blocking the way, the heart has no choice but to split apart
from the shrinking it is doing.

Basically, heart/sap wood shrinks more than the pith as they both dry.

I thought pith was softer than the surrounding wood. Trees often
rot from the pith outward. And having poked an awl into both
the pith as well as the non-pith surrounding wood. The pith felt
softer. And in the 3" diameter pieces of fruitwood trunk, with
the pith almost centered, the pith is very very small - in some
cases you really have to look closely to see it.

So why not drill out the pith in the bottom of a hollow vessel
and plug it with heartwood or sapwood?

charlie b

Darrell, it seems to me that "whatever works" is often erroneously
driven by "because we can" and perpetuated by "because we ought to" An
example might be larger deeper hollow forms with narrow necks and
openings. IMO, some of us would be better served to hollow easily thru
a wide open bottom and plug it later than to hollow thru a narrow top
requiring much greater effort and expertise plus special tooling and
perhaps a need to add a difficult collar. Where is it written that "good
turners don't plug"? Plugs can be turned and decorated, maybe we need a
new term to describe them. eg. "closing ornament" OK, OK, So YMMV


Turn to Safety, Arch
Fortiter


http://community.webtv.net/almcc/MacsMusings

On 9 Sep 2006 04:53:04 -0700, "cad" wrote:

Well I may be incorrect on it being harder, but as another person said,
it is smaller. This makes it more dense and harder, to compress. I use
the wrong words sometimes and am not as eloquent as some here.

snip

Last year someone here, I think it might have been George, said something about
stored energy from "wind shakes" or something.... and that as the log dried the
heartwood sort of twisted or unwound the tension...
That ring any bells out there?
Mac

https://home.comcast.net/~mac.davis
https://home.comcast.net/~mac.davis/wood_stuff.htm

Arch
I hear you, brother. Just because "we can" does not mean "we should." On the
other hand, sometimes it is good to do a turning "just because I can"
because it extends turning expertise. The piece itself may be contrived or
ugly but what is learned may be beautiful. That said, a jar shape that is
turned in two or three pieces can be as demanding as one done in a single
piece, can look as lovely, and will be appreciated as much by the eventual
owner. One wonders if we turn for the beauty of the wood and the creative
experience so much as for the accolades of fellow turners who say "how on
earth did they do that?" (Mutter under breath, "and why would anyone
bother?"

--

______
God bless and safe turning
Darrell Feltmate
Truro, NS, Canada
www.aroundthewoods.com
"Arch" wrote in message
...
Darrell, it seems to me that "whatever works" is often erroneously
driven by "because we can" and perpetuated by "because we ought to" An
example might be larger deeper hollow forms with narrow necks and
openings. IMO, some of us would be better served to hollow easily thru
a wide open bottom and plug it later than to hollow thru a narrow top
requiring much greater effort and expertise plus special tooling and
perhaps a need to add a difficult collar. Where is it written that "good
turners don't plug"? Plugs can be turned and decorated, maybe we need a
new term to describe them. eg. "closing ornament" OK, OK, So YMMV


Turn to Safety, Arch
Fortiter


http://community.webtv.net/almcc/MacsMusings

You know Charlie, the point Arch and a few others just brought up, I
totally forgot about.

Enjoy it.

How ever you do it, its the doing that makes it so rewarding. Whether
you think its a beauty or crap, the experieince is what matters.

There are no rules here except for safety sake. Dont do anything that
risk your health when you turn, but by all means try new ideas you come
up with and have FUN.

I made a couple of large urns, composed of many pieces individually
turned, then glued together. They are beautiful to me, but that process
certainly didnt conform to any rules. I just thought it up, and did it.
I thoroughly enjoyed it too as I saw each new piece fit on to the total
piece.

It would certainly be along the same lines of doing what you proposed.

cad
charlie b wrote:
cad wrote:

Hi Charlie,
This is the scurge of the turner.

snip

This is what happens to wood that is drying when it contains pith. Pith
wood is much more dense than the surrounding growth wood that envelopes
it. The non pith heart wood cell's are spread out from each other more
than that compared to the tightly packed pith cells.

So as both dry, the cells of the heart wood travel more, and the mass
of it shrinks more, as the water occupying those spaces is eliminated.
The pith though, has less water, and since the cells are more compact,
dont travel much to cling to each other. So it shrinks very little.

All that heart wood is shrinking and getting smaller, and the pith is
in the direct path of where the heart wood needs to go. So since the
pith is blocking the way, the heart has no choice but to split apart
from the shrinking it is doing.

Basically, heart/sap wood shrinks more than the pith as they both dry.

I thought pith was softer than the surrounding wood. Trees often
rot from the pith outward. And having poked an awl into both
the pith as well as the non-pith surrounding wood. The pith felt
softer. And in the 3" diameter pieces of fruitwood trunk, with
the pith almost centered, the pith is very very small - in some
cases you really have to look closely to see it.

So why not drill out the pith in the bottom of a hollow vessel
and plug it with heartwood or sapwood?

charlie b

The pith is the innermost core of the tree and is soft and spongy. This is
usually quite small in any but young trees, as the size is reduced as the
woody tissue grows. 'Coring out the pith' is a common saying but it is
usually a lot more tissue that needs removing as the surrounding area is
prone to checking if not already having done so while the tree was still
standing. Any other part that would be open for debate as to whether it is
softer or harder would not be pith. The area around the pith could degrade
and become softer for different reasons and perhaps that is where the
confusion lies.

"cad" wrote in message
oups.com...
You know Charlie, the point Arch and a few others just brought up, I
totally forgot about.

Enjoy it.

How ever you do it, its the doing that makes it so rewarding. Whether
you think its a beauty or crap, the experieince is what matters.

There are no rules here except for safety sake. Dont do anything that
risk your health when you turn, but by all means try new ideas you come
up with and have FUN.

I made a couple of large urns, composed of many pieces individually
turned, then glued together. They are beautiful to me, but that process
certainly didnt conform to any rules. I just thought it up, and did it.
I thoroughly enjoyed it too as I saw each new piece fit on to the total
piece.

It would certainly be along the same lines of doing what you proposed.

cad
charlie b wrote:
cad wrote:

Hi Charlie,
This is the scurge of the turner.

snip

This is what happens to wood that is drying when it contains pith.
Pith
wood is much more dense than the surrounding growth wood that
envelopes
it. The non pith heart wood cell's are spread out from each other more
than that compared to the tightly packed pith cells.

So as both dry, the cells of the heart wood travel more, and the mass
of it shrinks more, as the water occupying those spaces is eliminated.
The pith though, has less water, and since the cells are more compact,
dont travel much to cling to each other. So it shrinks very little.

All that heart wood is shrinking and getting smaller, and the pith is
in the direct path of where the heart wood needs to go. So since the
pith is blocking the way, the heart has no choice but to split apart
from the shrinking it is doing.

Basically, heart/sap wood shrinks more than the pith as they both dry.

I thought pith was softer than the surrounding wood. Trees often
rot from the pith outward. And having poked an awl into both
the pith as well as the non-pith surrounding wood. The pith felt
softer. And in the 3" diameter pieces of fruitwood trunk, with
the pith almost centered, the pith is very very small - in some
cases you really have to look closely to see it.

So why not drill out the pith in the bottom of a hollow vessel
and plug it with heartwood or sapwood?

charlie b

I like to make solid wood, as opposed to face framed ply, furniture so
I'm familiar with wood movement - tangentially, radially and axially
(sp?) - at least when the wood is in the form of a board. If you don't
accomodate wood movement, which will ALWAYS happen due to changes in
ambient air's moisture content, your piece will blow itself apart or the
joints will open up eventually. And I'm familiar with internal stresses
that can lurk in a board and all the fun an games they can cause as you
do your stock prep (flat parallel faces, flat straight edges square to
theflat faces, ends square to both the edge and the faces) for a piece
of furniture. I've had a board "wishbone" as well as "cross legs" and
the scary one where the thin cut off dives into the saw’s throat plate
opening when being ripped. The riving knife takes care of MOST of these
potential kickback initiators. BUT - furniture making does not
typically involve green wood or cross sections of a part of a tree. We
are a patient lot, willing to wait a year or two for our stock to
stabilize BEFORE we begin to work with it (I've got a Bartlett Pear log,
en buole, under a tarp on my driveway which won't be ready to work with
for another 12 to 18 months.)

BTW - I've got, and have gone through, Hoadley's book. The Readers
Digest Condensed Version in Lee Valley's handy pamphlet that comes with
their shrinkage wheel is less expensive while still providing the info
needed on how much and where shrinkage will occur. IIRC, the LV info
also has the map of the US with relative humidity ranges contours -
handy if you live in Florida and are making a piece for someone who
lives in New Mexico.

I like Frank Klausz's explanation of the affects/effects (never can keep
those straight) of changes in moisture content in wood - thinking of
the grain as rubber bands- the "inside of the tree" grain being less
"stretched" than those towards the outside of the tree. Handy analogy
to minimizing cupping problems with joints when it comes to parts
orientation like dovetailed drawer sides to front joints (IDIOT - Inside
of Drawer Is Outside of Tree).

So let's talk about a cylindrical piece of wood 3" in diameter, the pith
running down the center of the cylinder and the grain perfectly
symetrical around the longitudinal (long) axis. And let's say, for the
purposes of this discussion, that we're talking about fresh cut wood -
specifically a fruitwood (which is what I'm working with and perhaps the
Worst Case Scenario) - with almost no pith (ie less than 1/32" in
diameter). Furthermore, lets have a 2 1/2" diameter hole down the
center that stops 1/2" from one end of the cylinder and leaves a "side
wall" thickness of say 1/4". If you made no attempt to control the
drying process, what would probably happen as it dries?

We know that axial changes are neglible (the length of the cylinder
won’t change much, if at all. Tangential changes are the killer when
working with plain sawn boards - twice that of radial changes.

The diameter would get smaller right? But what's that do to the
stresses and strains in the wood and how does the wood deal with them?

As the inside diameter gets smaller the wood cells on the inside face of
the wall shrink as they lose captured - intercellurer water? As long as
the strength of the stuff that hold cells together isn’t exceeded
everything hangs together.

The cells on the outside, which were "stretched" (rubber band analogy),
should "relax" a little, stay at about the same or lower tension - or -
it doesn't want to get "shorter" and must get squeezed/compressed.

So the change in wall thickness will be small because the wall is only
1/4” thick and the rate of drying between the inside wall and the
outside wall should be about the same so the inside and outside diameter
should just get smaller - assuming you sealed or caused the lost of
moisture via the end grain to be greatly reduced/minimized.

But what about the solid 1/2" thick "bottom", with the pith in the
center? Ah -there's the rub! Now the contraction - green to dry - if
over the full 3 inches rather than over 1/4”. Seems that cracking
usually begins in the bottom of a closed turned vessel - pith or no pith
- probably because you’ve got two relatively large surface areas of end
grain close together- which dries much faster than the side grain in the
walls.

I’ve noticed that turned green “weed pots” which have only a small hole
drilled into them seem to split crack almost anywhere - often starting
half way up the side “wall”

So we’ve got the inside wall losing moisture and getting smaller, the
outside wall losing moisture a little faster than the inside because of
more surface area being exposed to the air that has less moisture and
getting smaller faster than the inside wall. But if we could slow the
moisture loss at the outside wall so that if shrinks at the same rate as
the inside wall problem one would be solved?

It’s that damned all end grain bottom that raises all the hell.

Now I’m going to step a bit Outside The Box
Perhaps we should turn to samurai sword makers. For decorative
purposes, they create wonderful patterns in the steel by differing the
rate at which areas heat and cool. They do this with special clays -
probably refractory clays. They paint their designs on the pre-heat
colored blade using clay instead of paint and they carefully control the
thickness of the clay in different areas by the number of layers of
“paint” - thick means it heats more slowly than bare steel - and cools
more slowly as well so it won’t discolor as much as bare steel.

So maybe painting the end grain with different thickness of “end sealer”
would be a possible solution. But do you want the wood farthest from
the center to dry more slowly than wood closer to the center - or the
other way around?

Maybe I should’ve started with a cylinder so only the end grain of the
walls was of concern rather than adding the addiotional variable of the
end grain.

I used to do a lot of work at a main frame “data center” and hung out
with a lot of structural and electrical engineers doing computer models
of structural or electric distribution systems. One guy - a Phd - who
worked for GE Nuclear was working on the characteristics of the
interface between the cylindrical walls of a containment vessel and the
domed top of the vessel. The cylinder and the hemispherical dome they
understood - but where they joined was a BIG unknown - not something you
want to leave to chance if containing radioactive material is a
concern. Any structural engineers out there?

charlie b

They are beautiful to me, but that process
certainly didnt conform to any rules. I just thought it up, and did it.
I thoroughly enjoyed it too as I saw each new piece fit on to the total
piece.

'Play' is an important and large part of what we call creativity. Dan

"mac davis" wrote in message Last year
someone here, I think it might have been George, said something about
stored energy from "wind shakes" or something.... and that as the log
dried the
heartwood sort of twisted or unwound the tension...
That ring any bells out there?

Wasn't me, but FPL mentions that the stresses the twig endured persist into
the log, especially in softwoods. Take a look at time-lapse photography of
growing plants and watch them twist and turn.

"charlie b" wrote in message
...

BTW - I've got, and have gone through, Hoadley's book. The Readers
Digest Condensed Version in Lee Valley's handy pamphlet that comes with
their shrinkage wheel is less expensive while still providing the info
needed on how much and where shrinkage will occur. IIRC, the LV info
also has the map of the US with relative humidity ranges contours -
handy if you live in Florida and are making a piece for someone who
lives in New Mexico.

So does the FPL Wood Handbook, which you can download free or buy printed.
It'll answer a lot of your questions.
http://www.fpl.fs.fed.us/documnts/fp.../fplgtr113.htm


But what about the solid 1/2" thick "bottom", with the pith in the
center? Ah -there's the rub! Now the contraction - green to dry - if
over the full 3 inches rather than over 1/4". Seems that cracking
usually begins in the bottom of a closed turned vessel - pith or no pith
- probably because you've got two relatively large surface areas of end
grain close together- which dries much faster than the side grain in the
walls.

So put the thing on stickers like you do with your other wood. It's not
that the wood dries rapidly that causes the problem, but that it dries
unevenly.

Even the hole in bottom method , an old remedy, isn't as effective as simply
allowing drying stress to equalize.
http://i35.photobucket.com/albums/d1...e/4c0d5a44.jpg Thin
material, where a split is more difficult to organize, which leaves room by
its shape - dished - for the circumferential stress to push the wood into
air makes the difference in rate between end and face grain irrelevant.

On Thu, 07 Sep 2006 23:55:31 -0700, charlie b
wrote:

I've read several posts that warn against working with stuff
that has The Pith in it. Seems like cracks and splits will start
there. And it'll be a hole in the bottom of your piece - making
it impossible to hold water - though why that's important, since
most turned vessels / hollow forms will never actually hold
any liquid, is an issue is a mystery to me.

So what is it about pith that makes turning anything with
pith in it a No No!?

charlie b
I like to make natural edge bowls from the crotch area of
trees...(often just limbs), and I like them to be deep enough, so I
often just keep a close eye on the blank and keep some thin CA glue
and wood dust handy. If a crack 'seems' to start, I dust, fill, add
glue, sand and re-turn...etc... I 'almost' never lose a bowl, and
seldom does it detract so I'd be reluctant to show it.
.....and people buy them! So, it is just one more type of bowl to have
on the table. I make sure to have 'clean' items with no fill and no
cracks also, but where I show, al least, the weird, rough, off-center,
spalted, natural edged pieces sell better than perfectly symmetrical
stuff....so my CA glue is my constant companion.

Now all this being said, it makes a BIG difference which kind of wood
is being used. Some woods crack and check badly....some hardly at
all...and the thickness you prefer and the moisture content when you
start are critical. It jus ttakes experimenting...

Charlie, it sounds like from all the reading you're doing that you
already understand it better than many others here!

Bruce B and Dan B got it right, and you got most of the rest of the
story in your long post.

Most hardwood trees don't have a softer center ("pith") unless there's
some disease working on it. I notice that just about all the termite
infestations I've found are in the center of logs (stuff near the
surface is bark beetles or others, usually not termites until very late
stage), and many fungal infections as well. This could be because the
larger trees already have some cracks running inside the center of the
log from wind stress, weight of the tree or whatever.

Many people don't realize that only the outer surface of a tree is
actually growing. Excepting disease, if a tree doesn't have "pith" in
its small twigs then it won't have "pith" in its big trunks either. the
cells can't just change into another type. If you cut up a tree that
does have pith, the center of a log will have a pith that's only as
large as the pith found in its smaller branches.

The center (heartwood and sapwood) is just there acting as straws.
They're full of water, but not actually growing, dividing, etc. Despite
all the water in the center of a tree, I think there is some shrinkage
that happens to the oldest cells in the center. The biggest trees I've
cut up have often had big cracks in the center and I think this is
probably one big reason that infections can start there. Since this
happens pretty frequently people call it pith even though it's really
an infection of some sort.

As others have noted, this isn't the cause of cracking, cracking is due
to differential expansion rates in the three directions: longitudinal,
tangential and radial, and the residual stresses are greater than the
wood's failure stress.

Regarding the issue of untwisting, there are a lot of woods whose
growth produces spiral grain, and these are the ones that untwist the
worst as they dry, however it's also true that trees have differential
growth on the top and bottom of branches under heavy loads. Look up the
terms "compression wood" and "tension wood".



charlie b wrote:
I thought pith was softer than the surrounding wood. Trees often
rot from the pith outward. And having poked an awl into both
the pith as well as the non-pith surrounding wood. The pith felt
softer. And in the 3" diameter pieces of fruitwood trunk, with
the pith almost centered, the pith is very very small - in some
cases you really have to look closely to see it.

So why not drill out the pith in the bottom of a hollow vessel
and plug it with heartwood or sapwood?

charlie b