What Is It About Pith?

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.

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.

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

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...

Mac

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 > I've read several posts that warn against working with stuff

snip

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

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.

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 th> cad wrote:

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

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

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?"

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 certa> cad wrote:

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.

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

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

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.

So does the FPL Wood Handbook, which you can download free or buy printed. It'll answer a lot of your questions.

> >

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.

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.