Pithy Question

[Olaf Vogel]

I turn a lot of green wood and have finally worked out the right drying techniques to prevent cracking.
But (almost) every time I leave the pith in the form I have problems. And often the patterns around the pith are really interesting.

Many of the books, articles etc suggest to just leave it out, work around it etc.
However, none say why.

Has anyone figured out how to leave it in?

Thanks
Olaf

[John Keeton]

Many of the books, articles etc suggest to just leave it out, work around it etc.
However, none say why. Olaf

Olaf, at the risk of sounding too simplistic, I think the sources recommend leaving it out for the simple reason that it nearly always generates cracks. As to why, I guess I never really needed to know that. Many folks that do endgrain turnings soak the pith with CA glue, however, in that instance, it is a very small area.

Wish I had an answer for your real question - "how to leave it in".

[Prashun Patel]

I'm not an expert, but I always thought it was bkz the pith was the weakest and most porous part of the wood. Because there is a difference in the rate at which the pith and the rest of the wood loses moisture, shrinkage occurs nonuniformly and causes stresses that lead to breaks at the weakest points.

If you want to keep yr pith from cracking, I bet you have to a) find a way that causes its moisture release to be as slow as the rest of the wood, and b) strengthen it vis-a-vis the rest of the wood, so stresses don't impact it preferentially. Glue tries to address both of these.

[Thom Sturgill]

I recently heard a description of the problem that makes a lot of sense. When the tree was young it was exposed to a lot of flexing around what is now the pith. That created a pattern of fractures which never heal. They may close to the point that you cannot see them and then open back up during drying, but they've always been there. This would also explain why CA glue, which can use capillary action to seal close joints, would often work.

[David Reed]

Wood rounds including the pith crack when drying because the majority of the movement is in the radial direction; that is the radius of the log decreases. Since there is much less shrinkage in the tangential direction. The radius decreases and the circumference decreases less. This is why flat sawn wood is more mobile than quarter sawn.
This is the same principle when old mitered joints (ex picture frames) open at the corners. The wood shrinks along the width but not the length (OK slight difference but the geometry is similar). The points of the frame stay together and the width decreases, pulling the inner portion of the joint apart.

[Reed Gray]

I can hardly remember when I have gotten a log that did not have cracks off the pith before I tried to make bowls out of it. This includes trees that I have watched come down. In the drying process of whole logs, which is next to impossible, as the log dries, it shrinks. The outside and especially the ends dry faster than the inside which creates stress. Stress is relieved by cracking.

I did have an old logger tell me that the cracks that are in the tree to start with are wind shake, or cracking to relieve stresses from high winds.

If you want to include the pith, be prepared to use epoxy and super glue to fill the cracks.

robo hippy

[Olaf Vogel]

Thanks for the feedback.


Just to try and understand whats happening.
- clearly the pitch is all end grain so would dry out quickly. I've been sealing all end grain with log end sealer, on the outside of the HF. That seems to work in many cases.
Because the wood fibers shrink in cross section, the outer layers should have the most distortion. Not the inner.

- my impression was that the pith is the same as the heart wood, hence the most dense and least wet portion. Which is why I was surprised at all the problems.
- I've read in a few books that in a live tree, most of the water travels in the outer, fresher layers, not so much in the

I'll try the CA route the next time, but will have to look for a cheaper alternative since most of my pieces are over 12". And its been a real pain with end grain HF's.

Thanks for the advice!

[David Reed]

Wood rounds including the pith crack when drying because the majority of the movement is in the radial direction; that is the radius of the log decreases. Since there is much less shrinkage in the tangential direction. The radius decreases and the circumference decreases less. This is why flat sawn wood is more mobile than quarter sawn.
This is the same principle when old mitered joints (ex picture frames) open at the corners. The wood shrinks along the width but not the length (OK slight difference but the geometry is similar). The points of the frame stay together and the width decreases, pulling the inner portion of the joint apart.

Not Earth shaking, but I reversed the drying/shrinkage concept. It is the circumference that shrinks the most, as evidenced by the fact that flat sawn wood moves more than quarter sawn. Oops. Anyway, there is a differential of movement which makes for some huge stresses and the wood has no where to go.

[robert baccus]

The pith of a tree was created as fast growing terminal stem tissue, the fastest growth of a tree. this results in big weak cells. it actually sends harmones down the stem to slow growth at the growing tip of branches. it has been isloated and defined. on the subject of cracks it used to be considered sinful to leave any pith in a turning blank. the large majority of my turning is endgrain and cracking is a real but rare problem. thanks to end sealers. i rough turn ASAP green and endseal the outside only(vases and urns) except for the rim area and the base which get a double coat. these are fair sized pieces!)10-15" in height. i akways try to center the pith for equal warping but endsealers are the secret--that and 4-10 months drying. the old logger was right-old large trees will commenly have heart cracks. good luck------------old foresrter

[Jon McElwain]

I went to a gallery in the Empress Hotel in Victoria B.C. a couple of years ago. They had some turnings by Davoud Khosravi, one of which was a large end grain hollow form pot with the growth rings concentric to the opening. Without looking in or under, I told the eager sales lady that her $6000 pot was cracked. She looked at me indignantly and asked what on earth I was talking about. I told her that if she flipped the pot over, she would find cracks near the center of the bottom of the piece. She gently picked it up (it was around 20" dia, and 25" tall) and flipped it over onto a cloth on a counter. Sure enough, radial cracks were present. I told her it was natural and to be expected, but I hope I did not get Davoud in trouble!

[Ken Whitney]

Not Earth shaking, but I reversed the drying/shrinkage concept. It is the circumference that shrinks the most, as evidenced by the fact that flat sawn wood moves more than quarter sawn. Oops. Anyway, there is a differential of movement which makes for some huge stresses and the wood has no where to go.

I was just about to comment on your first post, but then saw this. Right on the wood movement.

This is often very evident in roughed out bowls, where the end-grain portions of the bowl blank move much less than the side-grain portions.

The pith isn't woody tissue and consists of fairly thin-walled cells which would be long dead by the time the tree is bowl-sized. I always assumed that this provided a route for faster water loss in cut logs, exposing the center of the log to drying prior to outer portions and resulting in cracking as the inner portions of the wood shrink faster than the outer.

[robert baccus]

Just an add-on. in the past i coated all surfaces with endseal after rough-urning. started doing a lot of deep boreing and somewhere figured out that leaving the inside open and unsealed actually pulls the wood together(the outside and rim is coated). it took courage to do this the first time but now is standard practice. i still have a few cracks(mostly in boxelder#*%@) but not many. this speeds up drying time by 50-60%. good luck--------old forester