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

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.

What Is It About Pith?

Sep 08, 2006 21 Replies

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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:

What Is It About Pith?

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