A Turning Question

[James Baldwin]

Hi, I just turned two French Pastry Rolls. I turned them out of a couple of apple wood tree limbs I had. They had been drying for a year. When I checked the moisture content before I started they showed 17 to 19 %. They still had the bark on. When I finished turning them they showed 0%. Then I applied a mineral oil/ beeswax finish. Will they be prone to cracking? If they do crack I'll just make some new ones out of some kiln dried scraps I have around. Jim

[John Kananis]

You mention bark, were they entire limbs? If so, they'll have the pith still and that will cause cracks and/or deformities. Making the moisture content kind of moot.

Hi, I just turned two French Pastry Rolls. I turned them out of a couple of apple wood tree limbs I had. They had been drying for a year. When I checked the moisture content before I started they showed 17 to 19 %. They still had the bark on. When I finished turning them they showed 0%. Then I applied a mineral oil/ beeswax finish. Will they be prone to cracking? If they do crack I'll just make some new ones out of some kiln dried scraps I have around. Jim

Edit: I should mention that my time on a lathe is much shorter than some of the other folks here so you'll find more experienced answers than mine I'm sure.

[Edward Weber]

I share John's concerns and would also ask, were they end sealed while drying?
When you turn away the bark and reveal the inner wood, stresses within the branch can be released, causing checks, cracks, warping, etc.
Fruit-woods are notorious for cracking if not dried slowly. If they hadn't cracked in a year, you might be okay.

[Bill Howatt]

John and Edward have stated the reality of the situation. I find 0% to be a bit strange since wood that has been in a house for a while is typically in the 7% range.
Regardless, wood is wood and definitely lacks the uniform properties of other materials like steel so you just never know with certainty what it is going to do and this means you will have to "wait and see". There can be fine cracks in wood that are very difficult to see and this is very true in smaller diameter pieces. These cracks are not going to cause the rolling pin to fly apart but depending on what gets rolled (such as a meat patty) you may be concerned about bacteria uptake. For pastry, not an issue, IMO.
Your oil/wax finish on the end-grain could help if further drying is going to happen.

[Richard Coers]

No idea what kind of moisture meter you are using, but to get zero percent moisture you must live in a desert there in Idaho. Fruit woods love to crack. No idea how that survived 1 year.

[James Baldwin]

Yes they were turned out of a complete limb. They were not sealed. I was mostly using the wood for fire wood but just happen to grab a piece to see how it would turn.The pith is still in them. I'll keep an eye on
them. I have one in our house and the other one went to my sister-in-law. This the one I keep. My wife just used it and said that it worked pretty good. I did notice that it's got a very slight crown in the center portion. It wanted to flex a little as I was sanding it. It's about 20" long. Jim

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[Brian Brown]

I have found moisture meters to be very unreliable. I would worry about branch wood especially with the pith in. Branch wood grows commonly at about a 45ish degree angle to the ground. this causes the pith in most wood to be off center, and that causes some odd stresses. Cut the wood, and the stress can be released in unpredictable and random ways. Twist, warp, bow, and many others, but mostly cracking. Add to the mix the fact that it is fruit wood, and Yikes! I have had some luck with apple wood if it is trunk wood, cut with a 10% 45% 45% rule. Instead of cutting through the center of the log, right through the pith, cut with 5% of the diameter of the log on each side of the pith. 5% + 5% = 10%. Throw that slab in the fire pit. Seal the ends of the remaining 45% from each side of the pith. put it in a cool place, and let our Idaho desert take over. Please note that I have had very low experience, and high success with this simple procedure. The wood still moved and distorted, but didn't crack. Just my 2 cents from the more deserty desert on the opposite side of the state. By the way, are your going to Idaho Artistry in Wood in April? I went last year and it was amazing.

[Bill Howatt]

... I did notice that it's got a very slight crown in the center portion. It wanted to flex a little as I was sanding it. It's about 20" long. Jim
...

There are models that have a very definite crown in the middle - on purpose.

[James Baldwin]

I think I will. this the first I'd heard about it. I'm not that far away from it. About 5 miles. Brian are you local? Jim

[Brian Brown]

I'm In Idaho Falls.

[James Baldwin]

I've passed thru there a time or two. Jim

[Dave Mount]

If we assume for the moment that the 17-19% is close to where the piece started, then there is considerable shrinkage yet to happen, and if it was made from a branch in the round, pretty likely to split. Small diameters are better able to resist splitting when dried in the round.

The wood is most assuredly not at 0% after turning. My suspicion is that the oil finish is interfering with the moisture reading. Moisture meters sense electrical conductivity. Oil will interfere with the contact and therefore the reading. I'm guessing you have a pinless meter. A pin-type meter could access the wood below the finish, but of course you probably don't want to poke holes in your finished rolling pin!

To put some numbers on the shrinkage issue, the specifics vary some with the wood, but in general, wood doesn't start shrinking until moisture content is down to about 30%, and as someone else said, equilibrium moisture content for interior wood is often around 7% (depends on where you live, whether you have air conditioning, etc). On top of that, the shrinkage between 30% and 20% is less than between 20% and 10%, not to mention 20% to 7%. Here's a graphic from a paper by Gao et al (2022; find it here: https://www.mdpi.com/2073-4360/14/22/5045) who collected detailed shrinkage data for a species of pine (yours is not pine, but principles are exportable to your case).

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Even if the amount of shrinkage was similar in these two intervals, the total distortion is additive. The tension arises because the radial (from pith to bark) shrinkage is less than the tangential (along the growth rings) shrinkage. Wood dried in the round splits because the circumference of the piece decreases more than its diameter, so the outer growth rings become too short to cover the interior and a split results. At 17-19% moisture the wood may be flexible enough to manage the internal stress without cracking, but there's more shrinkage to come, and its likely it will result in splits (though not certain).

You often hear an adage that if you dry wood slowly enough you can avoid splitting. This is partially, but not fully true. Drying slowly avoids having a dry "skin" surrounding a wet, swollen interior. In a rapidly drying piece you not only have the radial/tangential problem, you have the outer part drying (and therefore shrinking) faster than the interior, so the outer part splits because it's too small to cover the interior. This is why we put coatings on green roughouts for twice turned bowls, to slow down the drying on the surface of the piece and the checking that will accompany that. If you dry slowly, there isn't such a gradient in moisture across the thickness of the piece, so you minimize differential shrinkage caused by differing moisture content. But you still get differential shrinkage from the radial/tangential issue, that's unavoidable.

Here's an example of distortion from radial v tangential shrinkage. This is a small cross-grain box I roughed out of holly, which has notoriously high overall shrinkage, as well as a very high tangential:radial shrinkage ratio (sometimes abbreviated TR ratio). This was a perfect cylinder which I roughed the green blank. The pic is taken of the end grain and though the grain lines are a little hard to see, but they run from lower left to upper right. Accordingly, the piece shrank more from lower left to upper right than it did in the radial orientation, from lower right to upper left. In the dried piece, the diagonal from lower right to upper left is now longer than the other diagonal. Another thing you can see in this pic is how curvature of the grain affects the distortion. At the left side, the grain lines are curved around to where they're almost horizontal, so the top to bottom shrinkage on the left side is almost entirely radial shrinkage, the lesser of the two. On the right side, the grain is diagonal; this side is shorter in the dried piece because tangential shrinkage (the greater of the two) is a significant component on the right side, so it ends up shorter.

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Best,

Dave

[Dave Mount]

Here's another example of the tangential v radial issue. This is a cookie of oak that was cut green and a single bandsaw cut was made from bark to pith. This allows the piece to dry without the stress created by TR differences. The piece undoubtedly decreased in overall diameter as well, but what you see is the wedge that opens up because the wood is shrinking more along the grain lines than across them.

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If you're interested in this stuff and other details on the material we call wood, get your hands on a copy of "Understanding Wood" by Bruce Hoadley.

Best,

Dave

[Dave Mount]

This horse is well past dead but. . .

It is possible to turn things from roundwood if you do something to allow the piece to shrink with less stress. One way to do that (which doesn't work for rolling pins) is to drill out the pith. By removing some of the bulk in the center of the piece, you can allow the outer portion to shrink in without drying to compress the wood around the pith.

Buckthorn is beautiful wood (even though it's a nasty invasive tree), but it typically doesn't grow very large, so getting workpieces from only one side of the pith is hard. This vase I roughed out of a full round, then drilled a 1" hole down the center to remove the pith (it's a little over 3" in diameter) from end to end. I let it dry, trued it up, then plugged the hole in the bottom with a piece of contrasting wood (only visible if you turn it over). Forgive the clunky shape, made worse by the camera fisheye effect. My bride wanted a smallish vase with a large plastic insert so it would hold more, which forced me to make the neck of the vase too thick for aesthetic pleasure. Looks like an old style milk bottle. But the wood is pretty.

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This picture of the drilled center is from another piece of buckthorn, but same idea.

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Hollow forms can be turned from roundwood because the fact that they're hollow allows the same kind of contraction. The only risk of cracking is at the base, unless you drill that out.

Best,

Dave

[James Baldwin]

I took the moisture reading after it was turned and before I applied the mineral oil/beeswax finish. Jim