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I turned apple on one previous occasion, and it cracked very shortly after coming off of the lathe. I've recently come upon a few hunks of apple and I'm wondering if I rough turn bowls, and put it in a drying chamber (after sealing the end grain), do you think it will survive the drying process without cracking? Anyone have any experience with apple; what is your approach for crack prevention? Thanks.
Lots of variables, hard to say. Things like how close you get to the pith, how warm you set your drying chamber, how even you keep the walls (including the bottom), how thick you leave the walls, etc.... For fruit woods, I let it rest in a yard waste paper bag before I add any heat. I use lawn waste bags because they are double thickness.
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Edward it is not perfect and there are no guarantees but I am a believer in boiling green fruitwood for 1-1/2 hour. A recent batch of Evergreen pear tangential/radial shrinkage 2.9 all of those that I rough turned and rolling boiled 1-1/2 hour then put in the shed to air dry had minimum to no cracking. All I rough turned and treated with Anchor Seal 2 and put in the shed to air dry severely cracked within weeks. Now that I have been getting into vacuum bagging I will be trying wrapping in a towel woods with tangential/radial shrinkage greater than 2.5 change to a dry towel daily, hookup the ShopVac to the bag and do it again. I can't wait to get some green Ironbark Eucalyptus to try vacuum bag drying. Leland
Love turning apple, I rough turn to the 10% thickness and a alcohol soak for a min. of 48 hours and wrapped in newspaper an set on a shelf to dry, usually about 2-3 weeks by checking the weight.
Wally
Thanks for the feedback.
I may be the heretic here, but I'm not a believer that T/R is a very big factor in determining the potential for roughed bowls to crack. For example, I turn a lot of sugar maple (T/R >2) and rarely have a drying issue, and find that paper birch (one of the lowest T/Rs among US domestic hardwoods at T/R = 1.4) is more prone to crack than sugar maple. Despite the high T/R of hard maple, I just put the rough turning in a paper grocery sack and set it on the shelf (no Anchorseal), and it's very rare to have a drying problem. T/R speaks to how much the bowl is likely to deform during drying, and hard maple bowls do deform more than birch during drying. But I'm of the opinion that cracking difficulties with certain woods stem more from their propensity to lose moisture quickly than their T/R.
I'm not saying T/R is completely irrelevant, and the grain geometry (e.g., how close the rim of the bowl is to where the pith was) is a factor too. But I would assert that most cracks in end grain start not because of the deformation from T/R, but from the surface of the bowl drying faster than the interior. Once a check starts, differential T/R shrinkage can cause it to pull apart, but I think it's the differential drying rate at the surface that starts the problem. This is governed by how fast moisture is lost from the surface of the wood, compared to the speed at which water moves from the interior to the surface. This difference is in turn governed in part by the conditions under which the bowl is held (e.g., RH and temperature of the surrounding environment, whether sealants are used). But it seems reasonable to assume that different woods have different innate moisture transfer characteristics that would factor in as well. No hard data to prove this, but it would be consistent with my empirical experience, which is that T/R is not closely linked to the propensity for roughed bowls to crack, yet some woods are definitely more prone to crack under the same bowl geometry and drying conditions.
I would say apple is in the mid-range of drying difficulty. For apple, I not only seal with anchorseal, but I'll put some shavings in the paper grocery bag I dry the bowl in, to further buffer the moisture loss rate. One thing with apple is that the trees I've dealt with sometimes have weak spots in the grain that lead to drying cracks forming along the grain lines (like wind shake), as opposed to end checking across the grain. Regardless, the biggest thing I focus on is slowing the rate of drying.
Another thing about apple is that, like cherry, there is a big difference in the amount of shrinkage in the sapwood versus the heartwood. Roughed bowls with a lot of both are more prone to problems with drying deformation because of that differential. A final note on apple -- I'm not sure I've ever had a crack in a natural edge apple bowl. That may be because I generally turn those fairly thin (circa 3/16") rather than drying a thicker rough (usually 10% of the diameter) or because of the different grain orientation (bottom of the bowl nearest the pith, rather than the lip of the bowl nearest the pith).
Redbud is the most cantankerous wood for drying bowl roughs I've ever experienced. In a half dozen attempts, I've never gotten a redbud rough bowl to dry without stupendous cracking (though I've not done the alcohol thing, or any other of the more involved processes). To be fair, the handful of blanks I've gotten have been from smaller, fast-growing trees, and their size/shape meant that I was not able to stay real far away from the pith, and that probably compounded the problems.
My $0.02 based on my experience -- your mileage may vary. I wish I had some data on the moisture transfer rate idea -- I'm just convinced it's a factor.
Dave
Dave - thanks for your detailed reply.