(apology: I'm sure this question has been answered many times on this forum, but I didn't have any luck with searching.)
I'm relatively inexperienced, but just starting to sell some things at a gift shop in Lexington. I was looking at a bowl and a candlestick I completed a couple of weeks ago with an eye to taking them to the shop, and found that there were cracks in both pieces. These particular pieces were from relatively new black locust, but I have the problem with old oak and even my favorite wood, osage orange, and practically everything else. I usually finish with danish oil and Minwax or beeswax.
  I suppose I can fill the small fissures with CA and re-sand and refinish them, but the bigger issue for me is that I don't want people to buy a piece and have it split/crack/check/whatever after they take it home.
  Is my problem that the wood is still drying? Do I need a less permeable finish like polyurethane to seal the moisture in (which I'd rather not use, as I like to be able to say everything is food-safe)?
  I'd love to discuss this. It's the biggest frustration I have with this work.

David

I am one of the few that makes once turned bowls.  I finish turn while green, the first time the wood is on the lathe.  As soon as the bowl is complete I weigh to the nearest gram and note the weight, then put in a paper grocery bag on the concrete floor (which is on ground).  I record the weight weekly until the weekly weight loss is less than 2%.  Then I move the bowl in the bag onto a table and continue weekly weighing.  When the weight loss again is less than 2% I remove the paper bag and continue weekly weighing.  When the weekly weight loss is near zero I consider the bowl dry enough to sand and apply finish.

On average, wood shrinks about 50% more along the growth rings than across them (some woods more, some less) so as the wood dries/shrinks the differential shrinkage stress causes the wood to move or warp.  I like the look and my customers usually say they do as well, but sanding a warped bowl is much more time consuming.

Because of this differential shrinkage and resulting warp, I doubt you can make things like pepper grinders from green wood.

No finish is a complete barrier to moisture movement, they only slow down the movement into or out of the wood.

An excellent book on wood, drying, and much more, which you might be able to find at your local library, is Understanding Wood by R. Bruce Hoadley.

David Thomson wrote on Dec 29^th, 2017 at 12:49pm:


Short answer is yes.
When you turn an item you remove material which does a few things.
One is that it relieves internal stresses in the wood and Two is that it abruptly exposes wet or higher moisture content wood the the atmosphere.

Relieving too much wood too quickly can cause the wood to warp and crack. This is why when 'roughing out" a bowl blank for twice turning it's best to leave the walls relatively thick. This allows the center to dry with less likelihood of the walls cracking. While the piece may warp, by leaving the walls thick you left yourself enough material to turn it again when dried, hence the term twice turned.

If wood dries too quickly, it can warp and crack.
This is why when lumber is fresh cut precautions are taken, like sealing the end-grain to slow the moisture release from the wood.

John Cepko wrote on Jan 1^st, 2018 at 5:53pm:


I use a calendar to dry mine