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Wood separating after work is done (Read 459 times)
 
David Thomson
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Wood separating after work is done
Dec 29th, 2017 at 12:49pm
 
(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.
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Louie Powell
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Re: Wood separating after work is done
Reply #1 - Dec 29th, 2017 at 12:59pm
 
When timber is first cut, it contains a lot of moisture.  As the wood dries, it has a tendency to crack.  There are things that you can do to minimize cracking, but its a physical reality that the wood must dry to the point where the moisture content in the wood is approximately the same as the environment it is in before the wood is stable. 

While you can rough turn items when the wood is wet, if you finish turn items in wet wood, there is a strong probability that you will either have cracking or warping, or both.  And for that reason, its not wise to sell items that are finsh-turned when wet unless you are looking for cracks and or warping as part of the final effect.  Also, its hard to sand wet wood, and wet wood doesn't finish well.

The best example of rough turning is bowls - many turners rough turn bowls, aiming for a final thickness that is about 10% of the overall diameter of the bowl.  The objective is to remove as much wet wood (the technical term is 'greenwood') as possible, and then allow the remaining wood to dry.  That will minimize the tendency to warp and crack, and because the walls of the bowl are fairly thick, that also leaves enough material to finish turn the piece after the wood has equalized with the environment.
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Don Stephan
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Re: Wood separating after work is done
Reply #2 - Dec 29th, 2017 at 8:01pm
 
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.
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Don Stephan
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Re: Wood separating after work is done
Reply #3 - Dec 30th, 2017 at 9:45am
 
Some points I forgot last night, David, my apologies.  While drying, I have experienced some cracking in once turned bowls.  Bowls with higher shrinkage ratios of course are more prone to cracking, especially with thicker walls.  But routinely I leave the wall thickness 3/8" to 1/2" in salad mixing bowls, so the wall does not have to be extremely thin.  Second, and this may also be true of twice turned bowls, as a general rule the bowl blank should not include wood at minimum within 1" of the pith, where cracking is more likely.  Finally, some woods are just plain ornery and much more likely to develop cracking, in my experience with once turned bowls.  Black oak and sweet cherry are two examples.

An on-line search using "black locust shrinkage" returned "The Wood Database" which reports that for black locust the tangential shrinkage (along the growth rings) is 7.2% and the radial shrinkage (across the growth rings) is 4.6%, for a ratio of 1.57, which I think is close to average across common North American hardwoods.

Learn all you can about drying wood and shrinkage rates and ratios, as this will always be helpful to anyone working with green wood.
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Re: Wood separating after work is done
Reply #4 - Dec 30th, 2017 at 2:30pm
 
David Thomson wrote on Dec 29th, 2017 at 12:49pm:
Is my problem that the wood is still drying?


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.



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John Cepko
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Re: Wood separating after work is done
Reply #5 - Jan 1st, 2018 at 5:53pm
 
And, to make things even more complicated, the inside of a blank is greener than the outside. Wood dries from the ends in toward the middle. So, the moisture is different even in different places of the blank.

The only real cure is time.
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Re: Wood separating after work is done
Reply #6 - Jan 1st, 2018 at 7:01pm
 
John Cepko wrote on Jan 1st, 2018 at 5:53pm:
The only real cure is time.


I use a calendar to dry mine
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Re: Wood separating after work is done
Reply #7 - Jan 2nd, 2018 at 7:12am
 
David

Some turners use moisture meters to check the dryness of timber before turning.  I don't have one, and frankly can't justify the expense because I usually turn only dry wood.

This past Spring, wife and I cut down an ornamental plum tree in our front yard.  I saved some of the larger pieces because plum, like most fruitwood, has a nice grain pattern.
I sealed the ends and left it in the garage.

This Fall, I cut a few pieces to make some small turnings.  Just this week, now that the weather has turned really cold (sub-zero here in upstate NY), the humidity has dropped inside the house and several of the pieces that I turned in October or November have now developed cracks.  In each case, the piece was a spindle and included the pith.  I knew better, but was hoping that the wood had dried enough that there would not be further shrinking - but I lost that bet.

It happens - just deal with it!
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