Green wood and cracking during drying

[imapseudonym]

When trying to estimate how a wood species will behave when drying after roughing, which properties of the wood are most relevant? For instance, the amount of shrinkage (and ratio of radial to tangential shrinkage) seems relevant since the more the wood moves (and the less evenly it moves) the more stress will be applied. That does not account for the difference in success I have had between butternut and cherry - the former of which behaves very well for me, the latter less so, despite fairly similar shrinkage numbers.

How flexible as opposed to brittle a species is would also seem to be relevant as that will allow it to bend with the stresses produced by shrinkage, but cherry seems to be more bendable than its failure rate suggests.

[MichaelMouse]

None of the above. The differential between loss through end grain versus quarter (face in between) is so great, other factors pale into insignificance.

Two things commonly done to reduce or eliminate loss of green wood turnings during curing are to control geometry by tapering or curving the walls of the turning to divide the mechanical stresses that form the first checks, and might lead to full cracks. The second, to control the rate of loss by control of the RH so that the interior moisture can replenish the lost outer more or less equalizing loss. Some folks use both, myself included.

Where's your cherry cracking? Its really prone to heart checks, many of which go unnoticed during initial turning. Haven't had much trouble with broad-bottom syndrome compared to comparable shapes in other species, so I can't point you to anything special there.

[robo hippy]

Drying is an art that some times follows some general rules, but is different for every location, wood species, and even pieces from the same tree. Some are more prone to cracking than others. Only real rule is that most of the time, if you dry it too fast it will crack, and if you dry it too slow it will mold. For drying a roughed out blank, make sure to round over the rims of the bowl as a sharp edge not only will cut you, but it is prone to starting cracks. You want the pieces out of sun, heat, and wind, so a concrete shop floor in a corner is generally good. Some totally seal twice turned pieces, some don't. There is boiling and steaming, and microwave drying, bagging methods, sinking them in the log pond, and others. Best bet would to be find a local turner with more experience and ask.

Once turned bowls are faster to dry than twice turned bowls. I wrap the rim in stretch plastic film, and start them off on the floor. Up to the wire rack in a couple of days, and dry and warped in a week.

I have only had pieces of butter nut once, as it is uncommon out west here. It was pretty easy to dry. Cherry is more crack prone than a lot of other woods.

robo hippy

[imapseudonym]

Thanks for the suggestions so far. I am really mainly curious about the 'why' though.

Apart from differential shrinkage rates, do some woods have a greater difference between end grain and side grain moisture loss? Does it correlate with open pore/closed pore woods? When you look at something like madrone (one of the crackiest woods) the shrinkage rates (tangential, radial and also the difference between them) are significantly higher - so that makes sense, the wood will move more so there will be more stress if it happens too quickly. Madrone does have good bendability while butternut's is poor so that must not be a big part of the equation.

[MichaelMouse]

Science is free at http://www.fpl.fs.fed.us/products/public...eader_id=p Specifically Ch 4. T/R shrink ratio normally about 2:1, throughout, which isn't all so important as you think. Annual ring orientation and interval are more important in governing distortion. Direction as in the figure.



Since "typical" woods have density differences between earlywood and latewood, the early shrinks more, pulling the late into the distorted pieces in the figure. More air, and therefore more room in the earlywood.

Lots of woods are borderline "typical" at best. Salicaceae, the willows, true poplars, cottonwoods and such are bulletproof in drying. I believe you could throw a rough out under the noonday sun and survive it with good geometry. Other woods, madrone, beech, yellow birch, can be a real challenge. I use both preventive methods on them. The figure is also a big player, but all relates back to the early/late difference and orientation of same

You can use simple science without "art" and dry most woods. No problem with sharp edges in the last few thousand of mine. Assume it's a west coast aberration I forgot when I moved east.



Can't outrun dumb, though. Here's what happens when you leave a rough sitting directly under a hot air duct for 36+ hours.



Direction of drying strain is pretty obvious when you see the wide outside of the crack and tapering in. Guess It wasn't a total loss - can serve as a "teaching" bowl.