Help with crack diagnosis - neophyte turner

[David Castonguay]

Hi folks:

I'd greatly appreciate some help with diagnosis of the cause of these cracks and prevention of cracks in the future.

BACKGROUND
I've been turning for 1.5 years in my spare time. Produced about 30 bowls of various sizes from blanks source from my firewood pile. All of them have been turned green to final dimension. The wood has been beech, maple, ash, white birch and ash (black and white). Many of the logs have been spalted.

The small to medium bowls (4 to six inches) have been relatively free of cracks. Recently,I turned a couple of bigger bowls (10 to 11 inch diameter; 5 to 6 inch tall) in a similar shape. The walls are about 1/4 inch thick and at the base the curve produced about 3/8 to 1/2 inch thickness.

The black ash bowl developed some small cracks along the bottom in the sapwood. (pictures attached). The cherry bowl turned in approximately the same size and shape did not crack. The cherry came from a diseased tree with some imperfections that I think may have come from a fungus or insect infestation. Both trees had been cut down for about a year.

Your advice is greatly appreciated.

David

cherry top.JPGcherry bottom.JPGash bowl top.JPGash bowl.jpg

[Don Stephan]

Once all the "free" water is out of wood, as the "bound" water leaves the cell walls shrink. Wood shrinks more along the growth rings than across them, and this differential shrinkage creates stress, leading to cracks. Free and bound water will migrate out even through the bark, so my expectation is that there will be shrinkage cracks along the surface of logs after one or months of drying, and I won't bother with such wood.

[John K Jordan]

Hi folks:

I'd greatly appreciate some help with diagnosis of the cause of these cracks and prevention of cracks in the future.
...
The black ash bowl developed some small cracks along the bottom in the sapwood. ...

Were these turned wet from start to finish or turned to a thicker wall, dried, then turned to final size?

With that shape the wood will be thick from the "corner" to the inside. Thicker wood can shrink and crack more while drying, especially if the rest of the bowl is thinner. The tangential shrinkage can be the highest on the bark side of the log, which includes the sap wood, and if dried too quickly on the surface while the inside is constrained by thickness it will often crack.

One way to minimize the problem is to shape the outside curve at the base so it follows the inside curve making the wall thickness is about the same all the way from the rim to the base. This can prevent the thicker areas from causing problems.

Another thing that can help when turning wet wood is to turn a little thicker, and seal the suscdeptible areas such as the end grain and the base in your case. This slows down the drying. Once dry, turn to finished shape. Slowing down the drying with sealer can allow the natural moisture gradient inside the wood to equalize and prevent the stresses on the outside that cause cracks. I use Anchorseal.

Rather than turn wet I generally cut and seal the blanks and put them up until dry before turning. This may take years depending on species and blank size but can eliminate warping and cracks regardless of the shape used. I weigh and record the weight periodically to check the drying progress. I record the weights and when the weight quits changing the blank is dry. If you cut a few blanks on a regular basis before long you will have more dry wood blanks than you can turn!

That's my problem. I occasionally have to pack up a bunch of blanks and give them away.

auction_wood_2018.jpg ambrosia_maple_IMG_20171202_175649_933.jpg drying_IMG_5757.jpg rack1_2_IMG_5762.jpg

JKJ