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Thread: Limb pith in bowl questions

  1. #1
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    Limb pith in bowl questions

    I don't turn many large salad bowls and I know it best to have defect free wood. But, my cousin wants a salad bowl and supplied a couple of pieces of silver maple from a tree that came down in their yard due to some recent storm damage. When roughing out the outside I came across a limb pith. In the photo below you can see the pith. There is a black streak that looks like a crack but no crack can be felt.

    If I carefully and slowly dry the bowl out over the next few months and no crack develops at the pith will it be safe for long term use as a salad bowl?

    Do I use CA glue now to help seal the pith or is that asking for more trouble once the bowl is dry and finished turned?

    Thanks
    Ricclimb pith.jpg
    Last edited by Ricc Havens; 05-19-2020 at 9:22 AM.

  2. #2
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    No pic attached.
    Brian

    "Any intelligent fool can make things bigger or more complicated...it takes a touch of genius and a lot of courage to move in the opposite direction." - E.F. Schumacher

  3. #3
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    Without a photo I can only offer that if you've rough turned it then let it dry and then decide what's the best way to deal with it.

  4. #4
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    Do you mean it has a knot on the side from a limb? Some of these dry fine without cracks, some don’t. I would seal with anchorseal and wait and see. Maybe she has some more pieces without defects.

  5. #5
    I almost never apply glue to a green turned bowl. I usually wax it heavily, let it dry and move, and then deal with the defect later. The reason is that if you constrain the knot by gluing it, there is a chance that the stress can be transferred elsewhere. In addition, depending on your wood, you may get other checks and cracks that you'd have to deal with later - or may make you decide to chuck it instead of chucking it.

    When you go to finish turn it, first mount it and true it up and turn away the wax - maybe half way to your final form. Then clean up the knot area with mineral spirits. After drying, you can use an epoxy to fill the void. I prefer slow setting, because it really seeps down into cracks and crevices. After 24 hours, you can then finish turn it.

    I prefer epoxy for a salad bowl because it's more water resistant than ca glue and better at wider gap filling.

    Now, realize that eventually, even that may fail with repeated washing and or abrasion.

  6. #6
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    oops - PHOTO IS NOW ADDED.

  7. #7
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    A knot that size is sure to develop more cracking, and gluing now is not going to prevent it.

    Cracking occurs for two reasons. One is that areas of the wood dry (and shrink) faster than adjacent areas, and the crack develops because the wet area is swollen and the dry area is shrinking and the wood can't handle the differential. This kind of shrinking is what you see on the ends of firewood shortly after it's cut to length, where the end grain is drying fast but the center of the piece is still wet and swollen. It's why we seal end grain, put bowls in paper bags, and other treatments for drying bowl roughs -- to dry the piece more slowly.

    The second kind of cracking occurs because no matter how slowly the piece dries, the orientation of the grain can't accommodate the dried shape. This is why you (generally) cut the pith out of a bowl -- because wood shrinks more along the grain than across the grain, the perimeter of a "circle" of grain shrinks more than the diameter, which can't happen geometrically, and a split develops to relieve the stress. This kind of cracking is generally inevitable, but the severity of it depends on the shrinkage characteristics of the wood (tangential/radial shrinkage ratio or "T/R ratio") and also to some degree its plasticity and resistance to cracking. Or, you can prevent it by preventing the shrinkage (e.g., polyethylene glycol treatment) or plasticizing the wood with heat (I presume, but don't know, that this is how microwave drying works -- never done it). Hollow forms can also include the pith and not crack, but that's because of their shape -- having the pith hollowed out allows the "circle" room to shrink.

    Your knot is essentially an enclosed pith; larger knots like this crack for the same reason a "tree cookie" does. In addition, there is also commonly some pretty tortured grain around knots that can also crack. The good news is, that tortured grain is why you've got such nice character around that knot.

    Seal it well and dry it slowly to minimize the first kind of cracking. Then, fill the cracks (voids) that do occur with epoxy combined with a filler that appeals to you. I usually use ground coffee, though that might be darker than you want for a wood as light as this piece of maple. Depends on whether you want to "quiet" the void visually, or make it a feature. There are all kinds of options, both organic things (wood flour, bark crumbles, coffee) and mineral (turquoise, mica powders) depending on your tastes. Epoxy is perfectly food safe (e.g., it is one of few ways you can cover a wood bar top and get past the health inspector) and provided you avoid bubbles, it will not create voids to catch food. Some people don't like the look of cracks or defects in bowls used for food, but I don't share that feeling. If you have hairline cracks (cracks that aren't open enough to get epoxy into), you can seal and stabilize those by wicking in some thin cyanoacrylate.

    Some of the coolest figure that occurs in wood occurs around knots. Celebrate it!

    Best,

    Dave

    Capture.JPG
    Last edited by Dave Mount; 05-19-2020 at 10:55 AM.

  8. #8
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    Quote Originally Posted by Dave Mount View Post
    ...Or, you can prevent it by preventing the shrinkage (e.g., polyethylene glycol treatment) or plasticizing the wood with heat (I presume, but don't know, that this is how microwave drying works -- never done it). ...
    Microwave drying, in my experience, doesn't help. It's best done slowly to avoid reaching destructive internal temperatures.

    However, boiling is likely to help. Boil 1 hour per inch at the thickest point, boiling longer doesn't hurt. (According the the boiling expert Stephen Russell.) From what I understand boiling softens the lignin and allows stresses to equalize.

    JKJ

  9. #9
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    Ricc as others have said, let it dry and then deal with it. That crack is too big for CA glue, you'll need something else (ie: epoxy), but wait for it to dry and finish cracking and then decide. Just because a bowl has a crack doesn't mean it's not useful. People joke and say "but it won't hold water". I ask them "When was the last time you filled a bowl with water" and that shuts them up. A bowl with a crack can still hold salad, pasta, chips, popcorn, veggies, bread, etc.

    Me personally I'd put a butterfly patch on the crack, and I may or may not fill the crack first. Some cracks are better left open and just stabilized.

    DSC_1431.jpg DSC_1446.jpg

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