Small cracks on inside of bowl

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Hi Guys,

I've rough turned a few bowls and finished some of them pretty nicely. The bowls are not that large (about 10" or 11") and I let them dry for about 5 months. After rough turning, I applied end grain sealer ONLY to the OUTSIDE of the bowl on the end grains. The bowls did not crack during drying ... however after finishing them, I've noticed a few small cracks on the end grain INSIDE the bowl (see pic). I can't see any on the outside.

Did I not let them dry long enough? Is there something else I've done wrong during the process?

Thanks,
Dawson
 

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How fast are you running the lathe while sanding? Those cracks looks like they could be from overheating the wood during the sanding process.
 
Not sure of the exact RPM, but its at it's slowest speed. I'm using small sanding pads that attach to the end of my drill to sand the bowl.

Is it possible that my drill speed is too high? Or perhaps I am applying too much pressure on the drill when sanding?
 
Something is causing the heat. I agree with Vaughn that is what it looks like. I use low speed with 3" disc's on mine and you don't have to put much pressure at all. Let the sandpaper do the work.
 
I know I've caused cracks from overheating during sanding. Some wood seems to be more prone to it than others. If you continue to have trouble after sanding with a little lighter touch I would guess these are cracks that just did not become visible until you put on the finish. I'm in a very dry climate (Phoenix) and can tell you that sometimes it seems all the tricks available to you are not enough to keep all the cracks out. If it becomes an ongoing problem I would coat your next blanks inside and out, and possibly wrap in newspaper or brown bags. You may even end up going the DNA route. Others can give you more info on that if you need it. I really hope for your sake it's just the heat build up from sanding. That would be the easiest thing to fix as it's just a minor change in technique.

Ken
 
Ken, you're comment about certain types of wood being easier to crack (due to heat) is interesting ... I never realized it before, but both bowls that have these types of "heat cracks" are Cherry. Perhaps that type of wood does not stand up to heat well, I'll keep trying and post some results.
 
Dawson, as I understand it, cherry (and other fruitwoods) is more susceptible to heat cracks than many other woods. So it sounds like your assumption is correct.
 
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