Old bark is very hard to get off as it tends to fuse to the tree, however the layer of bark is noticeably distinct and you can sometimes wedge up large chunks at a time but all in all you just try your best. A chisel works fine, even tried a angel grinder at one point, but again what ever works for you.
I've never really dried it in the oven though it does seem like a good idea. I'm not sure how long the treats last in dead bark but I'm assuming it's a fair while.
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