I have read that early pioneers would mix a batch of chopped,pulverized oak bark and put it in a pit or hollowed log. They would add the scraped and dehaired hides into the layers of bark. Then add water to it. The strength of the tannin would slowly strengthen as it leached out of the bark. ( if you put a hide into a strong tannin it can harden the surface and make tanning slow) They would then leave the hides in the tannin for months or a year or more for heavy hides (cow). An edge cut would show how deep or complete the tannin had made it into the hide. A small piece could be dropped into hot boiling water and if it didn't shrivel and cook it was tanned. It would then be oiled to finish.
The bark was sometimes changed after months with fresh but that was just to strengthen it again. So, I suspect the bark tannin is not good for bacteria growth like stagnant water and probably not good mosquito hatching either.
