I picked some fresh oak galls to extract juice from this spring. When fresh and green in April, our large oak galls from the California Valley White Oaks are very juicy and exude a clear liquid when squeezed. I put a piece of skin in the straight juice to test it and it case hardened, a phenomenon where the skin is shocked by the extreme astringency, becoming shrunken and stiff. In case hardening, the outside surface of the skin is so tight and contracted that the tanning stops or proceeds slowly because new tannin can't penetrate to the interior of the skin. In the video, I discuss the fact that fear of case hardening often leads to just the opposite problem, which is far more common, not using enough tannin. The experiment also provides a few other talking points.