I bought a small 75 lb anvil a while back at an auction sale and it was rusted from head to toe. I recently just got around to restoring it and decided to soak it in an apple cider vinegar bath for about a week to help take the rust off. I gave it a good rinse off after I took it out and it started leaking black from the hardy hole to the horn of the anvil. This thing is pretty beat up but the fact that it leaked black really surprised me. Anyone got any idea what it could be?