I have an urge to purchase a "vinegar mother" and make homemade vinegar with leftover red wine. According to what I have read, it is not a difficult process, and I have had great success with making kombucha, which is a similar technique, only uses tea and sugar rather than wine. I have a lovely dark blue glazed crock with a spigot at the bottom that would work charmingly, if I could verify that it would be safe to use. Is there any risk of the acid of the vinegar reacting with the glaze? Is there any way to test whether it would or not? Thanks, and any advise on vinegar making is appreciated, as well as good sources for buying a mother.
It probably won't hurt you, but I wouldn't risk it, just in case. Glazes for food containers are unlikely to contain lead, but may have other metals that are harmful in sufficient quantities. These include cadmium, barium, cobalt, strontium, chromium, and in the case of some older orange ceramic, URANIUM.
Blue glaze is probably copper or cobalt based. I found a reference on ScienceDirect about high blood cobalt levels in association with lead poisoning from glazed Greek earthenware. This indicates that cobalt can indeed be leached out of the glaze and absorbed into the body. Cobalt is eliminated rapidly, but is moderately posionous, and copper is problematic in large quantities. Which brings this back to where I started: it won't kill you, and probably won't make you sick, but it's not really worth the risk.
File it in the same category as eating egg salad left out in the heat a few hours?
One other caution: prolonged contact with acid may damage and discolor your formerly handsome crock, in addition to leaching out these metals. To check for this, you can leave white vinegar in the container for a week or so, checking for change in color of the vinegar and the container every day.
Acid promotes the leaching of lead from pottery glazes. Of course, not all glazes contain lead. The FDA claims that hardware-store lead test kits work on pottery: How can I find out if my ceramicware are safe?