I've worked with Terracotta vessels specifically made for wine (~120 gallons size). They are unglazed, the inside is sealed with bee's wax and the lid (stainless steel) is fixed to the opening with a very hard wax. They impart a fantastic flavor (honey-like of course), I've only used it on Viognier, no reds. The bee's wax can be reapplied or a tartaric acid (tartrate) finish applied to the inside, similar to the treatment of concrete tanks.