I couldn't resist posting on this. I make pottery for a living. Making a mash tun that would be big enough to do any good just wouldn't work. Making and handling it would be nightmare. If you do attempt to create one you would need to use specially formulated clay that can resist thermal shock. Usually it is called ovenware or flameware. If you are really serious I would be happy to answer any other questions you might have.
They were talking about using a kiln to ceramic coat a metal mash tun.