Early planning questions

[CSWolffe]

I have a couple of odd questions about home built ovens that I cannot seem to figure out on my own or on this site. please help me in my ignorance.
First, what exactly is 'fireclay' and where would be a good source? Masonry dealers or ceramic stores? When tracking down refractory cement(homedepot does not carry here) the counter help at a masonry dealer gave me an odd look when I asked him about it.
Second, how do the insulatory properties of vermiculite and 5/1 insulating concrete differ? Sub question, does the insulating concrete have any structural strength? Am I better off with four inches of vermiculite with a two inch concrete shell, or six inches of insulating concrete? I would of course cover either with a protective layer of stucco, and either method would include reinforcing metal mesh.
Thank you for any attention you can spare.