Subject: What is the best thermal ceramic insulator to keep heat in a vessel?

I am trying to devise a high efficiency heat exchanger.  I believe that a 
ceramic would be the best choice as an insulator, but I am having trouble 
finding out information on the insulative characteristics of the various 
types of materials.  I would like a clay with low shrinkage rate when 
fired, and that can be fired in a standard Kiln.  The device is relativly 
small, so I would not need a great deal of the material.  Any information 
that you can provide would be greatly appreciated.  Thank You.

Re: What is the best thermal ceramic insulator to keep heat in a vessel?