I had been experimenting with fusing glass in/on clay for a while when I ran across a link to John Groth a couple of years ago. He said he had been experimenting for 11 years (!), he'd formulated a clay with compatible Coe to fusing glass, and was in the process of patenting it (as noted above). However I haven't been able to find any mention online of the clay itself for sale. Funny, I've gotten so used to just Googling to find products that I'm embarrassed to say it has only now finally occurred to me to contact him directly! I will report back what I find.