caveat is the glaze
I spoke to the vendor who sold me my glass kiln about the same thing. He said that firing bisque is fine as long as you can get the temperature high enough. They make molds all the time. But he also warned that it’s not the clay that’s the problem, it’s the glazes. The fumes can leave residue in the bricks which will fire onto the glass in successive firings. They ran into that problem. Now they’ve got a kiln that leaves yellow “pee” colored spots on clear glass. UGH. He didn’t say specifically which glaze caused the problem, but it’s a risk I wasn’t willing to take, waxing my glass kiln that cost a fortune when I could buy a used clay kiln for a couple hundred dollars or just take to a local clay shop that will fire it for me in a proper clay kiln for $7, you know what I’m saying? Talk to Paragon, or whoever manufactured your kiln and ask them specifically which glazes they recommend so that it doesn’t expire your warranty. That’s what I’d do. Good luck!