I have made several molds to use with slumped glass. I use a mid range temp., cone 5, white stoneware and fire it to a bisque temperature of around 1800 degrees or about cone 05. I think that clays with colorants such as red earthenware might have too many impurities in it and cause gases to escape and contaminate the glass though this is just a guess. I did fire one mold in my glass kiln to 1700 which was the max temp. for that kiln. It seemed to work just fine. Ramp up slowly to allow any moisture to leave the piece. I roll the slab to about 3/8 inch thick. Be sure to score any piece that is being attached to another and use slip (dry clay and water) to moisten the joining areas. Allow to fully dry before firing to prevent the piece from blowing up. Use several coats of kiln wash on any surface that will touch your glass. As a former potter, I was able to throw some cylinders and bowl forms too. I haven’t time tested them yet so I do not know how they will hold up over time as compared to commercially made molds. I particularly like making one of a kind waves and other shapes.
I’m sure there are some artists who are casting molds from clay slip which would make a mold similar to those available from the commercial vendors. That is another coversation. I’d like to hear about that too. Good luck.