Glass Clay has been around in one form or another since 3000BC. In Richard La Londe’s book, Richard La Londe and Friends, he gives a formula Egyptian clay or Faience. The result gives a hand moldable clay that after firing becomes solid glass. It is a great article and I highly recommend this book for any glass enthusiast.
My ArtGlass Clay uses more modern materials that offer much more opportunities for a wider range of colors. I crush fusible glass to a fine powder, add binding agents to create my formula of glass clay.