Fusing Glass

Fusing glass in a kiln is a fascinating technique that enables artists to create unique and gorgeous projects. The following fusing rules and firing instructions should provide you with enough information to make a variety of projects, creating an appreciation for the complexities and potential of fused glass, and paving the way for more intricate designs and ideas.


Tested Compatible

Fused glass dishOtherwise, when the glass cools, one glass will pull on the other and cause the piece to crack along the seam. Even if the glass survives the cooling process, there is still a risk it might still crack if you try to reheat it in a kiln or even place it in a sunny window. Stress from incompatible glass is always in the piece. Do not try to refire broken incompatible glass.

  • All glass has a coefficient of expansion, or COE.
  • Glass manufactured specifically for fusing is often “tested compatible,” or guaranteed to be a certain COE.
  • The most popular fusing glasses are either 90 COE (Bullseye, Uroboros and Wasser) or 96 COE (Spectrum and Uroboros).
  • Always use compatible glass, which is known to have the same COE.


Slower Is Better

thermal shock glass

  • You can’t heat or cool glass too slowly. Going too fast can result in cracked glass or Thermal Shock ( see image to right).
  • A safe rate to heat is 15° per minute (900° per hour), although stacked glass 2″ in diameter and smaller can be heated at a faster rate.
  • Slow-cooling of glass or “annealing” depends on the thickness of the glass. If the glass breaks because it was heated too fast; turn off the kiln, allow the glass to cool, push it back together, and try again – at a slower rate. Breaks from thermal shock usually go straight across the piece and have a little hook near the edge. They can usually be repaired by refiring.


Glass Likes To Be ¼” Thick

When heating glass to full fuse, anything with less mass will shrink up, anything with more will spread out. This movement can be controlled somewhat by fusing slower, and not going to full fuse.


All Kilns Are Not Alike

  • There are some variances between kilns, especially mini kilns. Sometimes pyrometers are slightly off, and sometimes current loads vary.
  • Use firing schedules as a guide, but remember to check your piece frequently during fusing, and record changes in schedules as needed.
  • Prepare your kiln by applying kiln wash with a kiln brush. Apply one thin coat in each direction. Don’t forget to apply kiln wash to molds, too.


Take Good Notes

  • Use a project log to keep important information about your projects.
  • Keep track of what glass was used, how thick the glass was, the firing schedule and the results.
  • This helps repeat good performances and prevent bad ones.


Fusing Glass

Brittle Zone brittle Tack Fuse  tack Full Fuse  full

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