Understanding when Your Glass is at Increased Risk
Some glass is more likely to thermal shock than others:
- Large and/or Thick Glass
The thicker the piece, the deeper heat has to penetrate and the more difficult it is to keep the temperature even through the glass. The wider the piece, the more likely one side will heat more quickly than the other.
- Uneven Glass
Trying to keep the temperature at the center of a quarter inch piece of glass the same as in a half inch piece of glass poses some obvious challenges given that glass is such a good insulator.
- Light vs. Dark Glass
Anyone who has ever stood out on a summer day knows that dark colors absorb heat faster than light colors. The radiant heat in a kiln works faster on dark glass than light glass. If your piece has sharply contrasting areas of light vs. dark color then you will need to fire more conservatively.
- Glass with metal inclusions or an iridized surface
Metal (including dichroic films) absorbs heat faster than glass and can create a “heat sink” inside your work. Also problematic at times is the fact that the iridized surface of the glass reflects radiant heat. Both can contribute to thermal shock.
All of these challenges can be overcome. The first step is to understand that you have a challenge and the generic firing schedule that you have isn’t going to work. Once you recognize this you are ready to take steps to minimize the risk.