Initial Heating

What it does:

Heats the glass from room temperature to about 900° F (482° C).

Firing Schedule - Intial Heating

Things to consider:

For most glass firings, the only thing to worry about in the initial heating is thermal shock.   Thermal shock can happen when glass heats unevenly and part of the glass expands a lot more than another part.  Since solid glass doesn’t stretch, it breaks.

Here are some situations that can cause thermal shock:

  • Firing thicker glass too quickly can result in the outside of the glass expanding much faster than the inside of the glass.
  • Glass that is close to heating elements will heat faster than glass that is further away.  Examples include large projects whose edges are close to side elements and slumping projects that sit high on molds close to the top elements.
  • Metal inclusions, dichroic, and iridized surfaces all change the way heat is reflected and absorbed into your project, causing uneven heating.
  • Large areas of different glass – for example a circle that is half transparent and half opaque – will often absorb heat differently.

When considering the risk for thermal shock, remember that all your risk factors work together.  A thick piece of glass that is close to the side elements and has a large piece of copper foil embedded between layers is going to require much more caution during the Initial Heating.

Reducing Risk:

The best defense against thermal shock is to slow down the Initial Heating temperature ramp.  Fortunately, you can always slow down this step without any negative consequences.

You can also reduce the risk of thermal shock by ensuring that you load your kiln with the largest projects toward the center of the shelf, where the heat will be most even.

When the edge of your large project must be close to the side elements (when that’s the only way it will fit in the kiln), consider building a short wall of kiln furniture between the elements and the glass.  This will act as a heat baffle and protect the glass for direct shots of heat.


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