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G L A S S 1 0 1
Critical Temperature Ranges
A “Critical Temperature Range” is any temperature or temperature range in the fi ring cycle that has a high level of
potential for limiting the success of the project. Limited success can be expressed as overfi red, underfi red, breakage,
devitrifi cation or bubbles, just to name a few. It can be argued that there are numerous critical temperature ranges. To
keep things simple we are going to discuss the primary four: Heating Range , Process Range, Pre- Annealing Cooling
Range, and Annealing Range.
HEATING RANGE
The Heating Range goes from room temperature to the fi rst set of data in the Process Range. The only concern
during this range is heating the pieces too fast. Small pieces can normally be heated faster than larger pieces.
PROCESS RANGE
The Process Range is the temperature range where the material begins to visibly change. It is this stage that deter-
mines the fi nal shape of the piece. During the Process Range temperatures and hold times are key. If you are unsure of
the desired peak temperature you may want to start on the low end of the range with a longer hold time. This will help
insure that thicker pieces receive the proper heatwork throughout the entire piece.
PRE-ANNEALING COOLING RANGE
After the process range is through, it is desirable to cool the piece quickly for several reasons. The fi rst reason is to
stop the heatwork. This is especially important on a project such as a less then 100% fuse or a drop mold.
The second reason is that an undesirable reaction known as devitrifi cation can occur during this cooling period if
the kiln is cooled to slow. Devitrifi cation is a scummy white crystallization on the glass surface that is diffi cult if not
impossible to remove. Be sure to slow down the cooling before you enter the Annealing Range.
Opening the kiln lid to increase the rate of cooling, while practiced, is not always recommended. On certain
models the thermocouple is in the rear of the kiln and the temperature from front to back can vary causing one part of the
piece to enter the annealing phase before another.
ANNEALING RANGE
The fi nal critical range is the Annealing Range. This is the temperature range where it is important to cool the
glass slow so it can even-out in temperature. This process prevents stress from building in the glass which could cause
the piece to break.
Our pre-programmed fi ring schedules anneal from 960 °F (516 °C) to 800 °F (427 °C), which should be adequate
for most tested compatible glass. By incorporating such a broad range, the risk of breakage is limited. Be sure to keep
the lid or door of the kiln closed until the kiln reaches room temperature. Opening the lid too soon can cause pieces to
break.
Firing Processes
There are many different processes or techniques used for manipulating glass with heat inside a kiln. In this
manual we will focus on two, Fusing and Slumping. Other techniques include but are not limited to Drop Molds, Pate
de Verre, Casting, Painting, and Combing. For more information on using your kiln with these techniques please consult
your glass supplier.
GLASS 101