Skutt GlassMaster Kiln - Manual (Page 27)

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Critical Temperature Ranges
A “Critical Temperature Range” is any temperature or temperature range in the firing cycle that has a high
level of potential for limiting the success of the project. Limited success can be expressed as overfired, under-
fired, breakage, devitrification, 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.
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The Heating Range goes from room temperature to the first set of data in the Process Range. The only
concern during this range is heating the pieces too fast without adding steps to the program. Steps are hold
periods at designated temperatures that allow the piece to balance out during the firing. Small pieces can
normally be heated as fast as 800 F./Hr. as long as steps are added. With larger pieces you will want to slow
the rate and possibly add additional steps depending on the size of the piece.
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The Process Range is the temperature range where the material begins to visibly change. It is this stage
that determines the final shape of the piece. It is often a good idea to add a pre- Process Range segment to
slow the kiln down before entering the Process Range. If the kiln is firing too fast into the process range it is
possible to overshoot your goal temperature.
During the Process Range temperatures and hold times are key. If you are unsure of the desired peak tem-
perature you may want to start on the low end of the range with a longer soak. This will help insure that thicker
pieces receive the proper heatwork throughout the entire piece.
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After the process range is through, it is desirable to cool the piece quickly for several reasons. The first
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 devitrification can occur during this cooling
period if the kiln is cooled to slow. Devitrification is a scummy white crystallization on the glass surface that is
difficult 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 cer-
tain models the thermocouple is in the rear of the kiln and the temperature from front to back can vary greatly
causing part of the piece to enter the annealing phase before the part in the rear.
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The final critical range is the Annealing Range. Every piece of glass has an annealing point, this is a
point in the cooling cycle where the molecules in the glass realign themselves into a solid and stable form. It is
very difficult to know exactly where that specific point will be, so during this period it is critical to fire the kiln at
a slower rate throughout the range.
Our pre- programmed firing schedules in the Glass Fire Mode anneal from 1000 °F to 750 °F which
should be adequate for most stained glass. By incorporating such a broad range the risk of breakage is lim-
ited. 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.
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