Skutt GlassMaster Kiln - Manual (Page 26)

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The following guidelines just scratch the surface of the knowledge base associated with firing glass. We
highly suggest you take a firing class from your local distributor. If classes are not available in your area, there are
numerous books available on the subject that can be found at bookstores and on the internet.
What is a Firing Program?
Glass is very sensitive to changes in temperature below 1000 °F. If it is heated or cooled too quickly through
certain temperature ranges it creates stress within the glass which can
cause breakage. Firing programs are used to control these temperature
rates and limit the amount of stress created within the glass as well as
create the desired effect on the glass.
A firing program is composed of one or more firing segments that
dictate the heating or cooling rate throughout the program. Each one of
the lines in the chart represents a segment or hold time within a segment
and the slope of the line represents the rate of firing. A firing program
is either entered into a kiln controller or on kilns without controllers it is
replicated by turning up and down temperature switches.
Type of Glass
The art of firing glass has been around for centuries however, comparatively speaking, it has only been
recently that companies have begun manufacturing glass specifically designed to fuse together. Glass, like most
everything on earth, expands when exposed to heat and contracts when it is cooled. It expands at a measurable
rate, known as the COE, or coefficient of expansion, and as it becomes liquid it flows at different rates which is
referred to as it’s viscosity level.
These variables and a host of others must be carefully managed to create glass that can be fused together
without crazing, cracking, warping, or breaking. Always consult with your supplier of glass to determine if the
glass you wish to fuse is compatible.
Heatwork
Heatwork is a term used to describe the relationship of time and temperature and their combined effects on
glass. To a certain extent the two are inversely related. This means that the higher the temperature the less time is
needed to create the same effect and likewise, the lower the temperature the more time is needed.
This concept becomes most useful at the “Working” temperature range of glass. This is the temperature
range where the glass is fused, slumped or sagged. Most fusing glass will fuse between 1450 F and 1480 F. It
is possible to get the same results (or the same amount of heatwork), by bringing the kiln to 1450 F and holding it
at that temperature for 30 minutes as you would by bringing the kiln to 1480 F and holding it for only 10 minutes.
There may be other factors that make you choose one working temperature over the other such as the thickness of
the project.
Size and Mass
The size of the piece is one of the most influential factors for creating a firing program. One of the keys to
successful heatwork is having the entire piece go through critical temperature ranges at the same moment. When
a piece is thick it takes longer for the center to heat up than it does the outside of the piece. When it is a large
diameter, slight differences in temperature throughout the chamber of the kiln can cause the piece to expand at
different rates.
The key to firing larger and thicker pieces is to slow the firing rates through critical temperature ranges.
Determining how slow is often a trial and error proposition therefore it is best to start with a conservatively slow
program. More projects are ruined by going too fast than too slow.
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