GLASS 101 | 23 |
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 dealer. 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 dic- tate 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. All System 96 Glass 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 “Process” 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 (788 °C) and 1480 °F (804 °C). It is possible to get the same results (or the same amount of heatwork), by bringing the kiln to 1450 °F (788 °C) and holding it at that temperature for 30 minutes as you would by bringing the kiln to 1480 °F (804 °C) 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 suc- cessful 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 then 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. Determin- ing 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.
