When preparing for multiple firings of elements onto a prepared piece, you need to consider the order and temperatures of events so that you do not harm an earlier stage of the project. This blog entry will not give definitive temperatures, as that varies by glass and by kiln. Instead, it indicates what happens in progression from highest to lowest temperatures in approximate Celsius degrees.
ca. 1300C - Approximate liquid temperature
ca. 850 – 1000C - Glass blowing working temperature
ca. 950C - Raking and combing
ca. 850C - Casting
ca. 810C - Full fuse
ca. 790C - Large bubble formation
ca. 770C - High tack, low contour fuse
ca. 760C - Tack fuse
ca. 750C - Fire polish
ca. 700C – 760C - Devitrification range
ca. 700C - Lamination tack
ca. 600C – 680C - Slump and drape
ca. 650C - Vitreous paint curing temperature
ca. 600C - No risk of thermal shock above this temperature
ca. 540 – 580C - Glass stainers enamel curing temperature
ca. 520 – 550C - Silver stain firing temperature
ca. 550C - Glass surface beginning to soften
Slow rates of advance needed from room temperature to ca. 500C
These temperatures are of course, affected by the soak times. The longer the soak time, the lower temperature required. The rate at which you achieve the temperature also affects the effective temperature. Slower rates of advance require lower temperatures, than fast rises in temperature. These illustrate the effect of heat work.
The table shows for example you need to do all the flat operations and firings before slumping or draping. It also shows you can use vitreous glass paints at the same time as slumping and draping. This emphasises that the standard practice is to plan the kind of firings you will need for the piece and do them in the order of highest temperature first, lowest last.
In general, you do need to do the highest temperature operation first and lowest last. But there are some things you can do with heat work. For example, if you needed to sandblast a tack fused piece, but did not want to risk reducing the differences in height there things you can do. From the list above, you can see the glass surface begins to soften around 500C. It is possible to soak the glass for a long time around 500C to give it a fire polish, instead of going to a much higher temperature. You will need to experiment to find the right combination of temperature and soak length, but it can be done.
This article is to show that knowledge of what is happening to the glass at different temperatures, can help in “fooling” the glass into giving you the results you want without always following the “rules”. This may also be what it is to be a maverick glass worker. Use the behaviour of glass to your advantage.