This is based on Graham Stone’s work with float glass. The temperatures are applicable to float glass, and so need to be adjusted for any other glass, but illustrate the principle of how heating temperatures affect the glass. Temperatures in degrees Celsius.
10-250 Slow rate heating up. Risk of thermal shock. Venting often done in this phase.
250-500 Medium rate heating. Risk of thermal shock diminishing.
400 + Many glasses now tolerate fast heating up ramp rate.
550 Glass surface beginning to soften slightly
600 Safe from thermal shock above this temperature
610 Glass bending slightly, picking up texture.
680 Glass begins to stick to itself. Tin bloom becomes iridescent.
690 Fusing glasses reaching their softening points.
715 Glass beginning to stretch. Tack-fired pieces adhered by now.
720 Subtle devitrification and iridisation burn off becoming a factor with some glasses.
730 Softening point of float.
750 Edges no longer sharp. Tin bloom stretching becoming "frosty".
760 Tack fuse range for fusing glasses.
770 Float glass fused, but still "sitting up".
790 Trapped air can cause bubbles under sheet glass at this temperature.
800 Full fuse for most fusing glasses.
820 Fused float glass nearly flat.
825 Full fuse for float glass. Devitrification more pronounced.
850 Glass flowing.
950 Glass soft enough to "rake".
1000 Approximate liquidus temperature.
Based on Firing Schedules for Glass; the Kiln Companion, by Graham Stone, Melbourne, 2000, ISBN 0-646-39733-8, p24
Post revised 5th March 2014