Rates of Advance for Thick Glass
How fast can I heat up thick glass?
This is a frequent question. There are general answers and some detailed work has been done – mainly Graham Stone’s Firing Schedules for Glass, the Kiln Companion and Bob Leatherbarrow’s Firing Schedules for Kilnformed Glass, JustAnother Day at the Office.
A general approach which is generally successful is to work out the annealing part of the schedule first. I know, it seems backward. But if you know the speed at which you will do the final cool, you can use that speed as a suitable initial rate of advance for the un-fused piece. If the glass will survive the cool down rate noted, it stands to reason that it will survive through the brittle phase of the firing too.
You do need to be a little more cautious for the 6mm and 12mm schedules as given in the Bullseye chart for annealing thick slabs. As an example, Bullseye recommend a final cool down of 300°C for a 12mm piece. You will find eleswhere that an initial rate of advance of 150°C for 6mm and 12mm is acceptable up to 540°C, so should be adequately safe. Use this rate rather than the final cooling rate listed in the chart for these thicknesses. The final rates as given in the chart for even thicker pieces are suitable initial rates of advance.
These rates are for full fused pieces that are of a single thickness all over the piece. If your piece is tack or contour fused, you should consider using the final cool down speed for one and a half, or twice the actual thickness, as your initial rate of advance.
These rates may seem overly cautious, but you can experiment with other rates depending on the complexity of your pieces. The experimentation can be on clear glass assembled as the final piece will be. If you use float glass, remember that it is more acceptable of fast rates of advance when transferring the results to fusing glass.