What is it? When does it happen? Why does it happen? These are frequent questions.
Devitrification is the beginning of crystallisation of the surface of the glass. It can look like a dirty film over the whole piece or dirty patches. At its worst, the corners begin to turn up or a crackling can appear on the granular surface.
Devitrification occurs in the range of approximately 700°C –760°C. This means that you need to cool the project as quickly as possible from the working (or top) temperature to the annealing point, which is, of course significantly below this range.
There is evidence to show that devitrification can occur on the heat up and will be retained in the cooling. Normally this is not a problem as the advance on the heat up is relatively quick through this range. The quick advance does not (and should not for a variety of reasons) be as fast as possible. 300°C will be sufficient.
The devitrification seen in typical studio practice – especially among novices - results more often from inadequately cleaned glass than from excessive time at a particular temperature, or up or down through the devitrification range.
Temperature range for devitrification
Homemade devitrification solution