What is it? When does it happen? Why does it happen? These are frequent questions.
Dr. Jane Cook states that devitrification is not a category (noun), but a verb that describes a process. Glass wants to go toward devitrification; a movement toward crystallisation.*
Mild devitrification is the beginning of crystallisation on 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.
Differences in the surface of glass promotes precipitation of the crystal formation of silica molecules. This fact means that two defences against the formation of crystals are smooth and clean surfaces.
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 that it 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. A rate of 300°C per hour will be sufficient, as time is required for devitrification to occur.
The devitrification seen in typical studio practice results more often from inadequately cleaned glass than from excessive time at a particular temperature, up or down through the devitrification range.
It is often seen as a result of grinding to fit shapes. Even though the ground surface is cleaned, it may still be so rough as to promote devitrification. The surface must be prepared for fusing by grinding to at least 400 grit (600 is better). Alternatively, use fine frit of the same colour as the darkest glass to fill the gaps. This normally is applied in the kiln, so the pieces are not disturbed.
Dr. Cook suggests three approaches to devitrification:*
Dealing with it:
- Cold work
- Allow it
- Use it
Temperature range for devitrification
Homemade devitrification solution
Frit to fill gaps
* From a lecture given by Dr. Jane Cook at the 2017 BECON
[entry revised 26.7.17]