Devitrification

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.  This is distinct from the effects from an unstable glass or the crizzling as in a ceramic glaze. Divitrification can occur within the glass, but normally is a surface effect.

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. There are other factors at play also.  The composition of the glass has an effect on the probability of devitrification.  Opaque glass, lime, opalising agents, and certain colouring agents can create microcrystalline areas to "seed" the devitrification process.  One part of the composition of glass that resists devitrification is the inclusion of boron.

Devitrification generally occurs in the range of approximately 700°C – 840°C, depending to some extent on the type of glass.  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 by spending too long in this devitrification range, and that it will be retained in the cooling. Normally this is not a problem as the practice in kilnforming is for a quick advance on the heat up through this range.  The quick advance does not (and should not for a variety of reasons) need to 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:*
Resistance through:
 - Schedules
 - Flux
Dealing with it:
 - Cold work
 - Acids
Embrace it:
 - 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 25.9.19]