Saturday, 28 December 2024

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

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 in the composition of the glass, acting as a flux.

Visible devitrification generally occurs in the range of approximately 720°C – 830°C/1330F - 1525F, depending to some extent on the type of glass.  This means that 
the project needs to be cooled 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, causing movement in the glass and so working against any crystallisation.  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 develop.


Medical research into using a glass matrix to grow bone has shown that devitrification begins around 650C/1200F, but only becomes visible after 700C/1290F.  This has implications for multiple slumps.  Devitrification is cumulative, so the devitrification that may have begun on the flat piece will be added to in the slumping process and may become visible.  For me this has appeared as a haze on the edge of the slumped piece.  Avoidance of this effect is by thorough cleaning of the piece before placing it in the mould.

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 edges to fit.  Even though the ground edge is cleaned, it may still be rough enough to promote devitrification.  The edge must be prepared for fusing by grinding to at least 400 grit (600 is better).  Alternatively, use a fine coating of clear powder to give a new surface to the whole piece.

Dr. Cook suggests three approaches to devitrification:*
Resistance through:
 - Schedules
 - Flux

Dealing with it:
 - Cold work
 - Acids
Embrace it:
 - Allow it
 - Use it

Other sources of information:
Temperature range for devitrification
Homemade devitrification solution
Frit to fill gaps
Low Temperature Kilnforming at Etsy and Bullseye


* From a lecture given by Dr. Jane Cook at the 2017 BECON

[entry revised 28.12.24]

1 comment:

  1. thank you for the explanation...I have only had a cloudiness appear once when I tried mica flakes and assumed it was from the mica.
    Good explanation!

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