Devitrification

What does devitrification mean?

Devitrification, commonly called ‘devit’, is a gradual process wherein the disordered silica particles combine to reform into their preferred crystalline structure. It occurs as a result of prolonged heating at elevated temperatures.

 It can be regarded as a two step process:

• Nucleation, followed by
• Crystal growth.

Nucleation is the process of forming nuclei: wherein small numbers of atoms link together to create a site around which crystals will grow.

Initially these nuclei are so small that they don’t impair the passage of light, and so are invisible to the eye. Devit can be proceeding for some time before being evident.

Whilst crystal growth is actually a natural progression from the development of nuclei, the separation into two steps is made because the crystal growth does not always follow nucleation at the same rate: there being preferential sites where it will occur more rapidly than at others.

In soda lime glass, nucleation starts at below 750°C (1380°F) and can take place throughout the entire mass.

The rate of growth increases with increasing temperature to a maximum which is well above normal kilnforming temperatures.

As far as the kilnformer is concerned, it is only the visible effects which are of concern.

• Crystal growth will initially occur at the surface of the glass: preferentially at surface defects such as scratches or chips on the surface or edges of the glass.
• The hotter the glass the faster the rate of devit development. The face receiving the heat will be the hottest, so should be the first place for it to show.
• Dust sites on the surface have been shown to significantly enhance surface crystallisation on float glass.  
• As the crystals grow to a visible size they will appear first as a fine smoky effect, then a haze followed by a roughening and possible creasing of the surface.
• Because of the variable effects of fluxes leaching from the glass, devit won’t necessarily show a uniform effect, but can be patchy.

Tin bloom or devit?

In its early stage, devit can be confused with tin bloom. Making sure the tin side is down can help sort this out.

The cloudy effect often seen on the underside of a fired piece and originating from bedding powders such as whiting (calcium carbonate) or from binders in CF paper can also be confused with devit. Again, its position should give the clue. If it appears first on a cooler spot on the glass it is unlikely to be devit.

In side fired kilns, the edges of glass positioned very close to an element can become hotter than the body of the glass and can thus be a preferential site for devit.

Fusible and other art glass is also soda-lime, so is subject to the same process. However, because these glasses are designed for multiple firings, the glass chemistry can be altered to slow down the progress of devit.

Preventing devitrification

Some recommended art glass firing schedules call for greatly extended firing times, so surface sprays have been developed which retard the onset of visible devit. These apply a fine layer of lead or other devit retarding materials to the surface of the glass. These are absorbed into the glass when heated: in effect creating a different composition glass on the surface.

It is interesting that amongst Australian glass artists these same glass types are often fired with much shorter firing patterns than those recommended by the makers. Surface spray use here appears to be low.

Like other processes related to kilnforming, the devit process is time-temperature dependent.

Devit is a process which is part and parcel of kilnforming: it cannot be avoided, so must be lived with. To minimise its effects:

• Make sure that the glass is thoroughly washed and cleaned before firing.
• Minimise the incidence of scratches chips or rough edges
• Hold glass at elevated temperatures for the minimum time necessary for the particular process.
• Soak glass at the minimum temperature necessary to achieve the desired result.

At the risk of being repetitious, ‘devit should not be confused with tin bloom’. Devit, at a certain stage of development can have an appearance similar to tin bloom.

Tin bloom, if it is to develop at all, will appear after only a short time at elevated (slumping or fusing) temperatures: whilst devit can take some time to become visible to the naked eye.

How long does it take to appear?

For most kilnformers devitrification should not be a problem if firing patterns are kept short. It is useful to just be aware of how it develops and avoid as far as possible those situations which can cause it to become visible.

Obviously not all glass is subject to devit to the same degree. Glass makers can vary formulas to inhibit or retard the devit process. Art glass manufacturers, being aware of how glass goes through lmultiple firings at temperatures which promote devit, sensibly formulate their glass to retard its development. 

Makers of industrial grades of float glass generally do not design their product for kilnforming, although it is widely used for that purpose. However makers such as Viridian, do perform kilnforming processes on their glass so are more aware of these issues.

BUT, how do you know where a piece of glass has come from!