Devitrification occurs sometimes in glass fusing during the firing procedure.
The exterior of the glass forms a scum that is whitish in color instead of a smooth glossy shine.
Molecules in the glass alter their structure into that of crystalline solids.
It is an unwanted effect, although some individuals use it as an intentional artistic technique.
Often referred to as devit, it can generally be caused from holding glass at a high temperature for too long.
This makes some of the chemical elements burn off.
It can also be caused by the existence of any foreign residue on the surface of the glass.
The chemical makeup of some types of glass can make them more susceptible to devit than other types of glass.
Usually the darker more opaque the glass, the more chemicals it has and the greater chance it could develop devit.
Therefore clear or uncolored glass is less likely to have this dilemma.
There are ways to avoid devitrification. These would include:
1. Cleaning the glass surface to remove any unwanted residue. This is an important step especially if the glass has been ground with a grinder or cut with a saw.
2. Avoid glass with a high lime content. Devit is common in glass that has a high lime content.
3. Making sure the kiln is free from any dust or debris.
4. Limiting the time glass spends at a high temperature. Especially for glass with a high lime content or any opalescent glass.
5. Sufficient ventilation during the burn-off phase for any binders.
6. Allowing a rapid cooling to the annealing temperature once the glass reaches a desired temperature.
7. A devit spray can be made or purchased and applied to the surface of the glass before firing.
8. If firing window or float glass, place the tin side down against the kiln shelf.
If a fired piece has devit there are a few methods that can be tried to correct or fix the problem.
1. Cover the surface of the fused piece with a sheet of clear glass and refire the piece. This could cause the piece to be less stable. It has been used effectively with full fused glass with no problems.
2. Applying some of the devit spray and refiring can also be effective.
3. Sandblast, acid bath or polishing the piece with a rotary brush can also sometimes remove the surface.
Take a look at this fantastic
from Bullseye on fixing surface flaws with clear powder.
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Devitrification to Glass Fusing Made Easy