Devitrification

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Devitrification is a condition in the firing process of fused glass whereby the surface of the glass develops a whitish scum, crazing, or wrinkles instead of a smooth glossy shine, as the molecules in the glass change their structure into that of crystalline solids. While this condition is normally undesired in glass art, it is possible to use devitrification as a deliberate artistic technique.

Causes of devitrification, commonly referred to as "devit", can include holding a high temperature for too long, which causes some of the chemical elements to burn off, or the presence of foreign residue such as dust on the surface of the glass or inside the kiln prior to firing. The chemical compositions of some glass can make them more vulnerable to devitrification than other glass, for example a high lime content can be factor in inducing this condition. In general, the darker, more opaque the glass, the more chemicals it contains, and the higher the chance it might devit. Consequently, clear uncolored glass is least likely to devit.

Techniques for avoiding devitrification include cleaning the glass surfaces of dust or unwanted residue, and allowing rapid cooling once the piece reaches the desired temperature, until the temperature approaches the annealing temperature. Devit spray can be purchased to apply to the surfaces of the glass pieces prior to firing which is supposed to help prevent devitrification, however there is disagreement over the long term effectiveness of this solution and whether it should be used as a substitute for proper firing techniques.

Once devit has occurred, there are techniques that can be attempted to fix it, with varying degrees of success. One technique is to cover the surface with a sheet of clear glass and refiring. Since devitrification can change the COE somewhat, and devitrified glass tends to be somewhat harder to melt again, there is the possibility of this technique resulting in a less stable piece, however it has also been used effectively with full-fused pieces with no apparently problems. Applying devit spray and refiring can also be effective. Alternatively, sandblasting, acid bath, or polishing with a pumice stone or rotary brush can be used to remove the unwanted surface.

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In addition, glass can be formed by other techniques that are common for ceramics or metals, including polishing, sintering, sawing, grinding, drilling, cutting, and bending. For application to glass these techniques may need to be adjusted.