Common Glass Fusing Problems

Written by rebecca suzanne delaney
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Common Glass Fusing Problems
Fused and slumped glass bowl (strawberry's snack in a glass bowl image by AGphotographer from Fotolia.com)

Glass fusing is a popular way to create glass vessels, platters, bowls and jewellery. The technique of glass fusing began in ancient Egypt. Modern glass fusing is, however, much easier as man now better understands the science of fusing. A number of companies make speciality glass designed especially for fusing. A number of kiln manufacturers make digitally-controlled kilns making fusing more predictable and requiring less attention by the kiln.

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Raising Kiln Temperature

Glass is subject to thermal shock if the temperature changes too quickly when you are bringing the kiln up to temperature to fuse, cast, slump or fire polish the glass. If your fused glass cracks or explodes in the kiln, adjust your firing schedule so you raise the temperature of the kiln more slowly. You can tell that you raised the temperature too quickly if the cracked pieces of glass are fire polished and smooth when you take it out of the kiln.

Lowering Kiln Temperature

Glass is subject to thermal shock if the temperature changes too quickly when you lower the temperature after fusing, casting, slumping or fire polishing. If your fused glass cracks or explodes in the kiln, adjust your firing schedule so you lower the temperature of the kiln more slowly after the glass has fused, slumped or fire polished. You can tell the thermal shock occurred when you were lowering the kiln temperature if the edges of the broken glass are sharp and jagged.

Annealing

Glass must be held in the kiln at the annealing temperature for a period of time to release the stress in the glass. The annealing temperature depends on the type of glass used. The length of time needed to properly anneal depends on the size and thickness of the glass.

Devitrification

Devitrification results in an unsightly white scum on the surface of kiln-fired glass. Devitrification occurs when the glass is held in the kiln at too high a temperature for too long.

Jaggd Edges

Glass likes to fuse to a thickness of about 9mm (or 0.351 inches). In most cases, one layer of glass is less than 9mm thick. One layer of glass will tend to pull in when it is fused so the glass rearranges itself to be about 9mm thick. One layer of fired glass will be smaller and thicker than it was before it reached fusing temperature, resulting in jagged edges.

Incompatible Glass

Glass must have the same coefficient of expansion (COE) in order to be fused successfully. Incompatible glass will crack or explode when fused together.

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