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Fused glass, as the name implies, does not normally require heating the glass all the way to the liquid state. In a basic fusing process, the object will start as multiple pieces of glass heated enough that they bond together. This is typically done on a flat surface in the kiln, producing a fused “blank”. This blank may then be put back into the kiln on top of a mold and heated just enough for the material to sag into the shape of the mold. This process is called slumping or draping.

One of the tricky elements in this art form is that successful fusing requires only using glass pieces that have matching coefficients of expansion (COE). COE is a measure of how much the material expands or contracts in a given change in temperature. When two pieces of glass with different COEs are fused together and then cooled, the differing shrinkage rates develop very large thermal stresses in the glass. These stresses ultimately lead to cracking and breakage. Therefore, fused glass artists take pains to ensure that all the glass they use has compatible COEs.

Normal glass, whether window glass, wine bottles or stained glass, has widely varying COE.  Even just changing the color of a glass can change it’s COE.  However, several companies specialize in making “fusible” glass with tightly controlled COEs to help individual artists avoid this issue. Another big challenge in kiln formed glass is controlling the temperature of the glass. As many of us are aware, cooling hot glass too fast can cause it to shatter. This is also true if glass is heated too fast. This is called “thermal shock”.  It is caused by the stresses that develop when one part of the object is hotter than another part.  Moreover, even if a glass object does not shatter or crack with excessive heating or cooling, these stresses can remain and cause it to crack later. Because of this challenge, most modern glass kilns use a digital controller so that the temperatures and rates of change can be precisely controlled

A typical fusing process requires the glass to be heated to about 1475 °F. In order to avoid thermal stresses in the process, the heating and cooling rates must be set according to the thickness of the glass. In a typical project of, let’s say, a large bowl, these rates may be as low as 100 degrees F per hour. That means that an average fusing cycle is about 20 hours, and can be much longer.

For many projects, the fused blank does not come out exactly the size or shape that we want. When this happens, the piece will be “cold worked” which can include cutting it to shape, and grinding or polishing the edges. There are variety of tools we use to cold work our pieces – diamond saws, belt sanders and lapidary grinders to name a few. All of these tools use water to lubricate the cutting and keep the blades cool.

Once the fused blank has been cold worked, it is ready to be slumped. For slumping, we want the glass to soften but not melt. This requires another full kiln cycle, but with a lower ultimate temperature of around 1175 °F.

There are, of course, as many variations to these processes as there are types and colors of glass. That’s why we love fused glass, the art is in the process as well as the composition and the idea.

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