fusing process by anne collins

Created in Wales with love and passion.

When glass is heated above 1200º F / 820º C it becomes as soft as toffee, stretching and slumping as its resistance to gravity slowly relaxes. With more heat, the glass softens to a honey-like consistency as separate parts liquefy and fuse together into a singular form.

Fused and kiln formed glass are general terms used to describe glass that has been manipulated, shaped, decorated or formed in a kiln. Included are a variety of specific activities such as fusing, slumping, combing, fire polishing and laminating glass. A couple of centigrade differentiation in temperature can make a difference between success and disaster. If the glass gets too hot, like mercury and silver, it draws into itself and creates a bubble. Hotter again, and it will run like water.

My pieces are normally created using three to five layers of glass, one or more of the layers are smaller pieces, so as to create a pattern. I then assemble all the layers. This process can take one to two days, depending on the complexity of the pieces.

I then place the work in a kiln, which has been specifically designed for glass fusing, and fire it to approximately 870 °C / 1,700 °F.

Once the optimum temperature has been reached, and held for up to hour, the layers of glass 'fuse' together. The length of the firing process depends on the size of the pieces. For instance, jewellery will fire within 6 to 8 hours whereas the firing cycle of larger pieces; bowls, wall pieces etc., is 18 hours. The larger the piece, the slower the firing as the glass can crack or devitrify (crystallise) if fired too fast.

The kiln is then allowed to cool down. This again is a delicate process. Cooled too fast and the glass could shatter. Cooled too slowly, and the glass can devitrify.

I re-fire many of the pieces because I either want to add more glass onto the already fired layers or, I wish to achieve a different effect. A single firing has one effect, a double firing another.

Once the final fusing firing has taken place, the piece is then re-fired to polish it. This is called fire polishing.

The glass then requires additional firings over, or into, a mould. This is called slumping. The flat, assembled piece of fused glass is placed onto the mould and re-fired to the point of being soft so that it can fall into, or over, the mould. This takes about twelve hours and often two firings are required in order to ensure the glass achieves the desired shape.

When choosing the glass for each piece its extremely important that the glass is 'compatible'. I usually use Bullseye glass, which has a cohesion (expansion and contraction rate) of 90. Occasionally I also use Spectrum glass, which has a cohesion of 96. Or, when I work with enamels and float glass these have a cohesion of 80. It is very important not to put the different types of glass together as they will expand (when heating up) and contract (when cooling down) separately. This will cause the glass to crack in the kiln as it cools.

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