Glass is amorphous, meaning that “its molecules are not arranged in a regular, specific pattern — like those of a crystalline material — but are random in their configuration.” (Bulleyes TechNotes4, 2007) And, unless you have worked with glass in a kiln, as flame work, or blown, you might not know how finicky glass can be. As glass moves through different stages — from a cool solid to a very hot liquid and then back down to a solid again – the temperature must be adjusted many times during one single session of firing in the kiln. If the transformation is not gradual, the stress caused by molecules combating each other at different rates will create an unplanned and heartbreaking crack through its surface. Glass expands or contracts at a rate determined by its coefficient of expansion, so it is critical that an artist buys glass that is manufactured with a high degree of quality control. You cannot mix glass that have different coefficients; if one type expands faster than another, its molecules will push against the other and cause the piece to crack and break.
To give you an idea of how glass behaves as a kiln ramps up the temp, here is a short description of what happens at different stages, based on how I usually approach creating a piece of glass art. But before I can get into the fusing process, I need to prepare the design for my piece. After drawing intricate patterns on tracing paper, I cut out different colored glass pieces using a glass cutter, a bandsaw or ring saw for more intricate curves. These saws are covered with small diamond bits which grind their way through the tough edges of glass. Then, onto a properly prepared kiln shelf, I assemble the pieces on to a large bottom piece cut out to the intended full size. Next, I program the kiln to go through a series of stages, commonly referred to as ramps. There are three different numbers to specify. One, the rate — or how fast I want the temperature to rise (how many degrees per hour). Two, the temp I want the kiln to reach. And three, the hold — or how long I want the controller to keep the kiln at that temp. Each kiln has its own idiosyncrasies and there is much trial and error before you obtain the desired results. For the first ramp, I program the controller so that the temp moves up at a rate of 300 degrees/hour from room temp to 1000 degrees. Glass is subject to thermal shock as the temperature increases, so I am very conservative with the rates I use during this ramp, particularly as it moves through a strain point of about 920 degrees. I soak the glass (maintain a constant temperature) at 1000 degrees for a short while so the glass will not get stress. For my second ramp, I program the computer to increase the heat 400 degrees/hour, move up to a temp of 1250 and then hold for 30 minutes. This soak helps the glass to “settle down” and also squeeze out any bubbles that might get trapped beneath the surface. For my third ramp I program a rate of 600/hour and move up to about 1500 degrees. If you were to look inside the kiln you would see that as the temp reaches between 1250 and 1350 degrees the surface of the glass starts to smooth out, and any abrasions on the surface disappear. If the temp is held too long undesirable crystals will begin to grow. Between 1400 and 1500 degrees glass becomes a molten flow and when it later settles itself back into a solid it is completely smooth. For my fourth ramp I program the kiln to go as fast as it can from 1500 degrees back down to 1000 degrees, which I hold for 30 minutes so the glass can properly anneal. I use only Bullseye glass, but because I mix iridescent, transparent, opaque and other speciality glass I place a hold at a temp higher than what others usually do (at around 960 degrees). After this, I let the kiln slowly lose temp at 150 degrees/hour all the way down to a safe 700 degrees. From when the glass was initially placed into the kiln, it might be 12 or 14 hours until the glass slowly moves back down to a temp that is cool enough for you to reach in and remove the glass from the kiln. In some cases, I do not take the glass up to the full fuse point, but stop short, somewhere around 1400, so that the individual pieces of glass — placed on top of other glass — stick together while retaining their particular shape. This is referred to as a Tack Fuse. Artists frequently intentionally use a Tack Fuse so that pieces have dimensionality on the surface.
If the first fuse firing is successful, I select a mold and place it down on the kiln shelf. Then I program the kiln for a slump, and take the temp up only as high as 1250 degrees, which is where glass starts to soften, but maintains its original diameter. If placed in a mold, this is where the piece flops down to assume that specific shape. Again, it is important to soak the glass so it has time to adjust to its new molded position.
So working with glass, and understanding how it behaves as a material process is part science, and also art. As you admire an individual piece of kiln-formed glass, take time to appreciate how careful the artist must be while controlling the creation of their work.