The basic answer – glass that has undergone multiple heating and cooling segments to achieve the desired design. Okay, there are a few steps in the process….and it’s important to know they don’t always turn into the desired design. Sometimes they turn into something completely different than what you planned, but that isn’t always a bad thing!
First the glass is heated up in a kiln to a high enough temperature to soften and sometimes even liquify, depending on the effect the artist is going for. This heating is done in segments, slower in the beginning stages to avoid thermal shock. There are various holds throughout the process, including at the target or highest temp. Once the target temp has been reached and held for a period of time (usually a few minutes) the cooling begins with a quick drop to an annealing point. Failure to anneal long enough can result in instability of the glass – and it may never make it out of the kiln before breaking. Worse, the break could happen later (even days, weeks or months later!) Once the glass is held for a period of time at the annealing temp, the remaining segments will slowly cool the piece down. These segments are slow in the ramp down just like the first ones were slow in the ramp up, to avoid thermal shock during the cooling process. Normally once my kilns reach around 600 degrees F the program shuts off and I leave the kiln lid closed until the temp reads below 100 degrees F.
Some glass may be formed only using a single layer, although often times pieces are cut and placed together and/or layered before firing. Here are a couple of examples of smaller kilns – they don’t require special wiring. One is a Jen-Ken 15/6 and the other a Paragon Fusion 16. I’ve included a couple of photos showing taken during firing, one is a bowl slump and the other a fused panel that was later slumped.
Glass used in fusing projects is not the same as glass used in stained glass projects. Fusing glass is categorized by COE (Coefficient of Expansion) and the COE cannot be mixed in the same project. Some examples are Bullseye glass which is 90 COE, Spectrum glass which is 96, and other types including 104 (used in glass rods and bead making.) Some glass manufacturers like Uroboros and Wissmach carry both 90 & 96 COE glass. The higher the number, the softer the glass, meaning it will melt at a lower temperature. Mixing different COE glass together can cause the glass to form stress points which can result in breakage. While the pattern and appearance of the break is usually different than when related to annealing, the effect is similar; it can break anytime between when it is still in the kiln to any point later after cooling. Below is an example of a project I made with some scrap glass that I was not sure of the COE. Rest assure I have since gotten very careful about storing and labeling my scrap!
A fused glass project will usually go through multiple firings. The first firing is typically the fuse (or it may be cast in a mold.) Fusing can be done at several different levels, from a “tack” fuse where pieces retain more texture, to a full fuse where all layers become one smooth piece. Depending on what level of fuse the artist is looking for, temperatures, ramp rates and holds in the kiln will vary.
Once the glass has completed fusing and cooled it can be formed by “slumping” into a mold, draped over a mold, or dropped through an open ring. Molds are typically ceramic or stainless steel, although you can create your own molds using fiber paper or fiber board as well as molding materials like Castalot. Molds are first coated with a release material so the glass does not stick. This may be a wash that is brushed or sprayed on, or a chemical spray such as Boron Nitride (MR-97). Here are some examples of release materials and molds that have been coated prior to firing.
Sometimes no matter how careful you are, the project may not turn out! However not all “failures” are a waste. Some are just “happy accidents” and end up turning into something completely unexpected. Others may end up being cut or crushed and used as elements in other pieces, broken into pieces and used in mosaic work or even crushed and cast in the kiln making unique art glass. Unless your failure is suspect to possible COE incompatibility, don’t throw it away! Pot melts and screen melts are another good way to use up pieces that did not turn out as planned. Too many firings though, especially at higher temps can effect the stability of the glass so do keep this in mind.
Here is my favorite “happy accident” – a large bubble formed in the base of this bowl, possibly from the mold not having enough holes drilled in it for venting. However the effect of the bubble makes this a unique design that I’m not sure I could replicate if I try!