Fusing glass in a kiln is a fascinating technique that enables artists to create unique and breathtaking designs in glass. Fused glass is also referred to as kiln-formed glass, art glass fusion and warm glass. The “warm” of warm glass is between 1,100 and 1,700 degrees Fahrenheit (600 and 925 degrees Celsius). At these temperatures, glass softens enough that when pieces of glass are heated and pressed together, they will fuse into a single seamless piece. This is the underlying principle behind glass fusing.

Glass Fusing

Glass fusing is the process of using a kiln to join together pieces of glass. If you apply heat to glass, it will soften.  If you continue to apply heat, the glass will become more fluid and flow together. Two or more pieces of glass will stick to each other.  When the right kind of glass is heated and then cooled properly, the resulting fused glass piece will be solid and unbroken.

Fused glass is normally fired (heat-processed) in a kiln at a range of high temperatures from 593 °C (1,099 °F) to 816 °C (1,501 °F). There are 3 main distinctions for temperature application and the resulting effect on the glass. They are as follows:

1. Slumping

Firing in the lower ranges of these temperatures 593–677 °C (1,099–1,251 °F) is called slumping. Slumping is a categorical description of an area of techniques for the formation of glass by applying heat to the point where the glass becomes plastic. The increasing fluidity of the glass with temperature causes the glass to ^slump^ into the mould under the force of gravity. Glass is most commonly heated in an oven, often using glass in a sheet form and “slumping” it over a form or into a mould.

Moulds are generally made of high temperature plaster, clay coated with plaster or another release agent, graphite, sand mixed with a bonding agent, steel, or other materials. At the point where the glass has achieved the desired form the heat is quickly vented and the temperature reduced to prevent further movement of the glass and then it is stabilized at its respective annealing temperature and annealed.

2. Tack Fusing

Tack Fusing Glass refers to the effect that is obtained when two or more pieces of glass are heated to approximately 1350 to 1375 degrees F. This temperature range will result in any pieces of glass that are in contact with each other fusing together, while still allowing each piece to retain its^ original shape, size and thickness.

3. Full Fusing

Several pieces of glass fused into a single finished piece of uniform thickness by heating them to somewhere between 1450 and 1475 degrees F is known as Full fusing. At these temperatures your glass will have melted enough to combine and flow together into a single piece of fused glass. This piece may be a finished piece or a starting point from which you cold work, cut or reshape the piece prior to another fusing.

Techniques

Most contemporary fusing methods involve stacking, or layering thin sheets of glass, often using different colors to create patterns or simple images. The stack is then placed inside the kiln (which is almost always electric, but can be heated by gas or wood) and then heated through a series of ramps (rapid heating cycles) and soaks (holding the temperature at a specific point) until the separate pieces begin to bond together. The longer the kiln is held at the maximum temperature the more thoroughly the stack will fuse, eventually softening and rounding the edges of the original shape.

Once the desired effect has been achieved at the maximum desired temperature, the kiln temperature will be brought down quickly through the temperature range of 815 °C (1,499 °F) to 573 °C (1,063 °F) in order to avoid devitrification. It is then allowed to cool slowly over a specified time, soaking at specified temperature ranges which are essential to the annealing process. This prevents uneven cooling and breakage and produces a strong finished product. This cooling takes place normally for a period of 10–12 hours in 3 stages.

The first stage- the rapid cool period is meant to place the glass into the upper end of the annealing range 516 °C (961 °F). The second stage- the anneal soak at 516 °C (961 °F) is meant to equalize the temperature at the core and the surface of the glass at 516 °C (961 °F) relieving the stress between those areas. The last stage, once all areas have had time to reach a consistent temperature, is the final journey to room temperature. The kiln is slowly brought down over the course of 2 hours to 371 °C (700 °F), soaked for 2 hours at 371 °C (700 °F), down again to 260 °C (500 °F) which ends the firing schedule. The glass will remain in the unopened kiln until the pyrometer reads room temperature.