Glass Fusing

Fused glass is a term used to describe glass that has been fired (heat-processed) in a kiln at a range of high temperatures from 593º C (1100ºF) to 816º C (1500ºF). There 3 main distinctions for temperature application and the resulting effect on the glass.

Firing in the lower ranges of these temperatures 593º-677ºC (1100º 1250º F) is called slumping. Firing in the middle ranges of these temperatures 677ºC- 732ºC (1250º-1350ºF) is considered "tack fusing". Firing the glass at the higher spectrum of this range 732ºC -816ºC (1350º-1500ºF) is a "full fuse".

All of these techniques can be applied to one glass work in separate firings to add depth, relief and shape.

Fused glass consists mainly of silica. While the precise origins of glass fusing techniques are not known with certainty, there is archeological evidence that the Egyptians were familiar with rudimentary techniques ca. 2000Bc although this date is generally accepted by all researchers, some historians argue that the earliest fusing techniques were first developed by the Romans, who were much more prolific glassworkers. Fusing was the primary method of making small glass objects for approximately 2,000 years, until the development of the glass blowpipe. Glassblowing largely supplanted fusing due to its greater efficiency and utility.
While glass working in general enjoyed a revival during the Renaissance, fusing was largely ignored during this period as well. Fusing began to regain popularity in the early part of the 20th century, particularly in the U.S. during the 1960s. Modern glass fusing is a widespread hobby but the technique is not widely used for large scale glass production.

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 to avoid devitrification. It is then allowed to cool slowly over a specified time, soaking at specified temperature ranges which are essential to the  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.

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