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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 are 3 main distinctions for temperature application and the resulting effect on the glass.

Firing in the lower franges 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. 2000 BCE. 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. Michael and Frances Higgins began studio fusing in 1947, became nationally prominent in the 1950's, and continued until their deaths in 1999 and 2004. "Higgins Glass" is understood to denote their products. Modern glass fusing is a widespread hobby but the technique is not widely used for large scale glass production.

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 815ºC (1500ºF) to 573ºC (1000ºF) 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 516ºC (960º). The second stage- the anneal soak at 516ºC (960ºF) is meant to equalize the temperature at the core and the surface of the glass at 516ºC (960ºF) 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 371º C (700ºF), soaked for 2 hours at 371º C (700ºF), down again to 260ºC (500ºF) which ends the firing schedule. The glass will remain in the unopened kiln until the pyrometer reads room temperature.

Note that these temperatures are not hard and fast rules. Depending on the kiln, the size of the project, the number of layers, the desired finished look, and even the brand of glass, ramp and soak temperatures and times may vary.

Finished products

While fused glass techniques are generally used to create glass art, glass tiles and jewelry, the slumping process allows the creation of larger, functional pieces like dishes, bowls, plates and ashtrays. Producing functional pieces generally requires 2 or more separate firings; one to fuse the glass and a second to shape it.

 
   
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