Glass fusing is the process of using a kiln to join together individual pieces of glass to form one larger piece of glass by applying the propper amount of heat for a predetermined amount of time. When the proper amount of heat is applied to two or more pieces of glass, it will soften, become more fluid and flow together (or "fuse") 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. Glass “likes” to be 6mm thick when it is taken to full fusing temperatures. If glass is thinner than 6mm, the pieces pull in to become 6mm. If it is thicker, it spreads.

Glass Slumping

Many people also use the word "fusing" to include bending and shaping glass using the heat of a kiln. This manipulation can take many forms, but the most common is "slumping," where a mold is used to cause already fused glass to take on the shape of a bowl, a plate, or similar object. Other kinds of manipulation done with fusing techniques are "combing," which involves using a tool to distort the shape of the glass while it is hot, and "fire polishing," which uses a kiln to heat the glass just enough to make it shiny and smooth. Glass slumping should be done in a separate firing from fusing.

COE

Glass has a COE (coefficient of expansion). Pieces of glass with different COE are not compatible and should not be fused. They will almost always break.

Devitrification

Devitrification is when glass molecules start to crystallize around 1350º to 1400ºF taking on the appearance of a whitish scum on the surface of the glass being fired. This occurance is considered to be undesirable but is sometimes used as a finishing effect on glass pieces. To avoid this reaction it is recommended to minimize the amount of time at this temperature range.

Dichroic

Dichroic glass got its start in the 1880's in Germany with several scientists who discovered how to deposit thin metal layers onto a substrate. This scientific field of 'thin film physics' grew slowly until the 1950's and 1960's space age demands for specialized mirrors and lens coatings gave impetus to rapid development. Dichroic glass did not migrate to the art field until the 1980's. It began to grow in popularity in the 1990's as more artists worked with it and more companies began to produce an increasing variety of standard products for the glass artist.

The term "dichroic" comes from the Greek 'di' meaning two and 'chroic' meaning color. A sheet of dichroic glass will show one color when you look through the glass and a second color when you look at light reflected from the surface of the glass. The effect is achieved by vacuum vapor depositing multiple thin layers of various metal oxides onto a glass sheet. The number of layers range from 15 to 50 and altogether make a coating only 5 - 100 millionths of an inch thick. The multiple layers constitute an "interference filter" whose spacing and dielectric properties can be designed to pass and reflect specific colors.

Annealing

Annealing is a heating and cooling process by which the stress in glass is relieved and the molecules in the glass are allowed to cool and arrange themselves into a solid, stable form. Successful annealing is the key to creating glasswork that will remain stable and strong once it cools to room temperature.