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Fused glass, also known as warm glass or kiln-formed glass, is made when two separate pieces of glass are heated in a kiln until they melt, or fuse, together into a single piece. Typically, fusing refers to any glass that has been worked in a kiln. The process by which fused glass is cooled at a controlled rate is called annealing. Through the process of annealing, the molecules in the glass are aligned, thereby reducing residual stress and making the fused piece stronger than most drinking glasses.
When fusing two pieces of glass together, it is important that each piece shares the same coefficient of expansion (COE). Glass expands as it is heated, but not all glass expands the same amount. The COE is a measurement of the rate at which a particular piece of glass expands. If two pieces of glass with different COEs are fused, the resulting fused glass piece will crack as it cools.
Fused glass can be fired at three different temperature ranges to yield different effects. Slumping refers to the process of firing glass at lower temperatures — between 1,099° to 1,251° F (593° to 677° C). This process is used to shape already fused glass over a mold.
Tack fusing requires firing at temperatures of 1,251° to 1,350° F (677° to 732° C). This allows two distinct pieces to be joined together without either piece losing its original shape. Working glass in a kiln heated to 1,350° to 1,501° F (732° to 816° C) is considered a full fuse. The process of full fusing joins two pieces together seamlessly, creating one finished piece.
Fused glass finishing techniques include fire polishing, combing, and kiln casting. To fire polish a piece of fused glass, the edges of the piece are heated until they become rounded and smooth. Dragging a raking tool across the surface of a heated piece of glass adds surface texture and is called combing. Kiln casting involved fusing several small bits of glass, or frit, together inside a clay or stainless steel mold.
Generally, fused glass techniques are used in sculpture, glass tiles, jewelry design, and other ornamental art. Kilns for firing glass come in a variety of shapes and sizes to suit various types of projects. The inside of the kiln is typically lined with fire brick — this substance assists in keeping the kiln hot and supports the piece being fired. The amount of time required for the kiln to reach a particular firing temperature is called ramp time. When combining various firing techniques, the ramp time will be different for each step in the fusing process.