Glass Fusing Defined

Glass fusing is the process of stacking compatible glass to make a pattern or design. The stacked glass is then heated in a kiln until it melts together.

Because glass expands and contracts when heated and cooled, glass we fuse together must be compatible with each other. If they aren’t, stress cracks or breakage will result. We use glass that has been specially formulated and tested to be compatible.

Several manufacturers make fusing compatible glass, and each manufacturer has their own recommended temperatures to achieve specific results. We work with two primary families of fusible glass 96 COE and 90 COE. COE, (Coefficient of Expansion) refers to the rate at which a specific family of glass expands when heated and contracts as it cools. The expectation is that glass with the same COE is compatible with each other. It is believed it will expand and contract at the same rate during firing. For a fusing project to be enduring, all the glass used in a single project must have the same COE. Using the same COE will produce the most reliable and durable results. If incompatible glass is used together in a single project the glass will break due to internal stress, immediately or in time.

The firing process occurs in two basic stages: heating and cooling. During heating we take the kiln and glass project inside from room temperature to a target temperature of 1100°F – 1600°F (593°C – 871°C)depending on the type of fused “look” we want. At about 1365°F – 1385°F (742°C – 752°C) the glass will stick together. We call that tack fuse. At 1400F – 1425F (760C – 774C) the glass layers are fused, and the edges are soft and rounded. We call that a contour fuse. At 1465°F – 1480°F (796°C – 804°C), the different glass layers completely melt into one smooth piece. We call that full fuse.

Common Temperatures for Glass Fusing

Full Fuse 96 COE 1465°F (796°C), 90 COE 1480°F (804°C): At this temperature the glass pieces are melted together to form one smooth piece with visible, but not textural seams.

Contour Fuse 96 COE 1425°F (774°C), 90 COE 1400°F (760°C): At this temperature the glass layers are fused, and the edges are soft and rounded.

Tack Fuse 96 COE 1365°F (742°C), 90 COE 1385°F (752°C): Here the glass pieces will stick together. The edges of the glass will be square, shiny, and no longer sharp.

Slump 96 COE 1265F (685C), 90 COE 1265°F (685°C): At this temperature the glass will sag into and conform to the shape of a mold.

Once we have achieved the look we want, we begin the cooling stages.

Glass is sensitive to temperature change between room temperature and 1000°F (538°C). If we heat or cool the glass too quickly in this range breakage will result. We bring the glass down from the target temperature to 1000°F (538°C) quickly. Then we slowly bring the temperature down to room temperature. On the way down annealing takes place at 950°F (510°C) for 96 COE, 900°F (485°C) for 90 COE glass.

At this temperature we relieve the internal stress in the super-heated glass by holding it at the anneal temperature for a period of time. This allows all the glass in the project to equalize. How long this hold takes depends on the size and thickness of the project. The bigger the project and the thicker the glass the longer the hold. Proper annealing is crucial to the success of the project. Without adequate annealing the glass will break, in the kiln or later.

Anneal Temperatures Vary by Manufacturer

Each family of fusing material has its own temperature range and recommended hold time. Consult the manufactures published guides for specifics.

Once annealing is complete, we patiently allow the kiln and the project inside to slowly returned to room temperature.

Whether heating or cooling, it’s important to allow plenty of time for the glass to change temperature otherwise thermal shock, breakage caused by rapid temperature change, can occur.

If in doubt, fire slower.

Safety First: It’s important to note that the materials like glass and supplies like shelf paper that we put inside the kiln are special high-refractory materials designed to withstand excessive heat and are purchased at your art glass supplier specifically for kiln work.

Pro Tips:

Kilns fire differently, test fire your kiln and adjust the firing temperatures as needed.
Take notes and before and after pictures so you can repeat your successes and learn from mistakes.
Let large projects rest undisturbed overnight before opening the kiln.

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