The main ingredient in glass is silicon dioxide (commonly referred to as silica), which melts at a temperature of 1,700 degrees C. Sodium carbonate and calcium oxide are added to the glass to allow it to heat and melt at a lower temperature. When melted it has the consistency of cold syrup, and when cooled it becomes a brittle quartz glass.
Though possessing the same ingredients, not all glass is equal. Glass expands at different rates called the coefficient of expansion or C.O.E. Glass can only be fused together correctly if it has the same C.O.E. If you fuse glass pieces with unmatched C.O.E., the glass will crack when cooled.
Kilns used for fusing glass can either be glass or ceramic. Glass kilns heat one layer at a time from the top, while ceramic kilns heat multiple layers at once from the side. The author at Glass Fusing Made Easy suggests a glass kiln because it will heat the top of the piece at the same rate. Both glass and ceramic kilns are insulated on the inside with firebrick which heats quickly and cools slowly.
Before using the kiln to fuse glass, it must be put through a kiln wash. When applying the kiln wash, it is important to brush it on in one direction at first, then later brush it at at 90 degree angle to the first application.
Kilns need to be fired to 1,450 degrees F when fusing glass. You should heat top firing kilns at 900 degrees per hour and side firing kilns at 600 degrees per hour. After firing the glass, allow it to cool slowly in the kiln, leaving it for six to eight hours.
Tools you need for glass fusing include a kiln, dust masks, fusing gloves, safety glasses, haik brush, kiln wash and glass cutting tools (such as breaking pliers, oil, glass cutter, running pliers and other items). It is not necessary to own a kiln, as some glass shops, schools or ceramics shops will have a kiln available to use for a fee.
The process of fusing glass is not hard, but requires care. You need to cut the glass to the desired shape while keeping in mind the glass will melt and morph. Clean it well, as fingerprints will show up after the glass is fused. Set the bottom piece in the kiln and layer on top. Use a drop of honey or thinned white glue to hold pieces together while arranging them in the kiln.
The most common problems that occur in glass fusing have to do with the kiln. If there is glass stuck on the kiln shelf you need to remove it one of three ways: heat the kiln to 1,000 degrees with nothing in it, use a torch to heat the glass and allow it to cool and break off, or tap it with something metal. Another problem that may occur is the breaking of kiln coils. This can happen with regular use but you can repair them following manufacturer's instructions.
If the glass is cracked when finished it means that the C.O.E is too different. Never use glass for which you don't know the C.O.E. If the piece comes out cloudy or gray it means that it has devitrified and the kiln was not heated high enough. To avoid this, make sure the kiln is heated to 1,450 degrees F.