Any kind of glass can be used for slumping and fusing, but some glasses work better than others. Since it is likely that different pieces of glass will be used in the same project, it is necessary to make sure that the glasses are compatible – that is, that they expand and contract at similar temperatures. This prevents cracking or shattering when the pieces cool.
When starting out it is best to use glass which has been "Tested Compatible" by the company from which you purchase art glass supplies.
Glass for slumping and fusing can be purchased in 3/8" thick sheets as frits which are small pieces ranging in size from powder to 1/2" in width, as stringers which are long thin glass threads, as rods which are thicker cylinders, as shards and confetti which are glass slices a bit thicker than paper and as cullets, billets, patties, or dale which are different sizes and shapes of chunks of glass used in casting.
The basic slumping and fusing process consists of five main parts: the heating phase, in which the temperature in the kiln is raised from room temperature to the point where fusing takes place; the soaking phase, in which the temperature is held at a given point for a certain period of time.
The cooling phase, in which the temperature is brought down from its high point to just above the annealing temperature; annealing phase, a most critical step which relieves stress in the glass; and cooling to room temperature phase, in which the glass becomes cool enough to touch. Warm glass is a very rewarding and enjoyable technique; for more information, see your art glass supplier.