1 Answer

When glass is heated inside a kiln it expands, and when it cools it contracts. When fusing two or more pieces of glass together, they need to expand and contract at the same rate. Otherwise, when the glass cools, one glass will pull on the other and cause the piece to crack along the seam. Even if the glass survives the cooling process, there is still a risk it might still crack if you try to reheat it in a kiln or even place it in a sunny window. Stress from incompatible glass is always in the piece. Do not try to refire broken incompatible glass.

All glass has a coefficient of expansion, or COE.
Glass manufactured specifically for fusing is often "tested compatible," or guaranteed to be a certain COE.
The most popular fusing glasses are either 90 COE (Bullseye, Uroboros and Wasser) or 96 COE (Spectrum and Uroboros).
Always use compatible glass, which is known to have the same COE.

http://www.delphiglass.com/page/knowledgeBaseView?KBItemID=90 

http://www.artglass2.com/fusing-glass.html