Firing in the middle ranges of these temperatures 677–732 °C (1251–1350 °F) is considered as “tacking”. Firing the glass at the higher spectrum of this range 732–816 °C (1350–1501 °F) is a “full fuse”.
Compatible glass is most important for fusion. When glass is fused, unless the separate pieces are of the same COE (co-efficient-of-expansion) the pieces will shatter as the different types of glass cool. Different COE glasses will cause them to cool at differing rates, allowing stress fractures to appear where the piece is trying to settle together.
Frit (graded glass fragments) should also be of the same COE for the same reasons.
The stress in a piece of work can be observed by placing the item between two polarizing filters. This will show any problematic areas of tension that may be present.