Compatibility is a feature of glass especially important when combining multiple glasses together into one piece in a kiln. Fusing is the heat bonding of glasses at high temperatures (Bullseye will fully fuse in most kilns at temperatures above 1450ºF). Simply stated, two glasses are said to be compatible if, after fusing and cooling to room temperature, they are free of undue stress. Such stress can immediately or eventually lead to cracking.

Bullseye's testing method for compatibility is still unrivaled in the industry. It is based on a standardized base clear testing ("T") glass that has been monitored for consistency for over 15 years. It is against this "T" glass that all other Bullseye glasses are measured. Those which test as compatible to the standard will be compatible with each other and with all Bullseye glasses so tested and so labeled in all years past.

Tested Compatible glasses are popularly called "fusible" and are designated by an "F" after the 6- or 8-digit style code.


Although Bullseye glasses are popularly referred to as being of a "90" expansion, Bullseye does not encourage the use of this designation in describing its glasses.

The "linear expansion coefficient" is determined by a laboratory test which expresses the average expansion rate from room temperature to 572ºF (300ºC). It ignores the more important range of expansion for determining compatibility for fusing—the expansion through the annealing and softening ranges. It also ignores viscosity, an important element in determining whether glasses will "fit" each other on fusing. All "90" expansion glasses are not compatible.