The Fuse It - Test card is a convenient and money-saving device that tests the compatibility of glass. Cut a 1 1/2" section of clear glass (of a known COE) and a 3/4" square of the glass you wish to test and fuse together. Sandwich the fused glass piece between the two layers of film above a bright light. If there is no halo around the glass, the two glasses are compatible. If a light halo is seen around the glass, then this signals stress within the glass and incompatibility.