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 or form lines where the two glasses come together. This is referred to as stress. Over time, these cracks will increase and eventually the piece could separate.
Although larger cracks can be seen with the naked eye, not all compatibility problems are that obvious. In these instances you'll need to examine your glass with the Stress Tester. Just place the glass piece between the two strips of polarized film. Study the seams of the two different glass types. A halo will be evident if there is stress in your piece. The larger the halo, the more stress there is on the glass.
Includes two 2" square polarized sheets, each with a 2" frame around the sheet for easy handling. Also includes light diffuser and instructions.
Shown below is an example of a project with no visible stress. When viewed with the stress tester, you can see halo's where stress is present. The stress was caused by combining different COE's in the same project.