If you fuse two glass that are nor compatible, permanent stress is formed. This stress may break the glass immediately during the cooling phase. Even if the piece survives the cooling,the stress acts as fweakness in the piece. Any slight external force may be enough. A famous case was the IKEA vase. It was quite expensive and beautiful. The internal stress was so high that some serious accidents happenend when a customerdid nothing more that lifted it in hand. It exploded. IKEA has to withdraw the whole series. Stress has two major sources:- Inproper cooling, too fast or uneven- Incompatible glass. The stress can be made visible by using polarized filters. A filter blocks the light with rectangular polarisation.. Therefore two filters crossed blocks all light. If a stressec glass is placed between. it rotates the polarisation and a halo is seen. The stress is seen as brighter halo arpund the test pieces. Fusing test samples on clearPolarisation effect on the test piece.. Blue and green samples have bad fit, too much stress. The red matches the base glass well,the yellow is satisfactory. In this picture the test sample is placed between the filters. Note how the filters block all the light. The stress is the sample is seen as light halo. In this picture samples of the same glass are fused on two different backgrounds. On the right is the previous test piece. On the left the samples are fused on a green glass. The blues fit, but the red has a definive halo. The green base makes it look darker but it is there. Sun Glass Test If you brake a pair of polarized sun glasses, and adjust them to a right angle,you get a cheap tool for testing.