While this might look like lightning in a bottle, it is actually cord stress. Cord is generally invisible to the naked eye but can be viewed in polarized light. Cord is “an attenuated glassy inclusion possessing optical and other properties differing from those of the surrounding glass” (ASTM C162). Or stated differently, these colorful swirls are subtle differences in glass composition. It is can be caused by batching errors, problems with melting, furnace upsets, refractory erosion or insufficient mixing. High stress, tensile cord located on the exterior surface has greatest chance of causing a performance problem.