Last Updated: Oct 30, 2016 Views: 40
I forwarded your question to our Research Scientist, Glen Cook, who responds:
Opalescence in glass, such as is found in Barolac pieces, is an optical effect caused by the interaction of light shining through glass with very small particles (<1 micron in size) scattered throughout. The particles in the glass are either tiny crystals or pockets of glass that separate from the main glass, like a shaken mixture or oil and water; A similar effect can be created in a glass of water by pouring in a small amount of milk to cloud the water.
In glass the particles form after initially transparent glass is formed. That process is called “striking” the opal. It usually requires a reheat and hold at some intermediate temperature while the particles form and grow to the correct size to cause the effect, over seconds to several minutes. The lack of opalescence at the base of the piece could be 1) because the greater amount of glass concentrated at the base prevented that part of the piece from going through the required thermal treatment (since it would hold heat longer, heat up more slowly, and cool down more slowly than the thinner walls), and/or 2) that if the piece was puntied during forming, flame polishing of the punty scar would remelt the opalized glass (if it actually DID strike with the rest of the piece) and destroy the fine particles that had caused the effect without then redoing the heat treatment to restrike that glass.
Hopefully, this helps answer your question! Let me know if you need further info.