Sorry, wasn't being clear there. Insofar as I'm aware (and I could be wrong), all modern iridescent glass gets its shimmer from a coating. Fusible iridescent coatings are stronger and designed to keep their shiny after a few heats in the kiln. Iridized flat glass, like the type generally used in stained glass or mosaic, tend to be more fragile and the luster burns off in the kiln. Cheap stuff like certain aquarium gems and modern souvenir glass has a coating that barely holds up under sunlight.
Coatings on flat glass or most glassware are applied to only one side of the glass, and if that piece is clear or is a pale transparent, it may be possible to preserve the shiny by working it coated side down, like you would do with dichroic glass. I've tried that with Bullseye iridescent glass, but with poor results. I may have been doing it wrong, and more skilled glassworkers might do it better. At least with fusible glasses you can mix colors. Everything else gets to be monochrome.
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