The colors we see in glazes result from several sources, including some that are physical, rather than chemical. One source of color that most of us have encountered elsewhere, but not in glazes, is fluorescence. Fluorescent lights are extremely common. "Day-Glo" items of various sorts have been around for decades, and there are quite a few fluorescent minerals. Most "Day-Glo" materials use organic dyes and pigments, which are not suitable for use in glazes. Yet there is a broad range of fluorescent (and, recently, phosphorescent) materials that are within the reach of potters. Almost any clear glaze with a very small amount of copper in it-not even enough to produce a visible color in ordinary light-will have a pale yellow fluorescence under "black light" (long-wave Ultraviolet). You can get this effect by firing a piece with a clear glaze next to a piece with a copper green or copper red glaze. In fact, there are many such pieces in existence. Also, silver at a similarly low concentration is reported to produce a white fluorescence. The copper fluorescence, though easily visible to the naked eye in a fairly dark room, is not intense enough to be of much use. While I have not tested the silver fluorescence myself, I suspect that it, too, is not very bright.
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