The growth of crystals on glass, aka, devotrification, aka, devit, is not that common or easy to create. But, if devit does appear on your fired glass, there are several ways to remove it.
ONE SIMPLE WAY, for glass that’s flat, is to sift a very thin layer of Clear Powder (2 grains thick) over the entire piece until the surface appears white.
Then re-fire at 1425º–1450ºF (774º–788ºC) for about ten minutes, depending on the kiln and thickness of the glass. The resulting piece, like our tile on the right, will have a clean appearance.
This method is similar to working with overglazes, but Clear Powder has advantages: it contains no lead, it fires quickly, and a very small amount will get the job done.