I use underglazes on my sculptures, and I have no experience with glazes so this is a total newbie question. I bought some Potters Choice glazes and combined the colors as suggested in the catalog (i.e. two coats of one color followed by two coats of another). I made test tiles, fired to cone 5, and loved the colors and reduction effects the glazes produced. But I've just glazed some sculptures and the colors are noticeably different from the test tiles. I can live with the colors, but it made me wonder: Can you apply glaze over a fired glaze? If so, what happens to the original glaze coats when I fire the pieces up to cone 5 again?