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If a glaze has settled out, but has not gone rock hard in the bottom of the container, you can add CMC or bentonite, if you happen to have it. But especially if you're dealing with commercial glazes you probably don't have that lying around. However, you can also use Epsom salts to suspend your glaze. Epsom salts can be readily purchased in most drug stores. First you need to create a saturated solution of Epsom salts by dissolving them in a cup of warm water until no more will dissolve. Then add this solution slowly and carefully to the glaze while continuously stirring the glaze. It should require less than approximately one teaspoon of Epsom salt solution per gallon of glaze. The quantity will depend on the severity of the problem. If a glaze has gotten too hard at the bottom to mix back up, first try my favorite glaze-stirring tool, a handheld kitchen stick blender. If that doesn't work, drain all the liquid off, work on dissolving the solid into the Epsom salt / water mixture, then add the rest of the glaze liquid back in.