Tartrates
Harmless crystals resembling shards of glass that may form during fermentation or bottle aging (often on the cork) as tartaric acid naturally present in wine precipitates out of solution. Components of tartaric acid, including potassium bitartrate and cream of tartar, are less soluble in alcoholic solutions than in grape juice and solidify at cooler temperatures (such as those found in a refrigerator); can be avoided in finished wines through cold stabilization. Decanting and careful pouring can prevent transferring the crystals from the bottle into the glass.
