Periodically you may discover what may appear to be one of the following in the bottom of a wine bottle: Sand, glass shards or crystals. It may seem alarming and even objectionable at the time, but do not be concerned, as it is neither glass nor sand.
Tartaric acid is the natural occurring acid in grapes. Just as apples have malic acid and oranges have citric acid, grapes have tartaric. Tartaric acid, however, is unstable and has a tendency to precipitate in a crystalline form under two conditions; in the presence of alcohol and at low temperatures.
During the winemaking process, we try and induce the precipitation to take place in bulk, thus reducing the likelihood of it occurring in the bottle. However, due to multiple factors, crystals may appear in the bottle or on the under side of the cork, most notably following a chilling or refrigeration period.