This article is from the WineMaking FAQ, by malak@CAM.ORG (Don Buchan) with numerous contributions by others.
Tartaric acid crystals may fall out of solution to form a white,
crystalline sediment after a while, particularly if your wine gets
chilled. They're harmless and do not add any taste to the wine. To
avoid the problem, chillproof your wine for a couple of weeks in the
carboy in a cool to cold place -- an old fridge or a cold cold room is
appropriate. Desired temperature is 4C (36F). Rack off before allowing
the wine to warm up as the crystals may dissolve back into the wine.