Grapes naturally contain several acids including tartaric acid. They also contain potassium and calcium ions which form salts with the organic acids.
These salts can precipitate out of the wine to form a material called potassium bitartrate. This is a clear (sometimes red or brown), crystalline material. They are called tartrates also referred to as "wine diamonds".
Certain wines have a higher concentration of tartaric acid which may cause the formation of tartrate crystals. These crystals can form once the wine is bottled. Although not harmful, this crystal-like sediment affects the appearance of your wine.
To avoid the fallout of tartrates in bottled wine, a process known as cold stabilization or chill-proofing can be done to your fermented wine.
Not sure if your wine should cold stabilized? Do a sample test:
Pour a small sample of the wine (at least 12 oz) into a bottle and refrigerate for approx. 1 week. Proper cold stabilization requires a temperature of 25 degrees(F). Since that temp is difficult to achieve for the home winemaker, we recommend leaving it in a refrigerator for a one week period. You can check the sample by holding the bottle up to a bright light or inverting the bottle to see if any crystal-like sediments have formed. If not, your wine is considered to be cold stable and you can bottle at this point.
If your sample contains tartrates, you have a few options:
1. If you have refrigerator space, you can place the entire batch of wine in the refrigerator for at least a week.
2. You can also wait for weather conditions to achieve a temp of 25-40 degrees(F).
3. Do nothing - bottle your wine and keep it in an area that will hold a temp above 60 degrees. Usually the crystals will only form when the wine is exposed to cool temps. If they do form, simply decant (put into another container) when its time to serve the wine.