What is tartar?
Tartar is the name given to the crystalline salts of tartaric acid. Their chemical name is tartrate. These salts are formed when tartaric acid reacts with minerals (e.g. potassium and calcium). Essentially, tartar consists of potassium hydrogen tartrate and calcium tartrate.
The tartaric acid salts are poorly soluble in water and and therefore form crystals, i.e. their molecules join together to form a regular, lattice-like, solid structure; the technical term for this is: the salt precipitates.
The tartar settles in the form of the small crystals in the(stainless steel) tank or barrel during winemaking and can also collect at the bottom of the bottle or on the cork after bottling as the wine matures. Factors that favour the precipitation of tartar are a high alcohol content, an increased pH value (from 3.2) as well as low temperatures.
Tartar is not a wine defect, but it is also not a designated quality characteristic. Its occurrence merely means that the wine contains a high proportion of tartaric acid and may not have been completely stabilised. The formation of tartar can be largely prevented by treating the wine chemically (e.g. with metatartaric acid) or physically (with cold) before bottling.