What are those crystals on the cork or at the bottom of the bottle?

Tartrate crystals form when bitartrate ions from tartaric acid bind to potassium and sink out of the wine together as a solid salt.

Tartaric acid and potassium are both naturally present in the grape. As long as the wine does not get too cold, that salt stays dissolved. If the temperature drops, it falls out of the liquid and grows into a crystal. The Australian Wine Research Institute describes the shape as boat-shaped or lens-shaped. They sit on the underside of a cork if the bottle has been lying down.

Tartrate crystals are not a flaw: the Australian Wine Research Institute states that potassium hydrogen tartrate has no adverse effect on the quality of the wine.

A crystal is hard and stays in the glass. Pour the wine gently and leave the last centimetres in the bottle, and the crystals stay behind. If you let the bottle stand upright for an hour first, everything sinks to the bottom and you pour past it. With sparkling wine you sometimes see them sticking to the inside of the glass.

Wines that have been cold-stabilized before bottling have already lost their tartar, and wines without that treatment keep it in the bottle.

Cold stabilisation is a common cellar procedure. The wine is chilled strongly, the crystals settle out and are filtered away. The AWRI describes the accompanying test, in which a filtered sample is held at minus 4 degrees for three days and then examined for sediment. Producers who skip the treatment often do so to handle the wine as little as possible. You will see the result later on the cork.

Besides potassium hydrogen tartrate there is calcium tartrate: according to the AWRI it precipitates much more slowly and that precipitation does not depend on temperature.

The difference is of practical importance. Chilling helps with potassium hydrogen tartrate but not with calcium tartrate, so a wine that has been neatly cold-stabilised may still show calcium crystals later. In red wine you also sometimes see dark granules on the bottom. That is not salt but colouring matter and tannin that clump together over the years.

There are permitted additives that inhibit crystal formation, and the AWRI mentions carboxymethylcellulose with a maximum dose of 100 mg per litre.

Metatartaric acid also works, but only temporarily: according to the AWRI about three months at 20 degrees and a few years at 0 degrees. Potassium polyaspartate stabilised all wines in AWRI trials at 100 mg per litre. In its International Code of Oenological Practices, the OIV also describes cold treatment, electrodialysis and treatment with cation exchangers as recognised methods for stabilising wine against tartrates.