The crystals are potassium bitartrate. This is excess tartaric acid that is precipitating out. Potassium in the grape combines with tartaric acid to form a soluble potassium bitartrate. The solubility is reduced further by increased alcohol and the result is precipitation of crystals. If you chill the wine for a week or so, even more crystals will form.

These will not actually hurt the wine, but are unsightly and a nuisance to get out of the bottles once emptied. You have two choices. One, you can open the bottles and very gently transfer the wine to new bottles, or two, you can leave the wine alone and decant it before drinking. A chilled wine will take up less oxygen than a room temperature wine.

To prevent this from happening next year, cold stabilize the wine for two weeks or so before bottling it.

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