Tartaric acid crystals. Tartaric acid (C4.H6.O6) is found in fruits, especially grapes and is the principal acid in wine. It is used as a food additive, giving a sour flavour. Tartar acid is used to produce cream of tartar, which is used in baking, and Rochelle salt, which is used as a laxative and to silver mirrors. It is also added to other foods to give a sour taste, and is used as an antioxidant. The crystals in this sample have been grown artificially and probably derive from dark grapes or red wine.