Cream of tartar (potassium bitartrate, KC₄H₅O₆) — a fine white powder that is a natural byproduct of winemaking, deposited as crystals (argol) on the inside of wine barrels during fermentation. Used in natural dyeing as a mordant assistant alongside alum: it helps distribute alum evenly through fibre, keeps wool soft, and brightens final dye colours by creating a mildly acidic bath. Also used in baking as a leavening agent and to stabilize whipped egg whites.