blue fining, largely outmoded though still used winemaking process whereby excess copper and iron are removed from wine by fining with potassium ferrocyanide. The process works because soluble copper and iron form insoluble compounds with the ferrocyanide ion. A century ago, before stainless steel was widely available, winery equipment was often made of iron, copper, or bronze, an alloy of copper and tin. They would be attacked by the acids in wine. Wines containing more than 10 mg/l of iron or 0.25 mg/l of copper could easily form a haze, so blue fining was needed to remove the excess copper and iron dissolved from the equipment after prolonged contact with the metals.