Processing aid, potassium ferrocyanide is used to resolve discolouration in wine caused by a combination of tannin and iron. It was called 'blue fining', because it formed a blue precipitate called 'Prussian Blue', which was then removed by filtration

Excessive amounts of metals, particularly iron and copper, may be present in the wine, usually from contact with iron or metal surfaces. These result in persistent cloudiness and require removal by such special fining materials as potassium ferrocyanide (blue fining), long recommended in Germany. Cufex, a proprietary product containing potassium ferrocyanide, may be used in the United States under strict control. Phytates have been used for removing iron. In modern winery operations excessive metal content is rare, mainly owing to the use of stainless steel equipment.