The yellow (there is also red) blood lye salt (also yellow potassium, potassium ferrocyanide, potassium hexacyanoferrate or potassium hexacyanidoferrate) is a complex potassium-iron compound and is used in viticulture in the so-called blue fining used. It reacts with copper, though iron is present and binds both to an insoluble, blue substance that settles to the ground. Since it can also form hydrogen cyanide, the use is subject to approval. By means of a test solution in a small quantity of wine (10 ml) it can be determined in a "pretreatment on iron" whether before bottling a blue tint is required. The determination of the then possibly actually required blood lye salt requirement may only be carried out by authorized persons. Incorrect use may cause the wine error bitter almonds arise.