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 on the ground. Since hydrocyanic acid can also form, the use is subject to approval. Using a test solution in a small amount of wine (10 ml), a “pre-test on iron” can be used to determine whether before the bottling a blue enhancement is required. The determination of the blood lye salt requirement which may then actually be required may only be carried out by authorized persons. Improper use can lead to wine defects bitter almonds arise.