Copper sulfate is a fining agent used to reduce hydrogen sulfide and mercaptan aromas in wine. Hydrogen sulfide is a natural product of fermentation, but excess production can be unpleasant in wine. Hydrogen sulfide also has the potential to form other unpleasant smelling sulfur compounds that are more difficult to remove from the wine. Excessive hydrogen sulfide should be treated quickly to prevent the more difficult to remove compounds from developing. Copper ions bind with sulfur and form a compound that no longer has the offensive aroma. Copper treatments usually form a black sediment that needs to be filtered from the wine. Some commercial preparations have been developed that no longer require the filtration process though. Precise additions should be made with copper as excess copper ions are potentially poisonous from metal toxicity and strict limits are placed on permitted levels in wine.