Also known as blue vitriol, this substance was made by the action of sulfuric acid on elemental copper. The bright-blue crystals are soluble in water and alcohol. Mixed with ammonia, copper sulfate was used in liquid filters. The most common application for copper sulfate was combining it with potassium bromide for making copper bromide bleach for intensification and toning. Some photographers used copper sulfate as a restrainer in ferrous sulfate developers that were used in the collodion process.