Ferrous Sulfate Heptahydrate, also called copperas, is a bluegreen crystals; soluble in water, melts at 64 C. Fertilizer Ferrous Sulfate Heptahydrate Ferrous Sulfate Heptahydrate, also called copperas, is a bluegreen crystals; soluble in water, melts at 64 C. It is commercially generated as a by-product during the manufacture of Titanium Dioxide, or as a by-product of the pickling of steel. It is ...