IRON (Ferrous sulfate heptahydrate): Pale green coarse powder, mordant and post-mordant circa 6000 BC; saddens colors. Also a light gold to brown dye. Best on cotton and linen; use lightly on wool and silk; rinse very well as it is considered corrosive to those fibers. Good color changer in dye extract paints. Once called copperas or green vitriol.