Ferrous sulfate or iron is an optional mordant and is used as a colour changer. It has the added benefit of making naturally dyed colours more light and washfast. It is more often used with cellulose fibers like cotton, linen, rayon and hemp and should be used with care on protein fibers as it can make them slightly hard or brittle. Iron shifts a colour to a deeper, darker shade. If used in the mordant process that shift is more distinct than when used directly in the dyebath.
Iron should be used at 2-4% wof. More than that could damage the fiber.