Ferrous sulfate (iron) is an optional mordant and is used as a colour changer. Because it is a mordant the addition of iron increases the light and washfastnes of dyes. It is most often used with cellulose fibers like cotton, linen, rayon and hemp and should be used with care on protein fibers as it can make fibres 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. Higher rates may damage fibers.