Iron (ferrous sulfate) is a soluble metal that is the second most common mordant used for natural dyeing. Iron has the effect of darkening or "saddening" the colour, and in many cases will change the colour completely, for example it can turn the pink of cochineal to purple, or the yellow of osage to olive green.
Can be used as either a pre or post mordant. Very little is needed, only about 2-3% WOF.
Can also be used as the reducing agent in a Ferrous and Calx Indigo vat.