Ferrous (Iron) sulfate
Natural dye – mordanting and nuancing any fibers : iron is the basis of all grey and black shades by reaction with natural tannins. In a general way, it is used to darken colours and make them deeper. It slightly improves light- and wash-fastness from those obtained with aluminum mordants. Recommended use at 1-3 % of the Weight Of Fiber (WOF). A higher % may alter the structure of proteinaceus fibers and make them breakable.
Also, combinations of aluminum and iron mordants or a last (very light) iron bath after a aluminum-mordanted-colour will widely increase the number of shades.
Paintings and wood treatment : ferrous sulfate is used for reactions with tannins (formation of complexes) and its antifungal properties. It gives to wood tipsy finishes or darkens them according to species.
Other uses : Gardens and lawns (antimoss).