Ferrous: Ferrous crystal is used as a mordant, as a colour shifter, and to increase light fastness when used in combination with other natural dyes.
It is also used as a reducing agent for specific indigo vats. Iron is a versatile ingredient for the natural dyer.
Mordant: Use ferrous at approximately 5 to 10% to the weight of the fabric.
Its results may vary as per percentage used and the fabric composition.