Called “green vitriol” in old dye books, this is an essential of the dyer’s art. Iron after baths can take any yellow or gold and turn it to a soft green. Reds become burgundies, pinks become plums, and any tannin source (any weed or bark) can be used to make pretty greys, just by adding iron.
Usually used as an after-mordant modifier, where you just watch the colour and when it has changed “to taste”, rinse it. If used as a pre-mordant, ½ oz per pound is the usual ratio = 3%. Do not use too much as it can “weight” and damage delicate fibres.
All mordants we supply and use are from naturally occurring minerals that come from the Earth. When the fabric biodegrades the minerals will return to the Earth from whence they came. There are no residuals left on the fabric as the minor amount of fixative is locked into the fibre.
For more information please see the article "The Myth of the Bad Mordant."