Called “Blue Vitriol” in old dye books. Copper can be used as either a pre-mordant, at ½ ounce per pound fabric (3%), or as an after-mordant modifier, where you leave it to soak and take it out when the color has changed “to taste”.
Please use copper responsibly: do not dump your mordant bath, but re-use it according to the general formula of add only ¾ the usual amount, (assume ¼ of what was added for the last mordanting is still there).
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."