Copper
(Copper Sulphate, Verdigris, Blue Vitriol, Blue Copperas,
Bluestone.)
Copper is rarely used as a mordant. It is usually applied as a
saddening agent, that is, the wool is dyed first, and the mordant
applied afterwards to fix the colour. With cream of tartar it is
used sometimes as an ordinary mordant before dyeing, but the colours
so produced have no advantage over colours mordanted by easier
methods.