Wonky Weaver™ Cream of Tartar is a Dye Assist and can be added to the mordant or dye bath to soften wool, brighten shades, and change the colour of some dyes e.g. it will change the fuschia of cochineal to bright red.  Cream of Tartar works best with protein fibres like wool and silk and is rarely used with plant or cellulose fibres.
 
Recommended use is between 5 and 6% Weight of Fibre.