Potassium hydrogen tartrate (also known as cream of tartar) is a by-product of the wine-making process. It has several uses in natural dyeing.
It is naturally slightly acidic and can be used as a mild water softener in hard water areas
Some dyers find that it is also useful as a softener for wool fibres
A colour modifier, shifting the colours with some natural dyes. For example, cochineal will shift from pinks towards reds with the addition of potassium hydrogen tartrate to the dyebath or with modification after the dyeing process. Madder will shift towards orange