Also known as potassium bitartrate, is a sediment that is produced in fermenting grape juice. It is an optional addition to the dye or mordant bath, and is used to acidify the dye bath to effect a color change, and to soften wool fibers. It will dramatically shift the color of cochineal, madder and lac from their fuchsia and burgundy red shades to a brighter, redder shade. With madder, it will shift the hue to orange. It can inhibit the color development in logwoods and certain tannins, so it is rarely used with these dyes.