Potassium bitartrate, also known as potassium hydrogen tartrate, with formula KC₄H₅O₆, is a byproduct of winemaking. In cooking it is known as cream of tartar. It is processed from the potassium acid salt of tartaric acid.
 
In natural dyes it is a frequent ingredient to lower Ph in hard water, or make a solution acidic to get certain hues otherwise not achieved. It softens wool and is an assist in mordanting with aluminium at 10% WOF when it prevents the alum powder to precipitate at the bottom of the pot.