Why is Cream of Tartar so expensive? Can I make my own?
Cream of Tartar (AKA tartaric acid, chemical formula KC4H5O6, Potassium bitartrate) is formed during the production of wines made from grapes. For commercial production the crystals that precipitate out in industrial fermentation vats are collected and purified into the white powder that you buy at the supermarket. The process involves multiple stages of chemical purification, filtration and drying. The cost is high because the only known source or tartaric acid is from grapes, and because of the costs of processing involved in purification, as well as packaging, shipping and handling.
The same crystals will precipitate out from chilled grape juice but in too small of quantities to be useful at the home scale, unless you are making a lot of wine or grape jelly and have a chemical plant in your back yard.