Tartar is a salt or ester of the organic compound tartaric acid, a dicarboxylic acid. Muggles have a number of uses for tartrates.
Tartar has magical properties: half an ounce of it was used as a component in the making of
Belch Powder. 
Notes and references
"Tartrate" at Wikipedia
Label Collection book in the box set. Harry Potter Wizard's Collection ↑ The label reads: "
of Mummy, Mastick, Red Myrrhe, Olibanum, Ammoniacum, Oppopanax, Bdelium, each ʒii. Vitriol ℔ii. Honey, ℔ii. Tartar ℥1,ſs. Aquavitæ, gal.iii." which uses the apothecaries' system of weights.
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