What It Actually Is

Tartar, in its crude scraped form, is identified as potassium bitartrate, KC4H5O6, an acid salt of tartaric acid, itself one of the naturally occurring organic acids found in grapes and several other fruits. The identification is straightforward and essentially uncontested, since the substance survives in continuous use under its own name into the present day as cream of tartar, and its chemistry was worked out relatively early once eighteenth century chemists began distinguishing organic plant acids from the mineral acids that dominated so much of earlier alchemical practice.

What makes tartar historically interesting is less any ambiguity in its identity than its category: it was one of the clearest cases available to early modern chymists of an acid salt that plainly came from a living, fermenting process rather than from a mine or a metal, a distinction that fed directly into later debates about whether organic and mineral substances followed the same chemical laws, debates not conclusively settled until the nineteenth century synthesis of urea showed that they did. Tartar's own straightforward, uncontested identity, resting on an unbroken chain of continuous culinary and chemical use, makes it a useful anchor point in this atlas against which the genuinely contested named substances elsewhere can be measured.