Tartar

Tartar accumulates as a hard crust on the inside of wine casks as fermenting grape juice loses solubility for its own natural acid salts while the alcohol content rises, precipitating out a reddish or yellowish deposit that winemakers scraped and sold as a raw commodity, the crude material called argol. It was not, in its crude state, considered a laboratory reagent so much as a starting material: chymists purified it by repeated dissolution and recrystallization to obtain the whiter, purer form known as cream of tartar, and burned it in open fire to ash to obtain salt of tartar, a genuinely useful alkaline salt. Tartar's distinctive sharp, sour taste and its origin from fermented wine rather than from a mine or a metal marked it out, in the classification schemes of the period, as belonging to a different family from the mineral salts and vitriols, an organic acid salt long before organic chemistry existed as a discipline to name it.

Beyond its role as feedstock for salt of tartar, purified tartar found use in dyeing as a mordant and in medicine as a mild laxative and diuretic, uses that persisted into the modern era under the more familiar name cream of tartar, still found in kitchens today for baking and candy making, where its acidity stabilizes egg whites and prevents sugar crystallization.

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.