Salt of Tartar was made by burning crude tartar in an open fire until only a black or grey ash remained, then boiling that ash with water to dissolve its soluble alkaline component, filtering off the insoluble residue, and evaporating the filtered liquor down to a white, crumbly, strongly alkaline salt. The process, called calcination followed by lixiviation, was a standard operation across the alchemical workshop and produced one of the most reliably useful fixed alkalis available before the industrial manufacture of soda ash. Salt of tartar was prized for neutralizing acids in laboratory work, for making soap by reacting it with fats and oils, for glassmaking as a flux, and in medicine as a mild alkaline remedy for excess stomach acid.
Its causticity in solution and its tendency to absorb moisture and carbon dioxide from open air, gradually converting to a different, more stable form on long exposure, were both noted by careful early observers even without the gas chemistry to explain the transformation. Alongside soda, the related sodium based alkali obtained from certain plants and from natron deposits, salt of tartar was one of the two great fixed alkalis recognized by early modern chemistry, and distinguishing the two, one drawn from wine lees and one from ashes of certain other plants or from mineral deposits, was an important and genuinely useful piece of practical chemical knowledge well before either was understood in terms of sodium and potassium as distinct elements.