Salt of Tartar

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.

Salt of Tartar is potassium carbonate, K2CO3, an identification resting on solid ground since the substance survives in continuous industrial use under the closely related name potash. The historically important discovery, made gradually across the eighteenth century by chemists including Marggraf and later confirmed by Davy's isolation of metallic potassium itself in 1807, was that salt of tartar and common soda, though both fixed alkaline salts with broadly similar behavior, were fundamentally different substances built on two different metals, potassium and sodium, rather than variant forms of one single alkaline principle as earlier chemists had sometimes assumed. That distinction, drawn originally from careful comparison of crystal form, flame color and chemical behavior rather than from any direct observation of the metals themselves, is one of the clearer examples in this atlas of a case where alchemical and early chemical practice correctly separated two substances long before anyone could say precisely why they differed. Salt of tartar's own derivation, an organic starting material burned down to a simple, stable mineral salt, also illustrates how thoroughly the workshop's calcination technique could strip a complex living material down to a purely inorganic product.