What It Actually Is

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.