Aqua Fortis

Aqua Fortis was made by distilling saltpetre together with oil of vitriol, or in earlier recipes with alum or green vitriol, heating the mixture in a retort until a fuming, corrosive, yellowish liquor distilled over and was collected under water or in a cooled receiver. The resulting acid attacked most metals vigorously, dissolving silver, copper, iron and most other common metals into their respective salts while famously leaving pure gold entirely unaffected, a property that made aqua fortis the standard tool of the assayer for testing whether a supposedly gold object was genuine or merely gilded or alloyed. Engravers adopted it for etching, coating a copper or steel plate with a resistant ground, scratching a design through the ground, and letting aqua fortis bite the exposed metal to a controlled depth, a technique that gave the printmaking medium etching its name directly from the acid's own common French name, eau forte.

Refiners used aqua fortis at industrial scale to part silver from gold in mixed or alloyed bullion, dissolving out the silver and leaving a residue of pure gold, an operation called parting that remained commercially important for centuries. Its fumes and its corrosive action on skin and clothing were well documented hazards of the workshop, and early modern texts routinely warn handlers to work with it carefully.

Aqua Fortis is identified as nitric acid, HNO3, an identification secure enough that historians of chemistry treat it as one of the least contested named acids in the alchemical repertoire, its selective dissolving behavior toward silver over gold following directly from gold's exceptional chemical nobility, its strong resistance to oxidation, compared to nearly every other common metal. The acid's origin in distilling saltpetre is itself a reliable chemical clue supporting the identification, since heating a nitrate salt with a strong acid is exactly the modern laboratory method for liberating nitric acid, and the correspondence between the historical recipe and the modern preparation is close enough that no real scholarly dispute exists over what substance is meant.

What later chemistry added was an explanation, rather than a revision, of why the acid behaved as it did: the specific electrochemical nobility of gold, and the ready solubility of silver nitrate, were understood only once electrochemistry gave a theoretical account of metal reactivity that the alchemical assayers had already exploited empirically for centuries. The assayer's test itself, applying aqua fortis to a suspect gold object and watching whether it dissolved, remained in practical use well into the era of modern chemistry, a rare case of an alchemical-era diagnostic procedure surviving essentially unchanged because the chemistry behind it was correctly understood empirically from the start.