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.