I know that the UK assay offices still use fire assay in some
circumstances. As for nitric acid, Cellini also gives some discussion
to it, although nothing as detailed as Biringuccio in his
’Pirotechnia’. The origin of the acid, along with a few others, was
in the Middle East in the 8th century, by an “alchemist” called Jaber
bin Hayan, but whether it was used for assay at that time is unknown
to me. I don’t think the strong acids became common in Europe until
much later, perhaps as late as the 16th century.