Interesting, thanks for the link.
He must be using nitric acid below 70% purity if it's attacking the gold, that's why 98%+ is needed so that it doesn't destroy the gold bond wires going from the leadframe to the chip die. A 9:1 ratio of concentrated nitric acid to concentrated sulphuric acid works best for dissolving the chip epoxy package.
But yeah the 98% nitric is very hard to get, needs a government license apparently. The guy I'm working with is making it himself from 98% sulphuric acid and pottasium nitrate, but he's not in the UK so I'm still hoping to find a local source.