i used to do a lot of browning and rust bluing on shotgun barrels and old guns using similar chemicals, as you say its much more durable than modern chemical blacking and gives a blue tinge instead of pure black, i stopped doing it, as the fumes where to say the least toxic,especially when the bluing formular hit the hot metal. i doubt i would have seen forty if i continued doing it, you cant buy the chemicals now anyway, unless you are a registered buisness, i dont think spirits of nitre or nitrous ether as its now known, can be bought, i could never get any anyway, you will be hard pushed to find any chemical company now who even knows what it is. i made my own by distilling , dont know if it was exactly right, but it seemed to work ok.
Shaun
Was it worth the trouble, Ah, what trouble