Not the best of answers I know but is there a best solution , or can you just have a favourite ? I've been using the slightly altered version of 'ferrobronze'or french trade brown from Angiers book.
An old chemist I know gets me the ingredients, he did tell me that he supplied the sol. ferric chloride at a different weight than in the book, I can't remember why but as I'm running out at the moment I'll pay him a visit soon , find out why and let you know. I also add a drop or two extra hcl than the original formula but have had no issues with after rust.
It gives a lovely black and when I've experimented, I can get good results in under three days, start to finish. It also worked well to brown damascus barrels, again, no after rust problems. I wouldn't give someone their barrels back without a scald and neutralise but experimented by leaving my own or old scrap tubes hanging in a damp outbuilding to see what happened. I blacked a very old air rifle action, gave it a wipe with oil after the last carding and left it hang for a year. It was fine.
I'll probably find it's limitations one day and have to use another formula but for now it's my favourite.
I have the ingredients for mercury based salts but don't really want to mix and use them. Well not yet anyway.
