I am restoring an old BSA and have been trying to find a good method of recreating a rust blue. When I was a lad and you could walk into a chemist and just buy small amounts of any chemicals; I used a mixture from Dunlap’s Gunsmithing book that included mercuric chloride, for which I remember having to sign the poison book!
Times have moved on and I have no idea where I could now buy a small amount of mercuric chloride, potassium chlorate, spirits of nitre and potassium nitrate. Having seen the results of Birchwood Casey’s Plum Brown on Ed Bear’s various restorations, I thought I’d try it in place of my mercuric chloride formula. The key to producing a blue, rather than brown, is to boil the parts in water after swabbing the solution on and then rubbing with fine wire wool. Then back in the boiling water for a few minutes before swabbing on the next coat.
Well, I’ve just tried it and it seems to work – it even has the same mildly acrid smell when it’s swabbed on.
The photos show a couple of mild steel tommy bars that I just blued – one is straight from the boiling water tank, the other has been rubbed with 3 in 1 oil, which completes the process. They have had five applications and there is no trace of the brown patina that you get without the boiling.
These rust blues give a very durable finish – parts that have been polished bright will be etched to a satin finish that seems to hold oil and resist rust better than the modern caustic blues. They also will not attack soft solder as caustic soda does. The downside is that the process is labour intensive and messy. All that swabbing, boiling and carding.
http://s23.photobucket.com/albums/b391/Otley/Blueing/
Kind regards, Otley