SKB. Yes I have an express rust bluing formula for gun barrels that was once used extensively on this side of the pond, from start to finish it takes about an hour and a half giving a very fine rich deep dark blue colour. It fell out of favour due to its cost and toxicity with the formula being a closely guarded secret in the trade, I still use it myself for the occasional piece of work.
Quite surprisingly it was developed by the notable American Gunsmith, though Angier does mention it in passing in his book but may be there was a little rivalry involved because he does not elaborate much on the how and why.
