Belgian Blue (Herter's) was an Express Rust Blue.
Boiling and carding in betw applications of the soln to the warm surface of the metal.
'Express' in the fact that you do not have to wait for the soln to rust the surface of the metal.
The older Express Rust Blue solns (sometimes called Quick Rust Solns) often had Mercury compounds in them to make them work well,,that is rust. Mercury Bi-Chcloride was the most popular one found.
Up into the 60's and even 70's these solns were sold with the Merc content and no one said much.
Then the Green movement got it's start and the surefire to rust recipes were changed.
Even the old BirchWood Casey MuzzleLoader Barrel Brown,,the stuff where the hobbiest just heated the bbl warm w/a propane torch and swabbed the B/C on to get an instant rust forming. Then card it back, reapply, ect till it was how you wanted it.
That contained Merc BiChloride too. That's why it worked so well.
It made for a great Express Rust Blue soln! But somewhere along the way, probably the 1980's they changed the formulation to exclude the Merc.
Slow Rust Blue is the other side of the Rust Blue processes. The soln is applied to the room temp surfaces of the metal and then allowed to set. The rust forming naturally (Slow).
It can be accelerated by controlling the temp and humidity around it so that a coating of rust forms as quickly as a couple hours.
None of the Slow Rust Blue soln's I've used have had any Merc in them (that I'm aware of).
I stay away from any of the soln that may have mercury in them even though they work well.
Too many years exposure. Yes it does effect you.
I did try the Brownells 'New' Herters Belgian Blue when it came to market some years back (90's?). I figured by then it was reformulated to keep the MErcury out of it.
I tried a small bottle. Warmed a piece of brass and swabbed it with some of the soln and there it was. The nicest brite silvery plating on the brass. I figure it's mercury,,I'm not about to dig any deeper to find out, and they ain't saying what's in it other than it's Original Formula. Guess I should have figured it out.
Mark Lee's Express Blue soln for that operation and Laurel Mtn Slow rust blown (boil for blue) for slow slow rust blue is what I've used for the last 30 years or so.
No mercury in either. You can coat right over gold/silver inlays and not fear that they'll come out of the tank mercury plated. The mercury plating does NOT card or rub right off. It is there to stay unless you polish it off. You can imagine the problems.
In one shop I worked in early on they used Belgian Blue and discovered the plated inlay problem.
The TroubleShooters from the front office made a first attempt at removing the merc ... heat the tiny 14k inlays with the pin point flame of a torch. That did work a little. Burning off mercury in the shop,,that was a smart move looking back.
Then to finish off their fix, those in the know decided a rub down of Nitric acid on a small patch would polish them up nice and sparkly clean.
When they got done playing around destroying things,,
The parts were stripped and back to the engravers to polish repair the inlays and surrounding work and a better bluing process sought.
Swab on chemical cold blues...the only thing I have on the bench is a bottle of Oxpho Blue (Brownells) It does a decent job for screw heads and pins and the like on a touch up job. It certainly has it uses.
But anything else gets the full treatment.
Swab on cold chemical blue is nothing to do an entire gun with IMO.
It may look pretty good in certain light, but there always seems to be excuses for the overall way it looks.
You don't want to have to make excuses for your work.
