Zischang's rust brown/rust blue formula was nothing more than Hydrochloric acid & Nitric acid together with enough iron bits thrown in to disolve till it wouldn't take any more.
The 'iron' was generally said to be swarf and filings from bbl making process and other bench work. Iron nails of the period are often said to be used as well.
3oz of hydrochloric & 4 oz of Nitric acid,,,the amt of iron ends up being around nearly a 1/2 lb by the time the smoke clears. Do this outside. Add the iron in small increments so it can be consumed w/o a violent reaction. It does produce a lot of heat and acrid smoke.
When it cools down,, add the mix to a qt of distilled water and just let it sit for a few days.
Then strain it thru a cloth to get any bits out of it. and put it up in a clean glass jar. It keeps forever but will eat thru a steel cap eventually.
Yes it's A/R eccentially.
The Neidner Rifle Co used the same stuff to rust blue their custom work in the 20's and 30's. I think they tweeked the formula a bit by adding some 'Sweet Spirits of Nitre',,which is Etheyl Nitrite. Not sold anymore unless you have a license to obtain it of some sort..
I don't know what the added stuff was supposed to do, but you find it as a componet in many early rust brown and rust blue formulas.
The Zischang formula does make for a nice deep blue color when rust bluing.
But as it is Ferric Nitrate, it's the same main ingredient as L/Mtn.
I think a lot of the final look has to do with the polish of the metal, the metal composition itself, the techniques of the person doing the work, cycles applied, what is used to card with, ect.
