Rust bluing is a beautiful blue black finish that was used by firearms companies before 1940, when hot bluing was invented. Hot bluing is not better but it is much less labor intensive. Other bluing processes such as carbonia, charcoal, and nitre bluing also went by the wayside because of economic reasons.
Rust bluing is very popular today for custom gunsmiths and hobbyists because of the durability and quality of finish that can be achieved without too much equipment. Rust bluing is a method of making steel rust in a controlled enviroment. Brown rust is Fe2O3 that becomes black rust Fe3O4 when boiled in water. Mark Lee Express Blue #1, Express Brown #2, and Slow Rust #3 are true rust bluing solutions.
The main difference between the two blacking solutions is that rust is immediately formed with the Express Blue #1 but it takes hours for the rust to form with the Slow Rust #3 formula. Please refer to instructions for both and you will see that the procedures for each are quite different.
My solutions do not contain any mercury or selenium. In my experimentation I used mercury and selenium in solutions but found they offered no advantages but had the downside of being very toxic. In my experimentation I added different chemicals to try to affect the color of the finished bluing.
The surface texture and microscopic thickness of the coating determines how light is reflected and has the greatest affect on color. This color effect is seen in hot caustic bluing like the factories use today. If the metal is sandblasted it will look grey black, glass beaded or 320 grit polish will look blue black, and a mirror polsish will look ebony black.
The burnishing during the rust bluing process will usually produce the blue black color. If rust coatings are kept very fine, scrubbed with steel wool, and lightly buffed after the last coat the color will appear almost ebony black as seen on fine English shotguns.