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 environment.  Brown rust is Fe2O3 that becomes black rust Fe3O4 when boiled in water.  Mark Lee Slow Rust #3 solution and Mark Lee Express #1 are both true rust bluing solutions.  Either solution will produce best quality results.

The main difference between the two blacking solutions is that rust immediately forms with the Express Blue #1 but it takes hours for 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 very different.

My solutions do not contain any mercury or selenium.  In my experimentation I used mercury and selenium in solutions but found that 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 be grey black; glass beaded or 320 grit polish will be blue black; and a mirror polish will be 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.