Modern hot salts bluing is actually boiling the metal in a tank full of commercial caustic hot salts and distilled water bluing compound.
This is different then nitre bluing.
In this process, the metal is first boiled in a tank full of a cleaner to clean and pre-heat the parts, then they're immersed in the tank full of bluing chemicals for a specific time.
After they come out they're then boiled in a tank to stop the chemical process.
This is the modern bluing process and is used by all the gun companies and refinishing services.
The level of shine is determined by the level of polishing done BEFORE the metal is blued.
The big difference between a satin black Ruger and the brilliant deep mirror-like blue of a Colt Python was primarily the polishing done before bluing.
The commercial caustic salts process is the most durable of the blues, and the cheapest to apply.
Done right and properly polished before bluing and the appearance is outstanding.
Rust bluing is done by boiling the parts in pure water, removing them and swabbing them with a chemical that causes the parts to rust.
The rust is "carded" off with degreased steel wool, then put back in the water.
Each time the rust formed by the chemical is carded off, the blue color gets deeper.
Rust bluing is a durable blue but has a more satin blue finish instead of the shinier finish you can get with modern caustic chemical bluing.
Charcoal bluing is any one of a number of similar heat bluing processes in which the parts are usually placed in a steel drum with a mixture of bone meal, charred leather, and other often "secret" ingredients and heated in a temperature controlled furnace.
When the parts come out they have a deep blue finish that's attractive, but not as durable as the caustic process.
Colt notably used this type of process in the pre-war days, which was called "Carbona" bluing.
This process is very similar to the way in which steel is given a color case hardened coating.
Heat bluing is simply heating the parts until they change color.
When steel is heated it starts changing to a light yellow color called "straw" then through darker yellow, brown, purple, then to a brilliant blue.
The blue is a lighter, more delicate color than other heat processes, and is not very durable at all.
There are any number processes by which steel can be given a blue color, with many of the heat-type processes being very similar.
Due to the expense and time needed for most of these processes, most guns are blued with the commercial caustic salts process, with the others being reserved for high-dollar custom work or for restoration of valuable antique guns.