There are 2 commonly used but different methods of Rust Bluing.
Slow Rust bluing,,also called Cold Rust bluing
The other is-
Quick Rust bluing, also called Hot Rust bluing and Express Rust Bluing.

They use different chemical formulas to get the part to rust.
The Slow Rust method relies on the air temp and humidity to 'slowly' rust the part(s).
You can control that with the DampCabinet mentioned already.
But letting the natural air and humidity do the job works well as long as the temp is 75 or more and the humidity 85% and more.

When rusted to satisfaction (and it doesn't take a heavy crusty layer of rust, just a very fine reddish/brown coating), remove them and boil or steam as some do now. Then card, let them cool to room temp and reapply the soln sparingly but completely. Back into the DampCabinet for another cycle.
4 or 5 cycles will generally give you a nice blue. You can continue with cycles of rusting and boiling of course.

Quick Rust/Hot Rust/ express Rust bluing,,,
Warm clean parts to around 200*F and apply a coating of that soln to them. They will immediately turn a blue/brown. Place them right into the boiling water (or steam them). 15min and out of the water. Clear any water from the surfaces, card the part while still hot.

Rewarm the parts again if they have cooled down too much, and they usually will.
For warming the parts I use a small propane torch. Some use a heat gun.
The orig process used the water in the tank as it was boiling and never shut down during the start-to-finish operation. They  would place the carded parts back into the boiling water to heat them again.
Taking one at a time out quickly, clear any water on it , then apply another coating of solution to the parts.
Then right back into the boiling water to turn the brownish colored coating to blue/black.
15min in the water and out again, clear any water & card them.
Repeat over & over till the color suits you

It helps to have the rusting soln warm as well. I used to place the small amt I would be using for the job in a glass container and hang it over the side of the tank so it heated along with the water.

I use Laurel Mtn bbl brown for Slow Rust
Mark Lee's Express Blue for Quick/Express rust blue .

That's kind of a quick explanation of the process. 
Lots of techniques to tweak the process an get passed things like streaks in the finish, parts that won't 'take', etc.

I was fortunate enough to take Mark's bluing class at Pine City Technical College about 40 years ago. Along with his great bluing products, his biggest secret was to blast the parts with 240 grit aluminum oxide dust at about 35 PSI. This made sure the metal was clean and allowed the chemicals to have a good bite on the metal. Before you card the parts, make sure they are totally dry especially around screw holes and dovetails or you will streak the metal. 
If you've ever had a chance to see his half scale Mauser rifle that's on his web page, you'll marvel at such a work of art produced by that group of guys.

Chemically, the red rust formed in both slow rust bluing and Mark Lee Express bluing is exactly the same compound (ferric oxide), or basic red iron rust. 
However, while the chemical composition of the red rust is identical, the physical characteristic's of the rust are completely different. 
Slow rust produces a very tight, highly dense, and flat layer of micro-velvet red rust, while the Mark Lee product tends to produce a coarser red rust.
The question becomes whether the differences in the physical character of the rust creates a difference in the final product? 
In my experience, I can I see no difference in the final product. 
I’d be interested in the experiences of other forum members who have used both bluing methods.
Otto

What happened to Laurel Mountain Forge barrel brown and degreaser? That product was great!
I heard that the owners wife passed and he shut down the business, but that was second hand information.


Tom Klinger