Charcoal bluing (real charcoal bluing) can be done with very nice results but it takes a lot of labor and no small amount of practice.
Just putting the parts into a bed of coals and covering it will more often than not just leave you with an uneven colored coating of oxide. Often you'll be rewarded with a coating of scale on the steel as well where the air got at it. At times that scale can be an (almost) even blue color. Some that do the work that way look at that as their 'bluing'.
After the part(s) are in the charcoal for a while,,and it takes some experience to know when ,,it varies with the mass and shape,,they are removed one at a time and rubbed down.
A holder for each part is a near must-have to handle them through the process.
You can rub them down with burlap or canvas, or sisal(sp) rope pieces,,anything somewhat abrasive but not so much as to scratch the surface. You are burnishing the surface and color to even things out. The addition of a powdered polishing agent helps. Lime was an old standby. Some say that oil can be applied also during this stage of the process, I've never found that to work well,,but everyone has a bit of a different technique when it comes to finishing be it metal or wood..
Burnish the parts,,but remembering they are still 800+ degrees. So you will burn yourself badly if not protected.
Back into the pile of charcoal and buried as soon as you can and pull another part out. Do the same, and to the next. Around and around till the finish pleases you. Some will look done before others. Different steels mostly I'd guess. The heavier ones take longer to reheat and gain another color coating of course.
Add charcoal to the fire, don't let it get low and expose the parts to the air.
Hot work,,no getting away from the pile of charcoal especially if there's many parts to do.
Not like burying it and walking away for a couple of hours as most instructions tell you to.
