Charcoal bluing as practiced by Victorian gun makers and still used today by the original Artisans that perfected this method who where clockmakers. If you take a close look at the screw heads and other steel parts on a vintage European clock or pocket watch they have a polish and depth of colour that is a truly striking blue indeed.
It only uses one ingredient finely powdered charcoal and an Iron pan with a very controllable but small heat source applied to the pan. Each screw or part is polished to a mirror finish then covered with charcoal powder then the temperature is very slowly increased the slower the better. Each part is removed periodically to let the oxygen in the air cause the oxide layer to increase in thickness on the surface of the item being blued also any corrections in the mirror finish is done, with the finest blue colouring arriving at about 570 degrees F.
It is an Artisan skill which I have not mastered to the point that all my clock parts do not finish with the identical blue colour but are close enough, been trying to get it right for a lot of years now and running out of practice time.
