You can do the entire gun that way.
Just need a container big enough to completely dunk the parts in molten NItre Salts. Nitre can be either Sodium Nitrate or you can use Potassium Nitrate. I've even used Ammonium Nitrate and had good results.
A dangerous operation to work with if you do consider this. Even a small set up is dangerous and you need to take all precautions when working with the stuff. It can seriously burn.
It's heat temper color, nothing else. But it's done while the part(s) are completely shut off from the atmosphere and any oxygen in it. That way the color change and look isn't effected by oxide forming on the surface.
It's most effective visually when high polish is employed and extreme skilled flat, no ripple, sharp edged work.
Like you see on the Colt parts.
The process is also and sometimes primarily, a draw back/tempering of the steel after hardening.
The color a simple nice after effect.
The bright blues just happens to appear at the correct tempering temps.
It's the same process as 'Straw' coloring on auto pistol parts like Luger, ect.
Flat springs and V springs are(were) often drawn back after hardening in Nitre or even molten lead at around 750F. That temp will give you a dark blue color to the metal, But more importantly it will temper the fragile hard spring to a degree that actually gives you a 'spring'.
The name 'spring temper blue' comes from that process and color.
I think that's what was done on the Colt DA hammers. They were hardened. Then drawn back in the molten salts which left them with the bright blue color. Then the sides repolished to 'In the White' for a contrast.
Run the temp of the NItre to around 850+F and the color of the polished steel will come out a darker blue black. This is often used to mimic Charcoal Blue or Carbona Blueing on restorations.
More easily done in a small shop than running a small gas furnace and hoping for the best while rotating the parts in a cylinder sealed up with bone char inside and a little oil.
The temps that it runs at 850+F are the same for the Nitre ,the open hearth charcoal blue process, and the CArbona Blue American GAs furnace process to get about the same result.
Cimmaron Arms (Uberti made?) used to offer some of their SAA with the blued parts done up in spring temper Nitre blue. Very pretty.
But very fragile finish.
