Let's talk bluing

[Paladinpainter]

4 Scale,
I suppose "blue" is in the eye of the beholder, but to mine there is a substantial difference in color between a caustic and rust blued gun. Caustic 'blue' produces a flat black finish with very few blue highlights, while the old rust blue - like you find on a 1920s era DWM - is distinctly blue to blue/grey.
The chemicals used in today's rust bluing solutions and the old time ones are different. To my tired eyes, the modern rust blue solutions result in a black finish. Thats why, Markbritt's discovery that the addition of sodium nitrate to Brownell's Rust Blue would create blue highlights was noteworthy. (See his sticky in this section)
The old bluing formulas contained stuff like copper sulfate, ethyl nitrate, and mercuric chloride, chemicals you dont commonly find in the modern solutions. My view is that the inclusion of these chemicals creates a molecular structure on the surface of the steel that reflects the blue part of the spectrum.
I think if you were to put a 1920 Luger and a 1940 Luger side by side in natural light, you'd see a difference.
Regards,

[nukem556]

Actually, I "rediscovered" the sodium nitrate addition to the rust bluing procedure from some old gunsmith writings and posted pics on the result...markbritt followed along with more experimentation later.

[4 Scale]

Originally Posted by Paladinpainter

...there is a substantial difference in color between a caustic and rust blued gun....I think if you were to put a 1920 Luger and a 1940 Luger side by side in natural light, you'd see a difference.

We may be talking past each other a bit. I was not opining either generally or authoritatively on the appearance of rust blue vs. salt blue. i was offering one personal observation on a single pistol.

Attached are two photos in shade and two in sun (note dark shadows to figure out which is sun). The receiver/barrel is a recent rust blue, the frame is a 1938 Mauser original finish +90% blue. If I study very carefully, outside in the shade, I can detect a bit more of the color blue in the rust blue part, after allowing for wear on the frame, maybe. In bright sunlight I can't tell a difference.

I figured mixing bluing methods was OK on a shooter and was pleasantly surprised at how the color of upper/lower seems so close. I have no doubt you are right about 1920 vs. 1940 Lugers, I wouldn't know. My point was that at least in this instance I was able to "get away" with combining the two bluing methods on a shooter; to my eye the difference (when I can see it) is acceptable for a shooter.