Let's talk bluing

[David-NY]

I have been trying to get familiar with the different bluing methods used on Luger pistols and want to tap into (and perhaps even contribute to) some of the collective knowledge on this board.

One preliminary issue (based on some of the posts I reviewed) is to clarify precisely what bluing is -- and is not. A bona fide "blue" finish is a metal surface that has been oxidized to form a layer of iron (II) oxide (i.e. Fe3O4 or "black oxide") on its surface. Black oxide does not, in and of itself, impart any protection from corrosion. However, the process of creating a black oxide surface layer results in thousands of microscopic fissures in the surface of the metal. If the metal surface is then impregnated with oil, oil will fill in the fissures and pores and form a film over the surface of the metal. The oil film prevents oxygen from reaching the iron steel, which prevents further oxidation from forming.

The original form of bluing was "rust" bluing which involved creating successive layers of regular old red rust - i.e. ferric oxide, Fe2O3. When ferric oxide is heated over about 200 F it changes to Fe3O4. This is why one step in the rust bluing process is immersion in boling water.

I have a book that says that rust bluing stopped in 1918. There are other books that put the date at 1923. E.g. the discussion here: http://forum.lugerforum.com/showthread.php?t=32819

Later Lugers were blued using bluing salts. This creates a layer of Fe3O4 by oxidizing the surface of the metal in a heated solution that is already sufficiently warm that it forms black oxide rather than red rust.

Frankly there is very little actual (end result) difference between different bluing methods - charcoal bluing, carbonia, etc. All of these processes involve creating a layer of black oxide on the surface. The obvious difference (at least when dealing with Lugers) is going to be that on a rust blued gun the inside of the frame will remain in the white.

In the above link there is discussion of a "built up" bluing method used on some guns starting in 1923. What is this???? The reference to immersion in boiling water suggests that red rust is being converted to black oxide, but references to the finish flaking off don't really make sense. It is true that if the oxidation layer was thin the finish would not last that long (as metal wore off and got beyond the oxidation layer).

Any input?

[mrerick]

True bluing is a conversion process.

Lugers were blued using the Rust Bluing technique through the first quarter of 1937.

Many characteristics of Lugers that help prove authenticity are related to the bluing technique used.

Most "flaking" problems can be traced to improper surface preparation prior to bluing the metal.

[4 Scale]

I had an interesting recent "bluing" experience. Last month I acquired a 1938 mismatched Luger - frame and receiver matched, but DWM toggle, mismatched side plate and trigger, one grip is numbered the other seems a wartime beech replacement, and wartime (or at least, long ago) re-barrel with a WWII era Mauser barrel. The receiver was marred by whoever did the re-barrel. Given this pedigree the pistol is a "shooter".

As the barrel bore and exterior are excellent, I explored getting the pistol cosmetics improved. I had the marring fixed, not 100% but the pistol looks much better. Due to the location of the marring it was a choice of lose the proof marks on the right frame or tolerate some marring, and we chose to retain the proof marks/tolerate some marring. Charles Danner did the work on the marring and r-blue of the barrel/receiver and he did an excellent, fast, and very reasonably priced job on it. Charles does rust blue, not salt blue, so now I have a repaired and re-blued barrel/receiver to go with the salt-blued frame which retains over 90% of its original salt blue.

No matter how hard I try, I cannot tell that the receiver and frame were finished with separate bluing processes. As a relative newbie to the Luger lifestyle, perhaps my eye is not trained well enough yet but for now - it was an interesting lesson in how similar the end product of both bluing methods appear.

[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.

[David-NY]

I am actually now very curious to find out what sodium nitrate adds to the equation - is it just contributing that additional ion to help with oxidation? Or is something more complex going on?

I think you get better quality control with rust bluing because of the successive applications of the rusting solution. If there is an issue in the finish you are more likely to see it and to be able to correct it. In addition, it is easier to stop oxidizing once you have gotten the color you want.

Differences in color relate to the level of polish and the extent to which the surface has been oxidized. Ultimately we are talking about a variation that runs from the "royal blue" on an early 20th century Colt to the type of flat "black oxide" finish you might find on a military gun, like a S&W Victory model.

A bit of a change of subject, but has anyone found any good alternatives to nitre bluing? Nitre is a nice finish but (like color case hardening) it has just about no corrosion resistance.

[Paladinpainter]

Nukem556:
I am sorry to have mis-attributed the sodium nitrate discovery. I guess Mark and sodium are linked in my mind because of his research. I apologize.
David, the chemistry involved in the effect of adding sodium nitrate goes way beyond the 'D' I got in Chem.1 . But your question about nitre bluing raises a puzzling phenomena: why does an unoxidized piece of steel appear to change color as its heated from 400 to 600 degrees F ?
Regards,

[David-NY]

Paladin - read this: http://en.wikipedia.org/wiki/Temperi...mpering_colors

What is happening is a thin layer of oxide is forming on the surface, and the temperature at which that layer is formed determines what wavelength of light can pass through the film and then reflect back off the metal surface. Which is why a good polish is critical. And which is also why the finish is not durable - it doesn't take much to rub that thin surface layer off. As the saying goes, a color case hardened finish is only as good as its clear coat.

[Paladinpainter]

Interesting article. Thanks for enlightening me.
So can we infer that rust and caustic bluing are forming layers of translucent oxide of varying thicknesses? Further, that the thickness of the oxidized layer determines the color ?
I suspect that with rust bluing the use of some chemicals, sodium nitrate for example, effect the molecular structure of the oxide layer changing the way it transmits light. That's pure speculation on my part.
You had an earlier question about a more wear resistant alternative to nitre bluing. The only thing that I could come up with is vapor deposition. Some of the gun makers are producing intense colors using the process. I've read that the wear resistance of titanium coatings is excellent.
Regards,

[nukem556]

My two-cents worth theory is part of it may be that the sodium nitrate is slightly basic and may allow the acid in the rust solution to work more by corrosion that etching changing its reflectivity....then again if that were so, baking soda would work.dunno.

[David-NY]

Originally Posted by Paladinpainter

So can we infer that rust and caustic bluing are forming layers of translucent oxide of varying thicknesses? Further, that the thickness of the oxidized layer determines the color ?

Actually, no. Real bluing is not a layer. The thickness of the metal is the same before and after. With real bluing you are actually corroding into the metal. With heat coloring you are making a layer on the surface. The reason the heat coloring, color case hardening, cold blue, etc. don't do much to protect against corrosion is that you don't have all the pores, etc. in the surface of the metal. With a blued surface, all those pores get filled with oil and then help maintain a film of oil over the entire gun.

Originally Posted by Paladinpainter

You had an earlier question about a more wear resistant alternative to nitre bluing. The only thing that I could come up with is vapor deposition. Some of the gun makers are producing intense colors using the process. I've read that the wear resistance of titanium coatings is excellent.

That's exactly what I've heard and I've been thinking about using a titanium nitrate coating for small parts that are normally strawed.

I think I have a book at home that might have some info on the import of sodium nitrate in rust bluing . . .

[David-NY]

I don't know of any PVD coatings that are blue. I have seen some blue chrome, but only as decorative (bumper) chrome.

[ithacaartist]

I had an Ithaca SxS made by SKB the barrels of which were white chrome inside and black chrome outside. Plating adds dimension.
Yesterday I talked with a place in Florida that does black nickel, which I'd never heard of. I had been looking for a place that could brass plate some Zamak small parts for one of my Erma guns, to mimic straw color. 3 steps- -copper, then regular nickel, with a top layer of colored Ni.

[Paladinpainter]

David, Sig Sauer and Kimber are using PVD coatings on some of their guns. Some of them are rainbow like and I've seen one in electric blue. Here's a link to one of Sig's guns with a rainbow finish.
http://www.sigsauer.com/CatalogProdu...p938-edge.aspx
(I wonder if you couldnt duplicate the old Colt Royal Blue by using PVD.)
I agree that rust and caustic blues are invasive oxidation and not additive like plating. Most of the chemicals I can think of used in rust bluing are pretty corrosive. Its all the micro-pits they have eaten in the steel that give it that satin look and, as you say, provide a home for oil.
Regards,

[David-NY]

I don't think you can duplicate a nice blued finish with a PVD coating, at least not right now. I wish you could.

The process that has caught my attention is ferritic nitrocarburizing. But it's not really clear (to me) how that would like on a gun that had a high level of surface preparation.

[Paladinpainter]

David, in some of the images of nitrocarburized materials, the stuff looks pretty shiny. Melonite and Tenifer(?) finishes are used by a couple of manufacturers; my Glock has a flat/satin look to it. I've got no idea how it was prepared for finishing.
We still havent figured out why sodium nitrate and/or the other chemicals used in rust bluing impart a blue tone to iron oxide. At this point, I'm out of ideas.

John

[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.