doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Case Coloring

I thought the Doc Gaddy articles showed the film to be relatively simple chemistry arranged in a unique way. The colored film that forms might not be so unstable, some colors show centuries later if they are protected.

I'm not following what the temporary state is that is going to equilibrate. Aren't the colors primarily iron oxide and separate from the thin case of steel that was formed that may or may not be hardened. I don't think you need to quench to form iron oxide, but the quench seems to arrange the iron oxide in the way that shows as gun case colors.

I think there are things that affect the appearance of the colors, but I think all that means is it isn't always the toughest most resistant surface finish not that it's tearing itself apart to release stress. Even after the colors are long gone, there could still be a hard steel case on the part.

So an ideal way to remove case coloring and bring on that soft grey patina would be to take the action body and the forend iron to a tanning salon. Finally we have a legitimate reason for these places to exist!

Now that you mention it, on many faded old guns the only place the case coloring can be seen is the inside of the forend iron, the part that faces the underside of the barrels and is permanently shielded from the light.

Craig, there are something like 1000 varied forms of Iron oxides. Bond lengths, angles, electron energy levels, quantum effects, all shape the molecules and their reflectivity. Some are short lived forms. Every molecule desires to return to it's laziest condition. The quench can confine? molecules in an "anxious" condition. And they await an opportunity to go to a lower energy state. Sometimes the energy hump to cross is quite high, and sometimes not. Might even have 2 or more humps.
I don't think people are all that interested in coordination chemistry when they just want a pretty blue color.

I know there are many that do not believe in this stuff. But I am in the camp of colors transforming. They seek a lower energy level.

I am reminded of the effort to grow cubic tomatoes, to expedite packing.

I'm saying CC can't be photoreactive, but I am open to hear some chemistry to explain how it is reactive. Metals and metal oxides, far as I know, do not tend to photoreactivity. "Rusty" steel does not fade from loooong exposure to sunlight in my experience. The colors in CC are created by the thickness of the "stuff" layers. Thinner layers create yellows, middlin' layers make reds, and thick layers are blue to purple. Did I get that right? The actual light is reflected by the steel surface, but wave lengths other than the color we see are absorbed. Actual thinning of the "stuff" layer should, then, change the color, not fade it. Fading would, in my estimation, happen as the various areas wholesale loose their layers. So an area missing half of its "stuff" layer would be less bright than one with 100% coverage. Note how thinning bluing becomes less "bright" compared to fresh, 100% blue. This logic says that you don't wear off some of the outside of the layer, but loose the "chips" in a chip by chip fashion.

I'm not interested in proving this issue one way or the other, only in finding the truth. Looking forward to debate points to the above.

DDA

Most of us tend to think of color as residing in pigment or dye. Iron oxide in common rust is a reddish pigment; white lead is a white pigment and so on. The very thin surface layers that cause case colors are indeed very thin as noted above by Rocketman. He is right when he says that the colors are caused by the thickness of the surface layers, but it isn't a build-up of pigment that does it, it's wavelength interference.

Light passes through the surface layers, hits the substrate and is reflected back. Specific wavelengths are interfered with; others pass through and which are interfered and which pass depends upon:

1. the thicknes of the surface layer
2. the refractive index of the surface layer

I have no idea if UV or other energy will change the refractive index of the surface layers but it indeed possible as Clapper noted. The light doesn't have to change the thickness (or amount) of surface layer, it would just has to change its refractive index.

When we think of terms of pigment it is obvious that a heavier layer will be more opaque or darker, but the colors resulting from interference are generated by a different process entirely.

Think blue jays (the bird). There is no blue pigment in their feathers.

Then think the colors (blue to yellow) that you see on the surface of camera lenses. These too are interference colors generated by the anti-reflection coating on the surface (magnesium fluoride etc). A magnesium fluoride coating 1/4 wavelength thick will look blue in reflected light.

The rainbow colors generated by a spreading oil droplet on a puddle of water is the same phenomenon - the colors are not intrinsic to the oil; they are generated by interference of the light reflected through the oil from the oil/water interface.

The holy grail in case coloring is finding a non-thermal chemical way to create the surface coatings of proper refractive index. Then one could renew case colors without risking the integrity of the metal.

Note added in edit: Worn case color seems to become grayish - that is consistent with physical thinning. This doesn't represent "fading" in the common sense, it is the result of a shift in wavelength interference. In physical thinning the refractive index of the layer stays the same but its thickness decreases. Thickest layers will produce blue interference; then as the thickness decreases we see purple, red, gold, silver and finally grey. For a layer with an index of refraction of 1.5 we see blue at about 220 millimicrons, grey at <60. Note that the index of refraction hasn't changed. If photreactivity changes the refractive index then that would be a separate mechanism whereby perceived color would change.

I suspect the answer to these perennial questions is "all of the above". UV light degrades certain hues without mechanical contact, as pointed out by the display case observations. That is a simple enough experiment to perform. It implies a photochemical reacion.
I don't think this topic really needs to fall into either/or categories. Physics vs Chemistry.
The films composition and thickness can be changed by many things. It might be interesting to fire a shotgun a few thousand times in the dark, jigged so as not to touch the reciever, and then observe any changes. Chuck's hinge-pin discussion reminds me there's plenty of energy at firing to bend the action bars, and possibly disrupt the films. This would demonstrate brittleness I believe.
Then of course, there's just plain ole' oxygen, anxious to join it's Ferr(ous,ic,ite)brothers on the road to entropy.

Someone with a kiln could easily do experiment 1 at no significant cost.

Gnomon, I believe the ability of an iron oxide layer to interfere with light wavelength, refract, etc. to cause different colors to appear is well known.

But, as Doctor Gaddy pointed out, the example of an optic lense or drop of oil might not be good analogies to gun finish colors. Those examples are true optic occurrences, the color changes as the viewing angle changes because the light coming into the eye has passed through a slightly different thickness of oil or lense coating.

Gun cases colors don't change to any appreciable amount when it's viewed at different angles.

ClapperZ, if someone had a kiln what easy experiment would you be considering to show the variety, complexity, reactivity and instability of gun type case colors. Does oxygen have an energy favorable drive to react with iron oxide that's already formed on the surface of a gun.

Rocketman, Dr. Gaddy once told me of a test he had done on several pieces of identical steel that he had case colored. One piece went to an area that received the most direct sun, another into the house somewhere and another into a closet. There may have been more samples. After a year or two, comparison of the pieces showed no difference in the colors.

"The holy grail in case coloring is finding a non-thermal chemical way to create the surface coatings of proper refractive index. Then one could renew case colors without risking the integrity of the metal."

another voice in the wilderness...

I would color a bar, shield it with probably aluminum sheet on each end, and then expose the bar to high levels of narrow spectrum uv light until summers end. Pick your nanometer.

There is a continuous conflict between people that have observed fresh case coloring (without clear(ish) coatings) degrading in short periods of time, and those that say the surfaces are inert to light.
I suppose the bar could be laid in a tupperware box filled with argon or tri-mix to exclude oxygen.

I do not believe ferro-oxygen coatings are inert in our atmosphere. I believe there are great numbers of compounds captured in the films at quench, and some are very delicate. Delicate merely implying they can be changed in conformation or presence via outside energy, within the realm of what we might normally provide.

I worked in inorganic polymers during the cold war in an effort to protect carbon forms from neutrons. Even in the most extreme conditions, we had isomers, conforms, and stability issues.
I'm kinda glad it didn't work out, because ultimately it supported the limited use of atomic weaponry.

We should use this WWW of interested parties to cooperatively demonstrate, and collect these pieces like Drew and Pete have done for Damascus barrels.
Lots of deceased scientists findings have been refined and enhanced after their passing.
I used to have to re-do every experiment that came from a Russian publication. Even if it was re-heating H2O.(joke)