doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Composition of Case Hardening Colors?

I know the thin case is carbeurized steel, but what exactly causes the colors? And what, besides handling and wear causes them to fade?

I ask this because I just bought a Lefever H grade 12 ga. The gun is a small frame twist barreled model that weighs only 6lbs. 10 oz. with 28" barrels. It looks like it was very well cared for over most of it's life. It has way above average original condition, and the case colors in hidden areas are vibrant and at least 90%. However, it appears it was neglected or stored in a humid area in recent years. It did come from Georgia. There is a thin patina that has turned the barrels brown/black and has obscured much of the twist pattern. This patination is so fine that the metal almost looks as if it was sprayed with a thin coat of walnut stain. A similar thin patina covers the action and sideplates, but you can actually still see the mottling of a lot of case colors under the translucent patina.

I think I could possibly boil the barrels, card, and etch to reconvert the patina on the barrels to black and white without refinishing. The original black and white is still very evident under the forearm wood. But is there any way to remove the patina that is obscuring the case colors without also removing the colors?

Kieth I can't give you a scientific answer about what chemical process causes the various colors when steel is case hardened. I have however had good results bringing back rust blued barrels that have rusted as you describe simply by lightly carding off the patina with 0000 steel wool soaked in light machine oil. It might be worth a try and surely wont hurt anything.

Barrels are not case-hardened, but blued or rust-blackened. This is very different than the colors one sees on case-hardened receivers.

In real case hardening as one finds on receivers, a transparent film is created on the surface. This film has a varying refractive index and thickness and the colors one sees is the result of interference of light as it passes thru the film and is reflected out again. This is the same physical reason why "coated" camera lenses appear to be blue in reflected light; others orange. The coatings themselves are transparent but have different refractive indices.

In principle it is the same as the colors one sees in a rain-puddle that has a drop of oil spreading over the surface. These are known as Newton's rings.

Case coloring is the result of optical interference. It is NOT a pigment color - the layer is way too thin to be pigment-based color.

The transparent layers are very thin - measured in Angstroms. This is why they are so delicate.

If one used monochromatic light the perceived colors would tell you the thickness of the layer.

Keith,
The colors on the barrels are Ferric Oxide that have been converted to Ferro Oxide and the darker colors maybe some logwood that they were initially boiled in.THAT IS NOT COLOR CASE HARDENING ON THE BARRELS , ONLY ON THE ACTION PARTS.
HOWEVER, DO NOT MESS WITH THE BARRELS other than taking some 0000 steel wool and oil and cleaning them. This will likely bring a lot of the colors back.

I know that the barrels were originally rust blued and that's why I considered just boiling them to reconvert the ferric oxide back to ferro-ferric oxide. I wonder if purists would even consider that a refinish since all I would be doing is converting the original black oxide back to black again by the boiling. I do think that I'd need to etch them in ferric chloride solution after carding to bring out the twist pattern because the patina is darker on the barrels than it is on the frame, sideplates, and other case hardened parts.

But the big problem that remains is how to remove the patina from the case hardened surfaces without removing any colors that are still hiding underneath. If Tony was still with us, I might ask him to case color some polished pieces of mild steel so I could allow them to become stained with light rusting, and then experiment with ways to remove only the staining.

Some of the "patina" on the case hardened bits could just be oil, dirt, and crud. Have you tried wiping with something like CLP, then progressing to mineral spirits, alcohol, lacquer thinner, and acetone? Keep solvents off the wood, but solvents should not harm the case hardening or the associated colors. It is possible that the metal was coated with something to protect the case coloring, perhaps linseed oil or lacquer. Both of those will get darker with age and the linseed oil can have a tendency to hold surface dirt.

Some old threads here and elsewhere have suggested polishes like Flitz to clean CCH surfaces, but I don't think I'd risk it without a test on an old beater.

You might also try a stainless steel scouring pad with oil to gently clean the CCH surfaces. I have used it with some success on lightly rusted blued and CCH surfaces.

What about using copper wool (and oil) instead of steel wool? Seems to me like the softer copper wool would remove crud on the CCH or unwanted patina on the barrels without disturbing the desireable finishes.

All that I've done so far is to wipe the metal parts down with Ballistol. Some light rubbing with Ballistol on brown paper towel really didn't bring up any dirt or crud. As I said, the gun is really clean and appeared to have been little used and well taken care of until relatively recently. It also doesn't appear to have been coated with lacquer or linseed or anything to protect the case colors. I can try a little lacquer thinner on a sideplate, but I don't think there is anything there that will dissolve in solvent.

I have cleaned numerous guns in the past that had similar or worse patina using 0000 steel wool and oil or WD-40. Any colors that were hiding under the patina were either removed or left very faint like dark patches. But on those guns, the colors on the water table or inside the forearm iron were also mostly gone. I'm less inclined to do any hard rubbing on this one because I suspect there is a lot more that is worth trying to save. I'm wondering if the electrolysis method of rust removal would take away the patina and leave any case colors unharmed. These seem to be uncharted waters.

I never heard of the copper wool that Dave in Maine mentions. Sounds like something I'd like to try. I have used bronze wool in the past, but I haven't seen that for years either. Many thanks to all who have replied.

The copper being softer than the steel will transfer to the steel and you will get a yellow tinge to the steel.

Keith,
I think the case colors are very similar to rust...an oxide. Chemical rust removers would likely remove the colors as well.

That's what I'm trying to find out. I haven't found any actual reference to a chemical composition for the very thin layer that we see as irridescent shades of blues, purples, yellows, etc. I know for certain that the phosphoric acid rust removers will remove case colors. The ones that I've used are OK for exhaust manifolds and the like, but bite far too agressively for use on all but the nastiest barn gun.

Brownells has bronze wool.IME it works well .

The surface coating film of color case hardening is a complex mix of oxides of the various trace elements in the carbonizing medium and in the quench medium. As noted above, the visible colors are an indication of film thickness, not pigment. The same range of colors can be had by heating a piece of polished steel. If heated slowly, the color of the whole piece will progress from light blue to dark brown as the surface temperature increases and iron oxides form on the surface. If heated rapidly from one point, the colors will spread as bands of color. Note that in this case, the film is iron oxide, not the same complex assortment as in case coloring. Note also, that case colors can be had without actual case hardening. There are as many variations in color mix and pattern as there are variations in carbon packing, quench medium, and temperature and time control.

Since the surface film is thin and loosely bound, it is subject to any mechanical abrasion and/or acid attack. A top film is required to protect it - poly, wax, drying oil, etc. Anything that removes "rust" is very likely to attack CCH; either chemical or mechanical. The only advice I can give is to go slow and be careful. Keep experiments as small and inconspicious as possible.

Best of luck.

DDA

Dr Gaddy published SEM photos of case coloring. As Don states, this is very thin, only a few atoms thick. That is why a protective coating is always applied.

Pete

From some of the research I've done, the depth and magnitude of the colors has a lot to do with the exclusion of free oxygen within the pack, and during the time interval in which the contents of the crucible are dumped into the quench. This is apparently why Tony started getting more vibrant colors when he dumped the contents of his crucible faster and closer to the surface of the quench. I've read that ideally, carbon monoxide rather than carbon dioxide should predominate within the pack during heating. But I have never done this and am certainly no expert.

If the thin layer that gives us the colors really is comprised of various oxides, then reducing agents would almost certainly degrade that layer. Alternately, oxidizing agents would likely make the discoloration from the patina even worse. Surely someone somewhere has done a quantitative or spectroscopic analysis of exactly what this thin layer of surface colors is. Until I can find that, I think I'll leave the gun just as it is. I may look for some junk gun parts that still have good colors and lightly rust them, and then try to remove the staining by the electrolytic rust removal method to see if that might leave the case colors intact. I'm still interested in any other suggestions and thank all who responded so far.

If you are going to disassemble the gun, try ultrasonic cleaners.

As I recall, Dr. Gaddy found the "stuff" in the layers to be very complex and not necessarily uniform. The layer thickness is important to the color, not the composition. Differing compositions can/do create equal thickness layers which cause the same color. Given colors are not associated with given compositions, to my knowledge.

I agree that strong basic (high Ph) would likely attack CCH.

Interesting discussion!

DDA

keith, if you hadn't had a chance to read it, you may want to take a peek at: doublegunshop home page > scroll down to 'constants' > best reading material ever > index entries > vol. 7 iss. 4

Ain't that a laugh, can't figure out how to do the link thing. In a nutshell, seems you're dealing with iron oxide. I wouldn't try anything too abrasive or that might chemically react with iron oxide. If there's 'patina' present, you may not be able to recover the colors for various reasons.

Sorry about that if you are already familiar with this article.

Where might I find Dr. Gaddy's studies on case hardening? I've seen articles in Double Gun Journal on his techniques, but haven't seen any in depth analysis of case color chemistry.

On a side note, I do agree that these case colors can be somewhat fragile and may wear off rather quickly if not protected. But not always. Some case color jobs seem to hold up much better than average. For example, the case colors on my Thompson Center Hawken flintlock rifle's lock are still very vibrant and show almost no wear after over 30 years of use. This is the only gun I use for deer hunting, and it has been carried many many miles in all kinds of weather. Quite often, it is carried cradled under my arm or coat sleeve to keep rain or wet snow from getting into the pan. All of this rubbing from the sleeve on my coat plus exposure to the elements and black powder residue has barely put any noticeable wear on the colors.

Ken Hurst uses toilet bowl cleaner on a case colored receiver to create a "French" finish. It removes all the color and does not touch the case hardening.


Pete

I have noted that certain low end guns seem to retain their colors better than highly finished (well polished) surface of higher grade guns. Case in point I have a Hunter Arms Fulton that retains almost all of its color after 70 years while most of my A H Fox guns with about the same or less level of use are starting to fade or wear off. It seems to me that perhaps the surface preparation of the item being color case hardened would have a bearing on the durability of the case colors with courser surfaces retaining colors better. This is a very limited observation but I have seen many Savage 311's and Fox Model Bs that exhibit the same resistance to color wear. Both the Savage 311s and the Fox Model Bs have the same cyanide colors as the A H Fox guns. Any one have any thoughts on this?

That part one of Doc Gaddy's article seems to go into the chemistry pretty significantly, from about half way down to the end of the piece.

I believe iron oxide, colors, formed during the normal tempering of steel is what he implies is the known baseline. His article seems to detail how the traditional cch process arranges the iron oxide in a unique way.

It could be that you are wearing against some protective coating on your muzzle loader and not the colors. I've heard of many different coatings recommended for case colors. Some of them aren't too durable for their normal use, so I tend to wonder why they would suddenly protect metal colors very effectively.

The fact that different compositions can cause the same color is because the colors are optical interference colors and not due to the intrinsic color of the surface layer.

If you look at (not through) your camera or binocular lens you will see a blue or reddish color - that is NOT a pigment, but an optical interference color produced in the same way colors are on case-hardened guns.

The visible colors are the net effect of refractive index of the layer (causing different retardation at different wavelengths), the reflectivity of the substrate and the light once again bouncing outward thru the layers.

It's an unusual concept - case colors are not at all like the pigment colors of paint - they are the result of the physical properties of light and interference.

I think that's what Rocketman was saying. Supposedly, it's not the color of the 'stuff', it's the thickness of the layer that make the colors appear to be different.

Maybe though, there is a 'pigment' effect. The Gaddy article specifically points out that case colors do not change appreciably when viewed at different angles, which happens with your optical interference example.

Heat affected steel, like temper colors, seems to show changeable colors due to minute differences in the thickness of the iron oxide, but is otherwise a 'flat' layer. Case colors seem to have a component of optical interference, but those opaque 'globs' of iron oxide described in the article seem to be of big enough size to be a pigment and have some contribution to the final color.

In the end though, it seems to be just one ingredient, iron oxide, with very little other trace components. Interesting stuff. Not trying to be contrary, seems that true case colors are more complicated than eek torch colors.

"...Maybe though, there is a 'pigment' effect. The Gaddy article specifically points out that case colors do not change appreciably when viewed at different angles, which happens with your optical interference example...."

If the layer is uniform the colors do not change at different angles - you can try that on a binocular lens - usually those coatings, if bluish, are 1/4-wave

An oil drop on a puddle will demonstrate different colors because it isn't uniform thickness. The oil has the same refractive index over the entire spread-out drop but its thickness varies and the interference fringes consequently have different colors.

Certainly if there's a big "glob" of oxide on the surface that will act as a real pigment as far as color goes.

TwiceBarrel, I noticed the same thing about the colors on my Hunter Arms Fulton, and many other Fulton's I've seen. I don't know if it's the steel or the process they used, but their colors seem to hold up much better than Hunter Arms L. C. Smith guns in the same overall condition. They do look a bit like cyanide colors so I wonder if they used a bone charcoal pack method with cyanide added. I've read that this works but is seldom used because of the risk.

Craigd, my T/C flintlock never had any protective coating on the lock. They do use investment cast parts and the level of polish is not super high. Whatever the reason, the colors are attractive and exceptionally durable.

Keith,
I believe my Fulton is cyanide casehardened.

Chuck, I would have to agree that those are cyanide case colors. My Fulton has colors which look like more traditional bone pack colors, but they do have some of that distinctive "overlap" of color pattern which seems to be a characteristic of cyanide colors. That's why I thought they may have used cyanide added to the bone pack. As I said, I have read some reference to that, but I don't even know if it's really done. It would seem kind of odd that Hunter Arms would use multiple processes on a lower grade gun unless it was a change they went to to reduce costs.