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

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