Color case hardening steel

A. Supposedly a pretty good color case can be applied by first charring old leather and then grinding it to a fine powder. Take the part to be hardened and pack it in a steel container with a close fitting lid with a mixture of bone meal and the powdered charred leather. Put the lid on and heat to a red heat for fifteen or twenty minutes. Then immediately dump into a container of cold water with air bubbling through it (a small tube with holes in the bottom of the water container hooked to an air line). This is supposedly the method used by the earlier government arsenals, and produced a fair color finish and hardness. I haven't tried this yet but plan to soon. If you attempt it, practice first on some scrap steel as the time of heating and amount of bonemeal and leather used vary the result. If your need more info I will see if I can find the article and scan it for you.

Good luck!

"Old charred leather and bone meal". I could see how the ancients discovered this process! Thanks, sounds like you would get an authentic looking patina with this one.

I don't know how you would do this at home, but it sounds like it should best be done in an outside wood campfire. I also don't know about tightly packing anything and then heating to red heat, then plunging into cold water. I don't think OSHA was around when the alchemists were taking out entire city blocks.

A. Hi James. To me it sounds like the contents of the container rather than the sealed container. Sounds like the remnants of charred leather and bone meal go into the water with the color hardened item only because it's the easy way.

Regards,

A. Case Hardening

The following is a composite of two excellent posts on case hardening by subscribers Bruce Conner and Ward French.

It can be found at: http://members.aol.com/illinewek/faqs/case.htm

For readers wishing to pursue colour case hardening further, I strongly recommend a series of two articles by Mr. Oscar Gaddy on the subject, found in the winter 1996 and spring 1997 issues of the Double Gun Journal.

Bruce begins:

Case hardening involves putting carbon (or a combination of carbon and nitrogen) into the surface of the steel to make it a high-carbon steel which can be hardened by heat treatment, just as if it were tool steel or any other high carbon steel. Only the outer skin gets hard this way, the center is still tough and malleable. This makes for a strong part with a tough surface.

Ward continues:

Low carbon steel, i.e. steel with about 20 points or less of carbon, cannot be made to harden by heating and quenching, as higher carbon steels can. Low carbon steels are tough, soft and flexible. They wear quickly and batter easily.

Many parts, including gun actions in days gone by, were made with low carbon steel. It was cheap, strong and easy to machine. Unfortunately it would not stand up to the battering of use in the field. Case hardening added carbon to the surface skin of the steel part and left it in a state which could be hardened by quenching.

To case harden a part (the process is also known as pack hardening) the finished low carbon steel part is placed in a sealed container, packed with a high carbon compound. In the old days this was simply animal hide or bone. The container filled with parts and carbon bearing material was brought to a red heat and held at that temperature for a time determined by the size of the part. The time might be from a half hour up to several hours. As the bone or hide became carbon in the container, and a carbon rich gas formed, some of the carbon would infuse into the surface of the steel. Over time this would penetrate several thousandths of an inch, producing a high carbon surface on the low carbon steel part.

At the proper time the container is removed from the furnace and the contents dumped into a quenching bath, usually water with perhaps a surface coat of oil to lessen the shock of the quench. The high carbon surface skin becomes glass hard, but the low carbon body of the piece remains soft and very ductile and able to resist shock. Properly done it made a simple and very durable system for treating metal action parts.

Colors are produced when the steel surface is cooled unevenly, capturing the natural blues, oranges and yellows of cooling steel. Several methods are employed to do this. Stevens moved the parts into the quench in a jerky fashion, producing a barred effect of color. Perazzi did the same. In the London trade the quench bath, usually a barrel with soft water and a skim of oil, was agitated by stirring, or with bubbles of air, producing a mottled effect on the steel.

Bruce adds:

Color case hardening is done much the same way except that generally only leather and bone are used as the carbon source. I don't know why this works better than charcoal, but it does. You get more brilliant colors with them. The other thing you do is modify the quenching bath. You need a source of bubbles. LOTS of bubbles to really rile up the quench bath. Adding a bit of potassium nitrate to the water increases the brilliance of the colors as well, but isn't a requirement. You have to watch the temperature more closely with color case hardening or the colors won't come out well. Don't go over 1350 °F.

Kasenit and similar compounds are a lot easier to use and you can just use a torch. You heat the part up red, dunk it into the Kasenit compound and get a good coating of it sticking to the steel in the places you want hardened. Then reheat it up to a good red and quench it in water. This can be repeated to increase the depth of the case hardening. It works very well and is quite fast, but leaves a kind of dull grey color to the surface. For parts that are internal it works great and if you make the hardening deep enough, you can polish the metal and still have a hard surface.

Ward continues:

The colors have nothing to do with the effectiveness of the case hardening. Many, if not most, parts are hardened without colors. The surface takes on a dull gray look. The London makers usually polish this surface bright. It is glass hard, but without the decorative affect of the colors.

Anyone who has a case colored part should be aware that colors will fade on exposure to direct sunlight over an extended time period. Parts must be protected. Clear fingernail polish or a similar lacquer will protect the surface and a gun case or cabinet will do the rest. Case hardening was widely used on all lock parts except springs, and the process could be carried out even on the frontier with a minimum of equipment and knowledge.

Good shooting,

Ward and Bruce

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Ed. note: Thanks so much, Walter. The link to the original post no longer works; if anyone has a new URL, please advise. If not, we're grateful to Walter for consolidating the info while it was accessible.

A. The leather and bone composition is correct. I have tried this myself to case harden frizzens for flintlock pistols and rifles. It produced a hardness sufficient to resist a file. I tried several lengths of exposure to heat and got results from just the surface hardened to hardening to a depth where the part shattered when dropped like it was glass. There are many old world recipes using various components (sea salt, chamber lye (urine), calcium, etc.) but all of them included leather or bone or a combination of both (animal carbons). Hardening was usually "in the white" leaving a grayish or white metallic, frosty, look. I found an old English recipe "MOXON's Method" that I use to this day. It uses a tin with lid in which you fill with powdered bone, leather dust, charcoal powder and a small amount of calcium carbonate. The ratios can be adjusted for different effects but typically 3 of bone, two of leather, 1 of charcoal and one half carbonate. You place the item to be hardened into the mixture, add a little vinegar ⇦in bulk on eBay or Amazon [affil links] (it will effervesce). Let finish and stand so the mixture completely covers the part when done bubbling. Drill a small hole in the lid so you can observe the color of the mass. Cover & Heat until mass is blood red, hold for half an hour after reaching temp. Quickly remove lid and quench part in cool water with tongs, keeping it moving. For color, try an oil and water mix (make sure it is well agitated so the oil is in thorough suspension) instead of just cool water. This causes uneven cooling and renders different surface appearances. Remember, this can be dangerous and should be done outside where spills, splashes cannot damage or harm. Always wear a full face shield ⇦this on eBay or Amazon [affil links] , insulated gloves and apron ⇦this on eBay or Amazon [affil links] . Have a charged hose ready to wet you down if hot fluid splashes on you. Have fun, experiment but survive to enjoy it!

A. Dear Jon,

"The Machinist's Second Bedside Reader".
and "The Machinist's Third Bedside Reader".
both by Guy Lautard both have information on color case hardening processes. The information is not all-encompassing, however it will get you started.

A. In the winter 1996 and spring 1997 issues of the Double Gun Journal, color case hardening is explained in detail, these issues are available from the publisher at P.O. Box 550, East Jordan, Michigan 49727.

A. I use plain ole "bone meal" from the garden section. It imparts the carbon to the metal but I have not tried the Color Case Hardening. The hole in the tin will let you know when the bone meal has been burnt up as the steam will stop coming out. The meal will be black when it is opened.
Good luck.....

A. Hello:

I am not a gunsmith but in my Machinery's Handbook from 1942 a process for the type of coloring you are seeking is described. It involves boiling the piece in cyanide of potassium (dark cherry red) then dipping in clear water and moving vigorously. If not moved about in the water The mottled effect will not be obtained. I believe this may be the old process used by gunmakers of the past.

RFQ: I am trying to find a company that does this as I want to restore some old gun parts and do not want to try this at home.

Good luck.

A. I'm at the Lassen Community college for gunsmithing and we recently redid our receivers. Mine came out beautiful, amazingly well compared to others. What I did was heat mine in a furnace with 50/50 charcoal/bone (From Brownells [ Brownells home page (affil link) ] ) mixture to 1333-1375° for one hour then quenched it at around 1340°. The quench contained nitre salts, pine needles, pine cones (ground up), horse poo, bird poo, bird nest, and my roommates THC loaded urine. We also had an air hose hooked up to the bottom of our quench tank so there was water circulation. Colors came out spectacular.

A. The cyanide method produces less than brilliant colors. It's what Uberti uses/used on their revolvers. Gets the job done but is "blah". Charcoal 50/50 is THE way.