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The Processes of Color Case Hardening and Heat Treating – #022 of the Gun Room Podcast

The Processes of Color Case Hardening and Heat Treating – #022 of the Gun Room Podcast

An AYA gunsmith works on a shotgun.

Though a simple process in today’s manufacturing world, the history of heat treating metal and creating dazzling case colors is as rich as the results they give

I have always been fascinated by the depth of knowledge required to be a proficient gunsmith. Skilled individuals possess a working knowledge of mechanics, engineering, chemistry, and economics (if they are still employed), and can pull from any of those topics at will.

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Today’s topic in question, heat treating metal, requires one to don the hat of a chemist and engineer with the study of metallurgy.

Not all metals are created equal

Gunsmiths have to shape parts from metal, and not all metals are created equal, nor will one type serve appropriately in every situation. There are parts in guns that need to be very strong like firing pins while others like springs need to flex. Still, others require exceptional wear properties like hinge pins, where there is metal-to-metal contact. These properties are generally referred to as hardness, strength, and stiffness.

Hardness deals with surface durability. Strength with the amount of stress that can be applied before a part fails, and stiffness is the ability of a part to return to its original form. The properties of a specific gun part will dictate which of these characteristics are required. 

Over the last several hundred years, man has been improving the field of metalworking, in no small part with the help of modern technology, machinery, and production methods. More consistent and complex alloys can be created now than ever before. And modern machining allows parts to be made in metals that already possess the necessary properties that a final part may require. In the past, this was not so. Gunsmiths had to make do with techniques and materials at their disposal to create firearm parts, and, thus, needed to be good at changing the properties of the metals they utilized.

It may be overly simplistic to say that it is easier to shape soft metal. What I mean is, it is easier to shape metal when it is soft, instead of hard. But hard surfaces may be required for durability. Metals can have properties that make them so hard that even a high-quality file will seem to slide across without removing any material, which would make them very difficult to shape indeed. 

What is heat treating?

Enter the process of heat treating metal. In its most basic terms: a process of heating and cooling metal to alter its properties.

Temperature, time, and the presence of other materials like carbon, will dictate the final properties of the metal; the very things like hardness, toughness, spring-like character, or brittle nature required by varying gun parts.

One type of heat treating is “tempering” and is crucial to the production of springs and other gun parts. Untempered steel can be very brittle, and the process of heating the metal up to a specific temperature can change that brittle steel into something more spring-like. The duration of time the metal is heated, the temperature to which it is heated, and the speed with which the part is cooled determine its final properties. If you hear a gunsmith talking about heating up a piece of metal to a straw color, or flame blue, that is tempering. This process can also impart color to polished metal. This is most often used for small parts, like screw heads which can be flame-blued to produce a fine electric blue to purple color that is very appealing on certain guns. 

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What is case hardening?

Another heat treatment process that we hear a great deal about in the gun world is that of “case hardening.”

The case hardening process was used as a finishing technique on many firearms over the years and produces what is generally referred to as case colors. If you look at gun’s for sale ads, I am sure you have come across one that refers to the percentage of case color left on an action. 

Carburizing is the process of heating up a metal part in the presence of extra carbon. In the gun world, this is referred to as case hardening or color case hardening. For firearms parts, extra carbon typically comes from a combination of charcoal and bone, and you will often hear of bone charcoal case hardening. Without diving too deep, the metal is heated up enough that it begins to absorb carbon from the surrounding carbon-rich charcoal. The additional carbon enters the crystalline structure of the surface of the metal, and when cooled, or quenched, the new structure possesses more carbon making it harder. This change only occurs on the surface, while the bulk of the interior characteristics of the metal do not change. Essentially, we can have a piece of metal where the properties are not the same throughout (i.e. a tough surface finish over a softer yet less brittle interior). It also happens that a side effect of this process is the production of those oh-so-coveted case colors we are so fond of.

Bone charcoal case hardening takes a good deal of time, with parts needing to be carefully packed into a bath of charcoal and heated carefully up to the neighborhood of 1,300- to 1,400-degrees Fahrenheit. The resulting shell of hardened metal can be as much as 0.060-inch deep. The main drawback is that parts are heated to such a high temperature for so long, they are prone to distortion, warping, or cracking. Most actions, for example, are fixtured to metal blocks during the process so fragile top and bottom tangs remain true to their original shape. 

I mentioned above that improved metal alloys would play a role in this discussion, as would modern machining techniques. The steady march of progress has given us other options to produce the hardened metals we need for guns, without packing them in burned bones.

Modern firearm manufacturing materials and chemical case hardening

Modern gun manufacturing can take advantage of high-quality alloy steel. Production with modern equipment allows for the machining of gun parts that are made from steel that already possess the necessary qualities of hardness sufficient to meet the needs of firearms, and thus the only goal that remains is to produce the “colors” part of the process.

Chemical case coloring achieves this goal. Metals are heated up in a bath of chemicals and then quenched in an aerated tank of water or oil. This process gives the metal the case colors that so many folks love. Contrary to what some may think, this chemical process does harden the outside of the metal and typically produces a harder surface than the bone/charcoal method, though not as deeply penetrating.

Both types of case hardening will alter the metal, and produce colors, though the tones, specific colors (yellows, purples, blues, and reds), patterns, and shades from the processes do look different. Any colors on a gun in either process are very susceptible to wear. The colors are only an oxidized layer on the very surface of the piece and can accidentally be removed with chemicals (just like bluing on a barrel), or too much scrubbing with steel wool. This is why so many modern manufacturers are applying some type of clear coat over top of their case colors to make them last.

Lacquers and similar clear surface coatings can be applied after the heat-treating process to provide another physical barrier to protect colors. Taking it to the next level, the application of baked-on coatings, like Cerakote, provides yet another more durable finish to help case colors last. Cerakote is a ceramic polymer coating that is applied via a spray gun a few thousandths of an inch thick and baked to adhere. Generally, it creates a durable, abrasion, chemical, and corrosion-resistant surface. It is available in any number of colors and used throughout the gun industry, though the application of clear Cerakoe over case colors provides a durable alternative to the colors alone, ensuring they last longer. 

I personally have used a torch and some cold blue to produce colors on my first ever gun restoration project, a single-shot 20 gauge, and applied a clear spray on the finished action to try to get them to last. You can learn to do just about anything these days from a YouTube video or an online forum, although I do believe that Brownells Gunsmithing Kinks book had a section about the same process.

Anyway, these types of chemical colors are just that: simply chemicals applied in the presence of some heat to give the metal something that has the appearance of a case color. It’s not bone charcoal-colored, or chemical case hardening, and is not even hardening at all. Much like Steven Rinella’s comments about hunting big deer inside fences, the only reason that these “fake colors” have any value at all, is that the real McCoy classic bone charcoal colors are so rare and beautiful, and also so ephemeral. Case color disappears with use, is hard to replicate, and is a good indicator if a gun is original or restored.

I suppose that we are all looking for our own big buck of sorts and sometimes we all succumb to shortcuts from time to time. But then again, for my single-shot squirrel gun, it seemed a fitting choice to make the metal look new and pretty again.

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