A closer look at the function and fashion of various heat treatments and color case hardening methods.
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, economics, and they can pull from any of those topics at will. The topic in question, heat treating metal, requires one to don the hat of chemist and engineer; the study of metallurgy.
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. It is easier to shape softer metals. Specifically, 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.
Over the last several hundred years smiths have been improving their skills, in no small part with the help of modern technology, to become better at working with metal. More consistent and complex alloys can be created now than ever before. In the past, gunsmiths had to make do with techniques and materials at their disposal to create firearm parts of varying hardness.
Heat treating metal, in its most basic terms, is 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; things like hardness, toughness, spring-like character, or brittle nature.
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 heading the metal up to a specific temperature can change that brittle steel into something more spring-like. If you hear a gunsmith talking about heating up a piece of metal to a straw color, or flame blue, that is tempering. Screw heads can be flame-blued to produce a fine electric blue/purple color that is very appealing on certain guns.
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 are generally referred to as case colors. If you look at guns 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, and in the gun world this is referred to as case hardening or color case hardening. For firearms parts, this extra carbon usually 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 carbon-rich charcoal. The additional carbon enters the crystalline structure of the surface of the metal, and when cooled, or quenched, hardens. This leaves the surface of the piece hard, 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 durable and less brittle interior. It also happens that a side effect of this process is the production of those oh-so coveted case colors of which we are so fond.
Bone charcoal case hardening takes a good deal of time, with parts needing to be carefully packed into a ‘case’ of charcoal and heated carefully up to the neighborhood of 1300 to 1400 degrees Fahrenheit. The resulting shell of hardened metal can be as much as 0.06 of an inch deep, though care must be taken. As the 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 gun manufacturing can take advantage of high-quality alloy steels. Production with modern machinery 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 bath of water or oil. This process gives the metal the case colors that so many folks love. Contrary to what some 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 as the bone/charcoal method.
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 (applied to all case colored Upland Gun Company guns) provide yet another more durable finish to help case colors last. Cerakote is a ceramic polymer coating that is applied via 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 Cerakote 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. 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 case color. It’s not bone charcoal-colored, or chemical case hardening, and is not even hardening at all. But then again, for my single shot squirrel gun, it seemed a fitting choice to make the metal look new again.