Color Case Hardening is a process that carries a lot of mystique with it. In the early days of gunsmithing, before material properties were well understood and high carbon steels created, gun makers had to have ways to increase the pressure and wear capabilities of low carbon steel. To this end, gun parts were packed tightly in crucibles of high-carbon materials such as charcoal, bone, leather, and hoof material. Heating this to a high temperature created a carbon-rich environment which would deposit carbon into the steel. Quenching the metal after a period of heating would trap carbon in the surface of the material and harden it, creating a hardened “case” around the metal to create wear resistance while leaving the core of the metal softer to prevent brittleness and breakage.
A by-product of this process is the wide range of colors produced by the process. As seen on a variety of old single action revolvers, side by side shotguns, and lever action rifles, the case hardening process can produce a stunning variety of yellow, purple, blue, red, green and other colors on the surface of the metal. The result is striking and, today, has become associated with a high level of classic craftsmanship.
As a student currently studying gunsmithing, I decided that I wanted to give a custom finish to my CZ 75 BD Police handgun. It was my first handgun and I love shooting it, but ever since I upgraded to the CZ P-01 as my daily carry handgun, I never shoot the full size CZ. After all, with a limited ammo budget, it’s easy math to shoot the gun you want to carry and be proficient with. Somehow, though, every time I picked up the full size CZ 75, I fell in love with the now nicely worn-in handgun I had been shooting for years. I brought it to school with the intent to give it a high-polish and hot blue, until a teacher of mine suggested I look into color case hardening. Intrigued by the unique and highly mysterious finish, I agreed that it was worth pursuing.
The project started with a complete detail-strip of the gun. Every pin, screw, and spring was removed. This was going to be necessary for my original plan to blue the gun, so I had already completed that step. I covered any sensitive areas of the gun (such as the highly polished slide rails) with masking tape, and used a media blaster loaded with glass beads to strip the surprisingly tough Polycoat finish the gun shipped with. Then, using a backer, I sanded the finish to 400 grit to (roughly satin or semi-gloss). 400 grit is the most widely recommended finish for guns to be color case hardened, to result in the best depth, color, and durability.
Next I got to work machining a fixture to support the frame. Color case hardening involves temperatures of over 1,400 degrees for an hour or more. This kind of temperature can warp the frame, causing parts to bind or the frame to fail entirely. To remedy this, I machined three solid pieces of steel to block off thin and sensitive areas of the frame. This would minimize the risk of warping important dimensions of the frame, and have the added bonus of holding heat for better color. I first went to the lathe to turn a plug to fit in the recoil lug recess. A thin piece of rail was cut with slots to support the internal slide rail, and then drilled and tapped for a set screw to attach to the plug and hold both in place. Finally, a solid chunk of steel was machined to go into the magazine well and drilled for a cross-pin to hold it in place.
With this all set, I began the packing process. This is one part where the process becomes 50% science and 50% alchemy and superstition. Everybody has an idea about how the packing process works. The basics of it are to pack the gun tightly in a container covered in fine, carbon-rich organic material. For me, this meant finely ground charcoal with larger chunks of charcoal and graphite to fill space in the box. Looking on the internet, it seems there is no shortage of materials that people think contribute to the color: charcoal, bone, leather, hoof, horn, fruit pits, salt, urine, barium carbonate, paper, and many other objects have been suggested to increase the coloration or durability of the process, but there seems to be no consensus or “correct” mixture that is widely agreed upon.
After packing the gun tightly in the carbon environment, I started the pre-heat on the furnace. Set at 1500 degrees, it took a little under an hour to get up to heat. This gave me plenty of time to get to the next part of the process: Setting up the quench tank. There is a lot of voodoo about this process as well, and I have the feeling that any truth in the process will only be achieved through many tries and eliminating variables to find what works. The generally accepted idea is to quench in a cold container of water with some form of agitation to create a current. We quench in a large steel tank, similar to what would be used for bluing or parkerizing. A steel basket is dropped in the tank to catch the part when dropped while allowing the coal media to fall through into the bottom of the tank. To agitate the water, our simple solution: an aquarium bubbler from Petsmart. There is some theory that a powerhead to create a current rather than introducing bubbles to the water would result in differential cooling and increase the dramatic patterns in the part, but the bubbler served our needs. The air hoses were run to the bottom of the basket and held in place with simple zip ties.
Once the furnace reached temperature, we opened it up and put the crucible in. This took a team effort, as the metal crucible when fully packed was an obscenely heavy package. Opening the furnace long enough to get the crucible in place and close the door dropped the temperature several hundred degrees, so it was set on 1,440 degrees (our target temperature) and we started our one hour timer once the temperature had reached the full mark.
An hour later, it was ready to quench. The idea in quenching is to take the crucible straight out of the furnace and dump the contents in the quench container as soon as possible. Heat, pressure, and a carbon environment are all critical to preserving the colors and hardness. The crucible, once opened, will quickly allow oxygen to flow around the metal which will disrupt the color and turn it to a dull grey. It’s typical for gunsmiths to set the crucible so that the part drops no more than 4 to 6 inches before hitting the quench. This was where I ran into some trouble. The lid of my crucible had warped due to the intense heat and bound itself shut. When myself and a helper attempted to dump it, we found the lid stuck and it took two more people to help pry it open long enough to dump the contents. This delay could have resulted in some lost color, and is something to be considered next time.
The quenching process is an interesting one. Due to the extreme heat differences and burning carbon material, it produces a plume of smoke and ash that is very exciting to experience. The video above contains some awesome footage of this just after the 2:30 mark. This is the last step and, if everything is done correctly to this point, you’ll be able to retrieve your firearm and see the finish you’ve produced. The process will leave the metal completely bare and now quenched in cool water, a recipe for heavy rust problems. It’s best to leave it in the quench just long enough to handle, rinse the part very quickly to remove carbon sediment, blow it dry, and immediately get a light coat of a gentle oil on it. It’s been experienced by some that applying oil too soon after the quench is seen to dull colors, but waiting too long can result in rust which can be difficult to remove without damaging the finish. I have been told that Doug Turnbull, leading name in firearm finishes including color case hardening, suggests his customers use Rem Oil or CLP for maintenance on color case hardened firearms. Rem Oil is what I coated mine in within a couple minutes of removing from the quench.
All in all, I consider my first attempt with color case hardening to be a successful one. The colors could be brighter and more vibrant, but this will come as a result of experimentation and confidence in the process. The finish produced is still a nice, smooth metal finish ranging from a slate gray to incandescent purple, yellow, blue, and green. The rest of the pistol still needs to be refinished – a hot blue on the slide and a nitre blue on the controls, screws and other accents – but when it is done, this CZ will be a true one-of-a-kind piece of work.