When You’re Hot

Ever wondered how gunmakers and gunsmiths get that beautiful, mottled coloring you’ve seen on shotgun receivers and gun parts? Want to learn the ins and outs of this process? If so, there are several important things to know about what’s involved in color case hardening (CCH).

Safety First

The first equipment is quite expensive (a good kiln can easily run in the $2,000 range) and the process can be dangerous if you’re careless. The metal is heated to 1,400 degrees and must be moved around at a similar temperature, which creates a significant risk of fire and serious burns. Let me be blunt: if you don’t wear appropriate safety gear, you should stay away from this project. In this case, safety gear means a full-face shield, respirator, high-temperature gloves, an apron, long sleeves, boots and a set of tongs shaped to hold a crucible and substantial enough to bear the weight. The tongs and gloves alone can run $200 or so, but this pales compared to an ER visit for third-degree burns. And oh, keep an ABC-rated fire extinguisher handy.

Next, remember color case hardening is a hardening process, and when you harden critical gun parts they become brittle, which can have disastrous results — as in, the parts shatter and injure (or kill) you and anyone nearby. Because of this, it’s best limited to non-critical parts which do not experience strain while firing. While there are experts such as Turnbull Restoration that use proprietary techniques letting them safely color critical parts like rifle receivers, those are strictly off-limits for the rest of us.

Diving In

Now on to the basics of CCH, also called carburizing, a process which causes carbon to diffuse into the surface of steel, hardening it. This requires three things: a source of carbon; an environment in which the carbon can penetrate the surface of the steel; and a way to get the steel hot enough, for long enough, to absorb the carbon. Finely ground charcoal is the traditional source or carbon. While things such as peach pits and saddle leather are sometimes listed as sources for charcoal to create especially vivid colors, the most common types used are hardwood and bone charcoal, usually mixed in a 3:1 ratio of hardwood-to-bone.

The charcoal is packed around the parts inside a steel crucible, which must be closed, but should not be airtight. For this article, I used a crucible available from Brownells with a round body 4″ in diameter with two steel rings welded to give you a place to safely grasp it with tongs. While there are two sizes available, I chose the larger, though which size you need depends both on what size parts you want to color and how big your heat source is.

Historically, there are lots of ways to heat a crucible, but to reach and maintain the correct temperature, using a kiln is probably the best. In addition to being pricey, kilns are bulky and heavy and have higher electrical demands than a 110 household outlet can provide, so be prepared to find (or add) a source of 220 or 240 power to run one.

Repack the parts in the crucible with a 3:1 ratio of wood and bone charcoal. Make sure they don’t touch each other or the walls of the crucible, allowing ½” of charcoal on each side and 1″ at the top and bottom of the crucible. Heat the kiln to 1,350 degrees and hold it there for two hours, then reduce the temperature to 1,100 for one hour. When the hour has passed, use the tongs to remove the hot crucible from the furnace and quench by removing the lid from the crucible and dumping the contents into a cold water bath, for which you can use anything from a 5-gallon bucket to a large trash can.

For best results, the water should be both agitated and aerated — stirred up and filled with tiny bubbles. To accomplish this, I took a coil of copper tubing and pierced it with dozens of tiny holes, and then used a compression fitting to connect it with a quick-disconnect so I could use my air compressor to aerate the water. In retrospect, a larger diameter of tubing would likely have been a better choice, so next time I do this I’ll fabricate a larger aerator. Although I chose not to do it this time, it’s also smart to build a wire mesh tray to catch the parts so you don’t have to fish around in the tank to find them.

Obviously, a part at over 1,000 degrees hitting cold water is a shock, and this often causes parts to warp — so much so manufacturers who used the process often had one set of gunmakers who fit the guns “soft” and another who refit them “hard.” While nothing can eliminate the risk, fixturing parts one to another, as well as shielding them somewhat, can both reduce the risk and create more vivid coloring.

Once the part has been quenched, tempering it by heating it to 300-350 degrees for about an hour will reduce some of the brittleness caused by the CCH process, but it will likely still remain significantly more brittle than before — so again, limit the use to non-stressed parts.

For more info:
Brownells
http://www.brownells.com/
Ph: (800) 741-0015

Romanoff Int’l (gloves and tongs)
www.romanoff.com
Ph: (800) 221-7448
http://www.romanoff.com/

Special thanks to Hamilton Bowen and Pete Mazur.

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