Of the various metal finishes used on shotgun receivers, case colors are my favorite. I love case-colored receivers for the same reason I love figured wood stocks: like snowflakes, no two are exactly the same. Given a choice between an engraved receiver and one that’s a plain case color, I’d take case color every time, unless the engraving is exceptionally good.
There are two methods of imparting the swirled blues and browns of case color to steel. The gun at the bottom of this picture is a nice Cabela’s Dickinson Plantation Grade 28 that I’ve been shooting lately, and it has been bone charcoal case-color hardened, which is the traditional method and a medieval sort of process. To do it, you pack the metal parts in a box along with charcoal, leather, and bits of bone, then heat up it all and quench it. The steel absorbs carbon, hardening the surface but leaving the steel underneath softer, so it can flex.
I have seen this done at Connecticut Shotgun Manufacturing, but when I did, the quenching was more spectacular than what you see in this video: One man with tongs carried the glowing box and another took a mallet and knocked the top off it, dumping the contents into a vat of water. There was a big blast of steam when the parts dropped in—and the result was beautiful.
The other method of case coloring is bathing the metal parts in cyanide. This type of case coloring is common among Italian guns, such as the Caesar Guerini Woodlander at the top of the picture. To do it, you dunk the parts in a bath of boiling cyanide, but though the process produces similar brown and blue colors, it may not achieve the same quality of hardness that you get with the bone-case method. And, although the Woodlander is a good-looking gun, cyanide case color doesn’t look as nice as the more rare, bone charcoal color-case hardening, which results in richer, softer-edged colors, and a more organic look than the sometimes-harsh cyanide case colors.
Pretty as it is, case color does wear away, though. In my experience, the most durable finish for a shotgun receiver is coin steel, or its modern equivalent, nitrided steel. But I exclude stainless, which is probably the very best finish, but, aesthetically, I don’t think it belongs on a good shotgun, and I don’t think any silver receiver looks as nice as one that has been bone charcoal color-case hardened.