Ok so I know there is case hardened and a color case hardened so that's not my question.
Howdy
Actually, there is only Case Hardening. Sometimes it is called color case hardening, but there is really just Case Hardening. Before modern heat treating and heat treatable alloys became available, it was common to treat iron and steel to make the surface harder.
Case Hardening is only done to iron and steel. Steel is really nothing more than iron with a very small amount of carbon added to the alloy. Yes, there are other elements added too, but mostly it is a tiny amount of carbon that turns iron into steel. Too much carbon and the steel becomes brittle and breaks easily. But Case Hardening creates a thin layer of harder metal at the surface of the metal. The standard method was to take the finished iron or steel parts and heat them in a sealed crucible in the presence of carbon bearing substances like bone and leather. As the metal was heated, carbon from the leather or bone was infused into the outer layer of the metal, creating a thin layer with extra carbon in it. Typically Case Hardening is only a few thousandths thick. That's where the name comes from, it is only the outer case of the metal that gets hardened. The interior of the part remains more ductile and able to take shocks. If the entire part had been hardened all the way through in this matter, it would be too brittle and would break when subjected to shock.
The colors produced by the Case Hardening process have nothing whatsoever to do with the strength of the part. They are simply a by product of the process. However, during the 19th Century the gun buying public became so enamored of the colors that each company strove to achieve the most dramatic colors they could. Each company jealously guarded their secret processes for Case Hardening.
Modern heat treatable steels make Case Hardening all but obsolete. Ruger heat treats their frames so they are hard all the way through, not just hard on the surface. The 'color case' finish Ruger used to apply to the Vaquero was a chemically induced color. Uberti dips their frames in a hot chemical bath to create a 'case color' effect on their parts, but it is not true Case Hardening.
Kasenite is a modern surface treatment product that can be used to form a type of Case Hardening. The powder is spread over the parts, then they are heated. This does create a true Case Hardening, but it is not like the traditional method.
Brass cannot be Case Hardened, since adding a bit of carbon to the surface would not do anything. You can make it look blotchy if you want, but there is no such thing as Case Hardening brass.
P.S. A modern form of Case Hardening is still used on black oxide screws. It creates a harder surface on the screw, while retaining ductility inside.
P.P.S. If you want to create an aged look on brass, find a friendly Black Powder shooter and ask for some of the water he soaks his spent shells in. Just rub some on and let it sit. You will get natural aging. Of course, some modern brass framed guns come with lacquer on the brass to preserve it. If you want to distress the finish you have to first remove the lacquer.