[Jeff Higgins]

Originally Posted by id10t

The mottled effect that looks like an oil spill on water is actually color case hardening, not rust blueing. Involves heating and oil quenching.

For real differences, see https://www.africahunting.com/threads/color-case-hardening-vs-blueing.39031/


Personally, while I love nice wood and deep blue, I'm a "blank paint over parkerized metal" kinda guy

That link has a good overview of the differences.

The traditional method of "color case hardening" is known as "bone pack" hardening. Steels come in a broad variety of carbon content, with the "high carbon" steels being capable of what we call "through hardening" - heat treating results in the same, uniform hardness throughout the piece. "Low carbon" steels need to have some form of carburizing agent introduced to make them harden. In this case, that agent is bone meal. These pieces are literally packed in bone meal, then put into the heat treating oven. The carbon from the bone meal allows the piece to surface harden.

What we wind up with is a piece that is very hard on the outside, yet still malleable on the inside. Think hard boiled egg, but in steel. The vibrant colors are a result of the uneven nature of the carbon reaction on the surface of the steel. Every one will be different.