Color case hardening finish .vs Boiling...

[Doco overboard]

Somewhere I have some information that indicates cyanide as a possible element that was used to produce some of the vivid colors.
When packed in bone/ carbonaceous materials then quenched in a bath of water that a had a small bubbling vent that supplied air to keep the mixture rolling or bubbling.
As far as I recall , there was also maybe a few drops of oil or something in particular that was allowed to float on top that acted as an agent and helped to produce the colors.
Otherwise ,when done incorrectly a mottled gray appearance was usually the result without a bit of experimentation involved to learn the process completely.
And then finally, that it was a method that was usually guarded among the persons doing the work.
Not to mention the use of cyanide and the hazards associated with it.

Information

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[ssgross]

When packed in bone/ carbonaceous materials

The exact contents/mix/ratio of what metal was packed in before it went into the furnace would have been proprietary. Different mixes produce different types of colors.
Basic example...notice the colors aren't as vibrant as other examples.

This one is mighty pretty...but I don't think they say what's in their mix