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.