Charcoal Case Colour Hardening
About 25 years ago I built my first BPCR rifle, a 40-70 SBN rolling block. I have always had a fasination with casehardening so I started on the adventure of figuring out how to produce those beautiful colors. I am finally getting the kinds of colours that I enjoy and I seem to casehardening any actions I can get my hands on.
Here is a little video of the quenching process. Here is second video but taken at night so you can see the sparks and the red hot box before dunking.
Antique Rifle Restorations by the Gun Doctor 