Heat treated?

[caseback]

During disassembly of a landeron 48, I noticed a discolourasation of the main plate. It goes from black to blue to an orange/brown and then yellow. I know it might sound a bit silly, but it almost looks like this has been heat treated..? 

[Mercurial]

This is not heat treatment, it’s oxidation or tarnishing. 

If you look at some older nickel plated movements you will sometimes see darkened areas which if you play with the light show a rainbow of colours.

This is a similar phenomenon and it has happened at what looks like the edge of an area where some plating has worn off the brass plate. 

Best Regards,

Mark