Strange pattern
 

Has anyone seen a pattern like this on a sword blade?
What is it, and how should I treat it?
Any thoughts are welcome.

The sword is a Sumatran Cot Jang.

This is a laminated blade that has been produced from ferric material with varying levels of carbon, it is also possible that the entire blade was quenched during heat treat, rather than just the cutting edge. It is possible that the lighter coloured material contains some nickel, it could well be iron from Sulawesi.

Personally, I would simply oil & use steel wool on it, the variation in blade colour tells a story.

I'm with Alan. Oil and enjoy.

Thanks
 

Thank you both for your comment.

Albert, I'll add one other little comment. This blade was demounted for clean & stain, when it was remounted the tang was heated to permit insertion into the jabung used in the hilt to secure it, during this process the ricasso reached a heat of about 280-300C, this is not a bad thing & is quite common when remounting a blade. The colour in the ricasso is the evidence of this.