An acid exposure treatment involving the breakdown of raw copper, coating the blade or other steel part in a thin layer of matte copper. After the coating, the blade is then tumbled to achieve a stonewash appearance.
This finish is not permanent and will fade/patina over time. You'll eventually end up with an acid stonewash with copper fragments throughout after some use and abuse. Results may vary based on steel type.
Sharpening service required unless you wish to resharpen your own blade.
Is not applicable to D2 and some higher carbon steel types.