There are some Japanese knives that utilise copper as part of a decorative layering to create some stunning patterns like those in the Hatsukokoro Yorokobi, Nigara Hamono and Takeshi Saji knives and a common (and entirely reasonable) question we get is:
Does the copper oxidise and turn green?
Most of the time with copper, it does react with acids and can develop a distinctive green colour but does't typically happen with the copper used in Japanese chef knives.
A Hatsukokoro Yorokobi knife with copper layering - View all "rainbow" knives
We asked our friends at Hatsukokoro about this and the simple answer is that the lack of reactivity comes down to the very pure level of copper used in the knives so while it's not 100% non-reactive (that wouldn't be possible) it's as close as the blacksmiths are able to get and so in much the same way that a stainless steel knife is "stain-less" not stain proof, the copper used in Japanese knives will remain the striking colour in almost all cases.