Oxidation

Copper blocks can oxidize during time. And this mod adds the ability to copper items like ingots, nuggets, etc… to oxidize in your inventory. Item oxidation applies only when the item is in an inventory of an entity like the player or a zombie, and when it’s dropped.

Oxidation speed/chance can be configured via the gamerule copperOxidationChance, which was added since Copper Extension 1.2.1. More info here.

Here is the list of the oxidation chance per versions: