I use to work with copper that was 99% pure. We worked with copper plates and I noticed a few things. If copper gets wet it will start to show corrosion in as little as twenty minutes. Im not talking under a micrescope but right in front of your eyes.
Second if the plates were stacked on top of each other they would stay shiny for several months, even over a year, but if left in the open they would start to tarnish and dull in just a couple of weeks.
I dont know what any of this means but I thought I would pass along the observation.