Hi everyone, I was running what was essentially a proof of concept experiment to investigate the effect of polyethylene additives on the oxidation of copper. I've submersed several copper sheet squares in distilled water at 65C for several days, one bottle of which contained plastic additives from a microwave assisted extraction. 

I was planning on running this for a few weeks, and this was my first aeration period. However, when I removed the copper foil, the control experiment (image 1) had this darkened colour on its surface, whereas the additive experiment (image 2) looked essentially brand new. 

Now, my first thought was obviously that I had mislabelled the bottles. However, the additive jar was cloudy, and remains visibly so on inspection. The control jar is completely colourless, the only difference being the presence of air bubbles in the solution. When I pulled out the copper sheets from the control experiment, they were stuck together. Is it possible that being stuck together caused this dark colouration? It does appear on both sides, but mainly on one face. I hadn't noticed it until I'd pulled them apart and now cannot be sure if the dark colour only appeared on the stuck face, but this seems like a reasonable assumption. 

Could copper sheets sticking together at 65C in distilled water explain this discolouration? If so, what could it be? The metal acting as a thermal conductor and speeding up oxidation? Some kind of transfer? Any help would be appreciated!

Copper Oxidation Experiment Question