These receipts printed on thermal paper darkened when they were exposed to vapors from ProLink Chain Lube. The receipts are from a few days ago, and after lubricating my chain yesterday I tossed the paper towels wetted with ProLink into the same trash can which held these receipts. Today when taking out the trash I noticed that these receipts had darkened markedly, in interesting patterns. It surprises me that no region containing text was darkened.