For those of us tasked with managing masses of paper, the suggestion that humidity is not our friend is nothing new. But now we know that, in addition to stressing the paper fibres through their hygroscopic attraction, humidity also appears to affect the rate of yellowing of the paper, through particles called chromophores – http://www.heritagedaily.com/2014/06/vanishing-da-vinci/103502. While the study doesn’t appear to reveal how to reverse the process, it does again highlight the need to monitor the conditions in your archive carefully, and to ensure that there is an adequate movement of air through the collection.
(And no, the mittens I have to make are based on foxes, not foxing. And yes, I know what foxes say).