Calculator Woes: Dark Spots and Viscous Liquid on Screens.

[jam]

Hello everyone. I've been collecting calculators for a while now, but I've encountered an issue. Over time, some of them develop a dark spot in the center of the screen, which eventually spreads across the entire display. Others have screens that become rough and hard to read, almost like they're sweating a viscous liquid. Any idea what might be causing this? And, more importantly, what recommendations do you have for storing calculators?