trag said:

I don't think CRTs lose brightness if they're just sitting around, do they?   Or do little gas molecules sneak into the tube and oxidize the phospors or some such?  Contaminate the cathode?

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They dont lose brightness "persay" But... the older the CRT is/longer it sits dormant, there ARE impurities in the CRT and it will oxidize the cathode material in the electron gun.

This will cut-off/weaken emissions from the gun until the oxidized material gets burned off. 

This is called "waking up" the tube after a long slumber. (many decades). 

usually you have to run up the filament voltage to around 8V for a few minutes to a half hour to get emissions back up into the green again.