I will throw in my 2¢.
It is a fact that phenolic balls darken over time when they are not being used. I submit that it is not that they are "light-starved" but that they acquire a micro thin layer of oxidation. It doesn't matter if the balls are stored in a box in a basement or left on a windowsill in daily direct sunlight - they will darken.
Once the balls are brought out of retirement & you begin to play with them, the oxidation is rubbed off by the friction of the cloth during play & the underlying light color is once again visible.
A few observations that back this up:
- Cue ball shift knobs darken despite being exposed to sunlight daily. Buffing will lighten them up again.
- Cue ball ferrule blanks will darken despite being exposed to fluorescent lighting 24 hrs/day. Machining them when installed removes the dark "skin". They will darken at the same rate whether they are exposed to light or not.
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