Two quick opinions:
1. Until 1974 baseballs were covered with horsehide, not the presently used cowhide. The horsehide was more accommodating to signatures. Usually when the ball toned or darkened, it did so evenly.
2. Game used balls that have been rubbed with umpires’s mud are also more likely to age well. This makes the leather less absorbent and so the ink of signatures is less likely to lighten or disappear entirely.
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