In their defense, the early china clays were a much more stable compound (and clay-like) when new, but most have not aged gracefully. Dunno if they just dry out over time, or if some element breaks down when exposed to light / air / heat, but there is a clear degradation process ocurring in many instances.
Pretty much applies to all spirit molds, PGI'S greek keys, Pharaoh's, Desert Palms, and the early Dunes commemoratives.
Only time will tell if the 'new wave' of china clays will suffer a similar fate. CPS, new Dunes, Majesrics, Royals, and Bank molds all have a more plastic feel (presumably due to less silicates and carbonates in the material mix), so perhaps they will age more gracefully. Check back in 2025 to 2030 or so.