The 1980 has had nearly twice the time to tarnish as the 1998 (I don't think anyone else mentioned this).
This isn't quite a scientific statement, but silver-plated objects tarnish faster and differently than 90% silver. Likewise, it stands to reason that 95% copper will tone/tarnish differently than a copper-plated cent.
Tantum religio potuit suadere malorum.
"Just because you're paranoid don't mean they're not after you."
"Language is the source of misunderstandings."
-Antoine de Saint-Exupéry
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