Verdigris; the greenish or bluish coating that forms on copper when it oxidizes.
From Latin erugo, possibly related to 'aes' (copper) and 'aerugo' (rust of copper). This technical term was used in alchemy and metallurgy.
Erugo was super important to medieval alchemists who studied copper oxidation—they thought verdigris might help turn lead into gold, so they watched it very carefully!
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