copperas

Meanings

1. 1
   noun

   historical chemistry: iron(II) sulfate, a green crystalline compound used in dyes, inks, tanning, and other processes

   The workshop combined copperas with tannin to produce black ink.

From Middle English coperas, through Old French coperose from Medieval Latin cuperosa, a name for metallic sulfates. Despite its modern spelling, copperas usually meant green vitriol, or iron(II) sulfate, rather than a copper compound.

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