A green or blue mineral salt containing iron sulfate, historically used to make ink, dyes, and medicines.
From Middle English and Old French copperose, derived from copper-colored rose crystals. The name originally referred to the crystalline appearance of ferrous sulfate that resembles copper-hued rose blooms.
Medieval writers literally made their own ink by mixing copperas with iron-containing compounds—this is why old manuscripts sometimes turn brown or fade over centuries, as the chemistry continues to change. Your grandmother's antique letters are basically still chemically transforming!
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