What does copperas mean?
Copperas is the traditional name for ferrous sulfate, or iron(II) sulfate — a green crystalline salt once among the most important chemicals of pre-industrial Europe. Known also as green vitriol, it was prized for its remarkable versatility: combined with tannin from oak galls it produced the dark, durable iron-gall ink used for manuscripts over many centuries; it served as a mordant that fixed dyes onto wool and cloth; it was used in tanning leather, in making sulfuric acid, and even as a soil treatment. Despite its name, which derives from an early association with copper ores, copperas contains no copper at all. Today the word survives chiefly in historical, industrial-archaeological, and craft contexts, since chemistry has replaced it with the more precise name ferrous sulfate. For readers interested in the history of science, industry, or writing, copperas offers a vivid reminder of how a single unassuming mineral could underpin technologies central to literacy, textiles, and trade.
nounCopperas is ferrous sulfate (iron(II) sulfate), a green crystalline compound formerly used in making ink, dyeing cloth, tanning leather, and as a mordant; it is also known as green vitriol.
- Ferrous sulfate (iron(II) sulfate, FeSO₄·7H₂O), a green crystalline salt historically used in ink-making, dyeing, tanning, and horticulture; also called green vitriol.
"The dyer added copperas to the vat, and the wool slowly took on a deep, somber black."
"The scribe's ink was made from oak galls steeped with copperas and gum arabic."
"By the eighteenth century, copperas works lined the coast, extracting the green crystals from pyrite-rich shale for use in dyeing and glassmaking."
Copperas is normally treated as an uncountable mass noun referring to the substance; a plural form is grammatically possible but rare and generally unnecessary.
"Early accounts distinguish several copperases drawn from different sources of pyritic shale."
Copperas fooled chemists for centuries — its very name means 'copper-like,' yet this humble green crystal is pure iron sulfate, and it helped write history quite literally by blackening the ink of countless medieval manuscripts.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of copperas
Copperas entered English through Old French, ultimately from medieval Latin forms such as 'cuprosa,' meaning 'copper-like,' derived from 'cuprum,' the Latin word for copper. The name reflects an early belief that the green crystals came from or resembled copper compounds, although the substance is actually an iron sulfate. Its cognates include the chemical terms 'cupric,' 'cuprous,' and 'cupreous,' all built on the same Latin root for copper. The word has been in English use since at least late medieval times, when copperas was already a staple commodity of dyeing and ink-making.
How copperas is actually used
A historical and technical term rather than part of everyday vocabulary; today it appears mainly in discussions of traditional crafts, industrial archaeology, and historical chemistry. The name is now largely superseded by 'ferrous sulfate' or 'iron(II) sulfate' in modern usage.
Easily confused with copperas
Despite the name, copperas contains no copper — it is iron(II) sulfate, the name deriving from an early belief about its origin rather than its composition.
Copra is dried coconut kernel used for oil extraction, while copperas is a chemical salt of iron; the two share only a superficial resemblance in spelling.