Chalcanthum

In alchemy, chalcanthum, also called chalcanth or calcanthum, was a term used for the compound blue vitriol (CuSO₄), and the ink made from it. The term was also applied to red vitriol (a native sulfate of cobalt), and to green vitriol (ferrous sulfate).

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Some maintained calcanthum to be the same thing as colcothar, while others believed it was simply vitriol (sulfuric acid).

References

1. This article incorporates content from the 1728 Cyclopaedia, a publication in the public domain.
2. Webster's Revised Unabridged Dictionary (1913)