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.^[1] The term was also applied to green vitriol (ferrous sulfate).^[2]

Some maintained calcanthum to be the same thing as colcothar, while others believed it was simply vitriol (sulfuric acid).^[3]

The term "vitriol rubified" (Latin: reddened vitriol) refers to the reddish iron oxide residue (colcothar) left after heating green vitriol, and should not be confused with "red vitriol," a distinct mineral (native cobalt sulfate).^{[citation needed][3]}