[67] This refers to an iron sulfate commonly used in premodern times for blackening leathers and as a main ingredient in black inks (iron-gall inks). It is a common name in the vernacular for green vitriol. On historic name and uses, see e.g., the entry ‘vitriol’ on the Iron Gall Ink Website by Birgit Reissland and Frank Ligterink, hosted by the Cultural Heritage Agency of the Netherlands, https://irongallink.org/igi_indexee73.html, last accessed 3 December 2020; see also Jo Kirby et al., eds., Trade in Artists’ Materials: Markets and Commerce in Europe to 1700 (London: Archetype, 2010), ‘glossary’, 458. In purified chemical form known as ferrous sulfate, i.e., Iron (II) sulfate heptahydrate FeSO4·7H2O.