Editing Iron(II) sulfate (section)

===Historical uses===
Ferrous sulfate was used in the manufacture of [[ink]]s, most notably [[iron gall ink]], which was used from the [[Middle Ages]] until the end of the 18th century. Chemical tests made on the [[Lachish letters]] ({{circa|588–586&nbsp;BCE}}) showed the possible presence of iron.<ref>Torczyner, ''Lachish Letters'', pp.&nbsp;188–95</ref> It is thought that oak galls and copperas may have been used in making the ink on those letters.<ref>Hyatt, ''The Interpreter's Bible'', 1951, volume V, p. 1067</ref> It also finds use in [[wool]] [[dye]]ing as a [[mordant]]. [[Harewood (material)|Harewood]], a material used in [[marquetry]] and [[parquetry]] since the 17th century, is also made using ferrous sulfate.

Two different methods for the direct application of [[indigo dye]] were developed in England in the 18th century and remained in use well into the 19th century. One of these, known as ''china blue'', involved iron(II) sulfate. After printing an insoluble form of indigo onto the fabric, the indigo was reduced to ''leuco''-indigo in a sequence of baths of ferrous sulfate (with reoxidation to indigo in air between immersions). The china blue process could make sharp designs, but it could not produce the dark hues of other methods.

In the second half of the 1850s ferrous sulfate was used as a photographic developer for [[collodion process]] images.<ref>{{cite book |last1=Brothers |first1=Alfred |title=Photography: its history, processes |url=https://archive.org/details/b24886853 |date=1892 |publisher=Griffin |location=London |oclc=558063884 |page=[https://archive.org/details/b24886853/page/257 257]}}</ref>