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. Hydrates
Iron(II) sulfate can be found in various states of hydration, and several of these forms exist in nature. FeSO4·H2O (mineral: szomolnokite, relatively rare)
FeSO4·4H2O (mineral: rozenite, white, relatively common, may be dehydratation product of melanterite)
FeSO4·5H2O (mineral: siderotil, relatively rare)
FeSO4·6H2O (mineral: ferrohexahydrite, relatively rare)
FeSO4·7H2O (mineral: melanterite, blue-green, relatively common)
The tetrahydrate is stabilized when the temperature of aqueous solutions reaches 56.6 °C (133.9 °F). At 64.8 °C (148.6 °F) these solutions form both the tetrahydrate and monohydrate.Mineral forms are found in oxidation zones of iron-bearing ore beds, e.g. pyrite, marcasite, chalcopyrite, etc. They are also found in related environments, like coal fire sites. Many rapidly dehydrate and sometimes oxidize. Numerous other, more complex (either basic, hydrated, and/or containing additional cations) Fe(II)-bearing sulfates exist in such environments, with copiapite being a common example.
2
          
          
            FeSO
            
              4
            
            
              
            
          
          
            
              →
              
                Δ
              
            
          
          
            Fe
            
              2
            
            
              
            
          
          
            O
            
              3
            
            
              
            
          
          +
          
            SO
            
              2
            
            
              
            
          
          +
          
            SO
            
              3
            
            
              
            
          
        
      
    
    {\displaystyle {\ce {2FeSO_4->[\Delta]Fe_2O_3 + SO_2 + SO_3}}}
  Like other iron(II) salts, iron(II) sulfate is a reducing agent.