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From George Eastman House : Notes On Photographs

Greenish white crystals made by dissolving iron wire in nitric acid or by mixing ferrous sulfate with barium nitrate. Ferrous nitrate is soluble in water (1:6) and dilute alcohol. It was used as a developer in the collodion positive processes, giving a whiter color to the reduced silver.^[1]

1. ↑ Osterman, Mark. 2007. Ferrous Nitrate, Fe(NO₃)₂*18H₂O. In The Focal Encyclopedia of Photography: Digital Imaging, Theory and Applications, History, and Science, ed. Michael R. Peres, 77, Focal Press.