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From George Eastman House : Notes On Photographs

Ferrous oxalate was made by mixing potassium oxalate with ferrous sulfate. It is nearly insoluble in water, but solubility increases with alkalinity. In 1877 ferrous oxalate was introduced as an alkaline developer by both M. C. Lea, in America, and Wm. Willis, in England. Ferrous oxalate was one of the first developers for silver bromide gelatin plates and remained popular until the introduction of hydroquinone and metol.^[1]

1. ↑ Osterman, Mark. 2007. Ferrous Oxalate, FeC₂O₄*2H₄O. In The Focal Encyclopedia of Photography: Digital Imaging, Theory and Applications, History, and Science, ed. Michael R. Peres, 77, Focal Press.