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From George Eastman House : Notes On Photographs

A natural, colorless salt crystal found in some lake beds. It is soluble in water and glycerin but not in alcohol. When mixed with water, it produces a slight alkaline reaction. Its use in photography was principally as a pH modifier in gold toning baths, but it was also used as a restrainer in pyrogallic acid developers and as an accelerator in hydroquinone developers.^[1]

1. ↑ Osterman, Mark. 2007. Borax, Na₂B₄O₇*10H₂O. In The Focal Encyclopedia of Photography: Digital Imaging, Theory and Applications, History, and Science, ed. Michael R. Peres, 48, Focal Press.