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From George Eastman House : Notes On Photographs

Silver bromide gelatin emulsions were originally heated to increase particle size and, as a result, sensitivity. Charles Bennett was the first to discover the effect in 1878. The term boiling refers to heating the water surrounding the emulsion kettle to the boiling point. An alternative to boiling emulsions was to increase speed by introducing ammonia to the melt.^[1]

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