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From George Eastman House : Notes On Photographs

A substance used to keep another substance wet is called a humectant. Some photographic materials were more sensitive when exposed wet or damp making the use of a humectant very important. In the wet collodion process humectants such as honey, beer, glycerin, sugared preserves, and syrups were poured upon the plate to keep the alcohol-swollen nitrocellulose layer from drying out. Humectants were also important additives for gelatin emulsions and for keeping pigmented gelatin carbon tissues flexible.^[1]

1. ↑ Osterman, Mark. 2007. Humectant. In The Focal Encyclopedia of Photography: Digital Imaging, Theory and Applications, History, and Science, ed. Michael R. Peres, 86, Focal Press.