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From George Eastman House : Notes On Photographs

The asphalt solution used by Nicéphore Niépce has often been referred to as a varnish, though the most common use of the term in photography is the gum-based coatings used to protect photographic plates and prints. Varnishes made by dissolving gums in strong alcohol were first used to protect daguerreotypes. Paper prints were varnished to protect them from the atmosphere and to improve the tonality. Varnishes made with gum arabic, gum sandarac, and shellac were used.

In the collodion process a protective varnish was a requisite. The collodion surface was very fragile and prone to both abrasion and deterioration by exposure to the atmosphere. Unless there was a compelling reason to the contrary, all collodion negatives were varnished. Collodion positives on glass and early ferrotypes were always cased or framed and for this reason they can be found unvarnished because the photographer decided to leave the image a brighter color. When ferrotypes were offered in paper mounts they were susceptible to abrasion so they were varnished. Collodion images were usually varnished with sandarac and shellac varnishes though on occasion, waterbased gum varnishes were applied to collodion negatives. Gelatin emulsion negatives did not require varnishing, but early examples were often varnished until the practice was seen as unnecessary. The exception to this was when pencil re-touching was required, and for this a special mastic varnish was applied that would dry with a matt texture. See Re-touching.

Glass plate negatives of both the collodion and gelatin processes can also be found with a light yellow varnish applied to the glass side of the plate for the purpose of absorbing blue light when printing. This would retard the printing and help to increase contrast in the final print.^[1]

1. ↑ Osterman, Mark. 2007. Varnish. In The Focal Encyclopedia of Photography: Digital Imaging, Theory and Applications, History, and Science, ed. Michael R. Peres, 120, Focal Press.