The varnished truth: The recipes and reality of tintype coatings

The most popular photographic technique in the USA between 1856 and 1900 was the tintype, with millions of these objects created by photographers in established studios, by itinerant artists in portable workshops, and by amateurs working from 'how-to' manuals and journal articles. Whereas the fundamentals for this photographic process (collodion binder on a japanned metal support) were largely invariant, historical documents recommended a wide variety of protective varnish materials. A collection of 221 tintypes was analyzed using pyrolysis gas chromatography-mass spectrometry (py-GC-MS) to compare the components of actual tintype varnishes with recipes from the historical literature. Several resins in published tintype varnish recipes, including mastic, copal, and amber, are entirely absent from this collection and only five constituents - shellac, Pinaceae resin (Canada balsam or colophony), dammar, sandarac, and camphor - are detected alone or in combination. Each detected resin appears in historical recipes, but just 24% of the samples have varnish layer constituents consistent with published tintype varnish recipes. Forty-four percent of the tintypes have varnish constituents consistent with formulations recommended for other collodion images, but the varnishes of the remaining samples have no direct literature equivalents. The preponderance of shellac- and Pinaceae-based varnishes suggests that these correspond to inexpensive commercial varnishes, but tintypists may have developed their own preferred mixtures or simply used what was at hand. This first in-depth technical analysis of tintype materials suggests that the cheapest and most readily available materials were employed in the varnishing process and that the artists were not bound by literature recommendations.