Fluorescence Fails: Analysis of UVA-Induced Visible Fluorescence and False-Color Reflected UVA Images of Tintype Varnishes Do Not Discriminate Between Varnish Materials

Abstract

To determine if the commonly used technique of UVA-induced visible fluorescence could accurately identify varnish materials, a study collection of 221 tintypes with known varnish materials was digitally imaged. The CIE L*a*b* values of the fluorescence in the lightest light and darkest dark areas of the images obtained in Adobe Photoshop® showed no correlation between resins present and the color of the fluorescence, and fluorescence failed to differentiate between even quite disparate materials such as shellac and dammar. This commonly used examination technique should not be considered a reliable identification method for varnish materials: if identification of resins is critical, more accurate analytical techniques must be employed. False-color UVA images, a relatively recent addition to the digital imaging repertoire, also failed to differentiate between resins, indicating that the relative absorption/reflection of UVA light by the resins was also insufficiently characteristic to provide identification.

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