FTIR, Raman and XRF identification of the image materials in turn of the 20th century pigment-based photographs

Abstract

Artists at the turn of the 20th century experimented with a wide range of photographic techniques to create complex and unique prints by processes that are difficult to fully identify. FTIR, both in transmission and ATR, Raman spectroscopy and XRF were applied to characterize and identify the materials and processes used in photographs from the period in which pigment-based techniques were employed alone or superimposed to others such as platinum. FTIR in the transmission mode and ATR acquired in the high image density areas of the images allow to characterize the organic components in the photographic colloid, and the analysis of the low image density areas gives vibrational information about the organic compounds used for sizing the paper substrates. Raman spectroscopy allows the identification of the pigments, while the chromium-containing compounds, such as chrome alum used to size the paper substrates and the compound/s that chromium forms with the oxidized organic component in gum print images, were found to be below the detection limit of the technique both in samples prepared strictly following historic recipes and in hundred year old works of art. The advantages and limitations of the multitechnique approach used to identify and differentiate gum prints are discussed.