Specular Reflection FTIR: A Non-Contact Method for Analyzing Coatings on Photographs and Other Cultural Materials

Cultural heritage objects present a special set of challenges to analyze chemically. Often micro-sampling or even contacting the object is not appropriate or deemed too risky for fragile surfaces. Fourier Transform Infrared (FTIR) spectroscopy is a commonly used analytical technique for organic compound identification, either in a micro-sampling or Attenuated Total Reflection (ATR) mode, both of which inherently require some measure of contact with the sample. Presented here is a case study of the use of specular reflection FTIR spectroscopy, an FTIR collection mode that allows positive chemical identifications in a non-sampling and non-contact way. Although this paper focuses on the case study of coatings on 19th century photographs, the specular reflection FTIR mode is applicable to surface coatings on any artifact where molecular composition identification is needed but micro-sampling or physical contact with the sample have been ruled out. The main limitation of the specular reflection mode has traditionally been the lack of specular reflection FTIR reference libraries. As a result, the analytical project presented here first required generation of a specular reflection FTIR spectral library for historical coating materials used on 19th century prints by creating modern reference samples. The project is a result of collaboration between conservators at the Weissman Preservation Center at Harvard Library and materials scientists at Harvard’s Center for Nanoscale Systems to analyze original coatings on 19th century salted paper photographs in the collection of the Harvard University Archives.