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Quantitative Analysis of Thin-Film Organic Materials on Photographs Using Fourier Transform Infrared Spectrometry
In photography various organic materials are used to bind the light-sensitive material to a suitable backing. Binder thickness varies and depends on factors such as the application technique, the type of backing and aging effects on the photograph. The main purpose of this study was to produce a series of quantitative standards for each of the three principal binding materials - albumen, gelatin and collodion- so that reliable quantitative measurements of the binder thickness for historical photographs can be made using Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectrometry. Determination of the thickness of the organic layer on a photograph can help conservators identify the coating method and possibly indicate whether additional organic coatings are present on the photograph. A set of quantitative standards will serve to cross-calibrate ATR-FTIR instruments in different laboratories, thus making comparative studies more meaningful. Initial work focused on developing a method of preparing a uniform coating of each of the three organic materials with varying thicknesses on a satisfactory support. Satisfactory samples of albumen were prepared on a paper backing by rolling an albumen solution onto the paper and allowing it to dry thoroughly. Standards with a thickness range of 146 - 4518 µg/cm2 were created for albumen. Gelatin samples were prepared in a similar fashion and a set of standards with a thickness range of 1853 - 7055 µg/cm2 were created. All sample preparation methods used were unsuccessful in creating sufficiently thin collodion coatings. Furthermore, a study of numerous collodion photographs using digital and scanning electron microscopy indicated that the coating thickness for these photographs was too great to permit quantitative ATR-FTIR measurements. Consequently, no further work was done to prepare collodion standards of varying thickness. Calibration curves based on a normalized IR peak area measurement were obtained for the set of albumen and gelatin standards using different ATR-FTIR instruments. These were then used to quantitatively assess the albumen or gelatin thickness for a number of photographs from the Getty Conservation Institute collection. The inter-laboratory results showed a variation of less than 10% for the measured albumen thicknesses. The different instruments produced differences less than 20% for the measured gelatin thicknesses. Details of the standard preparation and the quantitative ATR-FTIR measurements, along with suggestions for additional studies, are presented in this work.