Coatings and Coatings Analysis

Specular reflection Fourier Transform Infrared Spectroscopy (FTIR) was used as an analytical technique to examine coatings in a non-contact, non-sampling way while on site in the collections. This presentation by Arthur McClelland discusses the development, use, and distribution of information gathered with specular reflection FTIR during the Harvard Salt Print initiative. 


Characterization of photographic coatings is an important tool for dating a photograph, identifying its chemical make-up, and determining if a photograph is a salt print or an albumen print. When applied by the original photographer or studio, coatings constitute an integral part of a photograph that may be considered part of the artist’s intent and working method. Identification of coatings can also aid in determining suitable conservation treatment approaches and indicate possible deterioration tendencies particular to each coating material.

Correct identification of coatings can sometimes be misleading to even a highly trained eye. For instance, differences in storage conditions over the years can lead to different coloration of the same coating. Much has been written about photograph coating analysis. Fourier Transform Infrared (FTIR) spectroscopy has long been used as an analytical technique, but traditionally in transmission or attenuated total reflectance (ATR) modes. For the Harvard Salt Print project, however, we used specular reflection FTIR for chemical analysis of coatings.

Despite the similar names, specular reflection FTIR and ATR-FTIR are fundamentally different. Specular reflection has traditionally been an unpopular mode of FTIR as the peaks in specular reflection FTIR spectra are not as well resolved as in ATR-FTIR spectra, making the data harder to interpret. In this project, interpretation of individual peaks was not needed because the spectrum as a whole could be used as a spectral signature for a specific coating.

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A Bruker Lumos FTIR microscope system using specular reflectance FTIR to analyze the coating on a salted paper print.












A spectral reference library for specular reflection FTIR, however, was needed to make matches. The Weissman Preservation Center at Harvard Library, in partnership with Center for Nanoscale Systems at Harvard University, created a spectral reference library for specular reflection FTIR of coated salted paper prints. The reference library is based on modern reference samples created from historic recipes. It consists of the most common coatings that were used on photographs in the nineteenth century, including albumen, beeswax, beeswax with lavender oil, casein, copal, Canada balsam, dammar, dextrin, gelatin, gum Arabic, sandarac, shellac, and white wax.

Standard spectral library search algorithms reliably matched spectra from the historic salt prints to the spectral library of modern reference samples. The matches were usually unambiguous for objects with single coatings. The distinction between gelatin and albumen coated samples, for instance, was clearly evident. This library continues to be successfully used to identify coatings on photographs from the Harvard collections.

Limitations to spectral library matches
Limitations of the specular reflection FTIR spectral library include the significant cotton paper contribution to the reference spectra. Since the library was built with the goal of identifying coatings on cotton paper, this is not an issue for salted paper prints, but is a limitation of the spectral library that should be recognized before attempting to apply it to other projects. We attempted to streamline the data processing steps for ease of use and only did a baseline subtraction of the data before matching it to the spectral library. We did not attempt to spectrally subtract the paper contribution since it was present in both the modern reference samples and the historic objects.

While spectral library search algorithms are better than humans at correlating unknown and reference spectra, the search algorithms tend to be biased towards matching the strongest peaks in the spectrum. This tendency leads to difficulties in distinguishing dilute albumen prints from albumen prints.

Principal Component Analysis
To take the analysis further, Principal Component Analysis (PCA) models are being developed with the specular reflection FTIR data set. The PCA technique suggests a potentially more robust approach to distinguishing between salted paper prints, dilute albumen prints, and albumen coated prints. Classical Least Square (CLS) and Partial Least Square (PLS) methods are also being investigated for more quantitative mixture analysis.

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