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 <i>non-sampling</i> and <i>non-contact</i> 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.

文化遗产文物的化学分析面临一系列特殊挑战。对于脆弱的文物表面而言，微采样甚至仅接触器物都可能不合适，或被认为风险过高。傅里叶变换红外（Fourier Transform Infrared, FTIR）光谱技术是一种常用于有机化合物鉴定的分析手段，可采用微采样或衰减全反射（Attenuated Total Reflection, ATR）模式，但这两种模式本质上都需要与样品进行一定程度的接触。本研究介绍了镜面反射FTIR光谱技术的应用案例：该技术作为一种FTIR采集模式，可在“非采样”且“非接触”的前提下实现准确的化学组分鉴定。尽管本案例研究聚焦于19世纪照片的涂层分析，但镜面反射FTIR模式同样适用于所有需要进行分子组分鉴定、但无法进行微采样或物理接触样品的文物表面涂层分析。长期以来，镜面反射模式的主要局限在于缺乏对应的镜面反射FTIR光谱参考库。因此，本项分析工作首先需要通过制备现代参照样品，构建一套针对19世纪版画所用传统涂层材料的镜面反射FTIR光谱库。本项目由哈佛大学图书馆魏斯曼保护中心的文物保护专家与哈佛大学纳米系统中心的材料科学家合作开展，旨在对哈佛大学档案馆馆藏的19世纪盐纸照片的原始涂层进行分析，本研究正是该合作项目的成果。