Ma Xiao, Pavlidis Georges, Dillon Eoghan, Beltran Victoria, Schwartz Jeffrey J, Thoury Mathieu, Borondics Ferenc, Sandt Christophe, Kjoller Kevin, Berrie Barbara H, Centrone Andrea
Institute for the Conservation of Cultural Heritage, Shanghai University, No. 333 Nanchen Road, Shanghai 200444, China.
Nanoscale Device Characterization Division, Physical Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899, United States.
Anal Chem. 2022 Feb 22;94(7):3103-3110. doi: 10.1021/acs.analchem.1c04182. Epub 2022 Feb 9.
Formation and aggregation of metal carboxylates (metal soaps) can degrade the appearance and integrity of oil paints, challenging efforts to conserve painted works of art. Endeavors to understand the root cause of metal soap formation have been hampered by the limited spatial resolution of Fourier transform infrared microscopy (μ-FTIR). We overcome this limitation using optical photothermal infrared spectroscopy (O-PTIR) and photothermal-induced resonance (PTIR), two novel methods that provide IR spectra with ≈500 and ≈10 nm spatial resolutions, respectively. The distribution of chemical phases in thin sections from the top layer of a 19th-century painting is investigated at multiple scales (μ-FTIR ≈ 10 μm, O-PTIR ≈ 10 μm, PTIR ≈ 10 μm). The paint samples analyzed here are found to be mixtures of pigments (cobalt green, lead white), cured oil, and a rich array of intermixed, small (often ≪ 0.1 μm) zinc soap domains. We identify Zn stearate and Zn oleate crystalline soaps with characteristic narrow IR peaks (≈1530-1558 cm) and a heterogeneous, disordered, water-permeable, tetrahedral zinc soap phase, with a characteristic broad peak centered at ≈1596 cm. We show that the high signal-to-noise ratio and spatial resolution afforded by O-PTIR are ideal for identifying phase-separated (or locally concentrated) species with low average concentration, while PTIR provides an unprecedented nanoscale view of distributions and associations of species in paint. This newly accessible nanocompositional information will advance our knowledge of chemical processes in oil paint and will stimulate new art conservation practices.
金属羧酸盐（金属皂）的形成和聚集会破坏油画的外观和完整性，给保护油画艺术品的工作带来挑战。傅里叶变换红外显微镜（μ-FTIR）有限的空间分辨率阻碍了人们对金属皂形成根源的理解。我们使用光学光热红外光谱（O-PTIR）和光热诱导共振（PTIR）克服了这一限制，这两种新方法分别提供了空间分辨率约为500纳米和10纳米的红外光谱。在多个尺度上（μ-FTIR约10微米、O-PTIR约10微米、PTIR约10微米）研究了一幅19世纪油画顶层薄片中化学相的分布。此处分析的油漆样品被发现是颜料（钴绿、铅白）、固化油以及大量混合的、小的（通常远小于0.1微米）锌皂域的混合物。我们识别出具有特征性窄红外峰（约1530 - 1558厘米）的硬脂酸锌和油酸锌晶体皂，以及一个非均匀、无序、透水的四面体锌皂相，其特征性宽峰位于约1596厘米处。我们表明，O-PTIR提供的高信噪比和空间分辨率非常适合识别平均浓度较低的相分离（或局部浓缩）物种，而PTIR则提供了油漆中物种分布和关联前所未有的纳米级视图。这种新获得的纳米组成信息将推动我们对油画中化学过程的认识，并将激发新的艺术品保护实践。
