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Micro to nano: multiscale IR analyses reveal zinc soap heterogeneity in a 19th-century painting by Corot



Xiao Ma, Georges Pavlidis, Eoghan Dillon, Victoria Beltran, Jeffrey Schwartz, Mathieu Thoury, Ferenc Borondics, Christophe Sandt, Kevin Kjoller, Barbara Berrie, Andrea Centrone


The formation and aggregation of metal carboxylates (metal-soaps) can degrade the appearance and integrity of oil paints and frequently poses a challenge to the conservation of painted works of art. Efforts to understand the root cause of metal-soap degradation has been hampered by the limited spatial resolution of Fourier-transform infrared (IR) microscopy (μ FTIR). Here, we overcome this limitation by leveraging two novel photothermal IR methods: optical photothermal infrared spectroscopy (O-PTIR) and photothermal induced resonance (PTIR), which provide IR spectra with ≈ 500 nm and ≈ 10 nm spatial resolutions, respectively. We investigate the distribution of chemical phases in the top layer of a French nineteenth-century painting at multiple scales (µ-FTIR ≈ 1011 nm3, O PTIR ≈ 108 nm3, PTIR ≈ 103 nm3 to ≈ 104 nm3), revealing numerous distinct and closely intermingled metal carboxylate species. The paint samples analyzed here are found to be mixtures comprised of pigments (cobalt green, lead white), cured oil, and a rich array of distinct zinc-soaps that form finely intermixed domains often much smaller than ≈ 108 nm3. We identify Zn-stearate and Zn-oleate crystalline (i.e., ordered, densely packed) soap phases with characteristic narrow IR peaks (≈ 1530-1558 cm-1) and a heterogeneous, disordered, water-permeable, partially hydrolyzed, tetrahedral zinc soap phase with a characteristic broad peak centered at ≈ 1596 cm-1 with additional broad spectral features in the ≈ 1570–1660 cm-1 spectral range. We show that the high signal-to-noise ratio and spatial resolution afforded by O-PTIR is ideal to identify phase separated species with low average concentration and to identify species with closely overlapping spectral features, while the nanoscale spatial resolution of PTIR provides an unprecedented view on distributions and associations of chemical species in paint. This work shows highlights PTIR and O PTIR ability to provide newly accessible nanoscale compositional information which is critical to advance our knowledge of the chemical processes occurring in oil paint and to stimulate new art conservation practices.
Analytical Chemistry


PTIR, O-PTIR, metal carboxylate, cobalt green, Jean-Baptiste-Camille Corot, Gypsy Woman with Mandolin.


Ma, X. , Pavlidis, G. , Dillon, E. , Beltran, V. , Schwartz, J. , Thoury, M. , Borondics, F. , Sandt, C. , Kjoller, K. , Berrie, B. and Centrone, A. (2022), Micro to nano: multiscale IR analyses reveal zinc soap heterogeneity in a 19th-century painting by Corot, Analytical Chemistry, [online],, (Accessed April 16, 2024)
Created February 9, 2022, Updated November 29, 2022