Uniformity, thickness and long-term behaviour of the coating are among the main questions that conservators need to address when they apply a protective treatment. An attractive aspect of FT-IR spectroscopy is that not only chemical characterization but also quantitative information can be obtained within a single spectrum, enabling an overall assessment of the entire coating-substrate system. Non-invasive FT-IR reflection spectroscopy is particularly advantageous when applied on metals, because it suffers less than on low reflective surfaces of diffusion and refraction phenomena that cause distortion of the spectra. On high reflective surfaces coated with optically transparent layers, a double transmission of the radiation may occur through the coating itself, providing high sensitivity and spectra quite similar to those in transmission. Based on the Beer's Law, a linear relationship exists between absorbance and thickness, providing information on the uniformity of the coating. In the framework of the CE Project IPERION CH, we explored the reliability of FT-IR reflection to monitor presence, uniformity and thickness of different protective films (acrylic polymers, nitrocellulose, microcrystalline waxes) on bronze and silver coupons, applied in single and multiple layers and with different concentrations. The absorbance of marker bands of the coatings (e.g. carbonyl and methyl/methylene stretching bands) was calculated as area, showing a correlation between band intensities and number of coats. Eddy current was used as cross method to measure the coating thickness in several spots. FT-IR reflection spectroscopy has proved effective in checking the uniformity of the investigated coatings, showing that no spectral distortions occur up to a thickness of at least 12 µm on average. In addition, the technique has proven its high sensitivity, as it is able to detect very thin layers (<1 µm) of organic coating. This feature is of great importance when reflectance FT-IR is used to monitor the removal of old coatings.
Non-invasive FT-IR reflection for monitoring distribution and thickness of coatings on metal artworks
B Salvadori;
2019
Abstract
Uniformity, thickness and long-term behaviour of the coating are among the main questions that conservators need to address when they apply a protective treatment. An attractive aspect of FT-IR spectroscopy is that not only chemical characterization but also quantitative information can be obtained within a single spectrum, enabling an overall assessment of the entire coating-substrate system. Non-invasive FT-IR reflection spectroscopy is particularly advantageous when applied on metals, because it suffers less than on low reflective surfaces of diffusion and refraction phenomena that cause distortion of the spectra. On high reflective surfaces coated with optically transparent layers, a double transmission of the radiation may occur through the coating itself, providing high sensitivity and spectra quite similar to those in transmission. Based on the Beer's Law, a linear relationship exists between absorbance and thickness, providing information on the uniformity of the coating. In the framework of the CE Project IPERION CH, we explored the reliability of FT-IR reflection to monitor presence, uniformity and thickness of different protective films (acrylic polymers, nitrocellulose, microcrystalline waxes) on bronze and silver coupons, applied in single and multiple layers and with different concentrations. The absorbance of marker bands of the coatings (e.g. carbonyl and methyl/methylene stretching bands) was calculated as area, showing a correlation between band intensities and number of coats. Eddy current was used as cross method to measure the coating thickness in several spots. FT-IR reflection spectroscopy has proved effective in checking the uniformity of the investigated coatings, showing that no spectral distortions occur up to a thickness of at least 12 µm on average. In addition, the technique has proven its high sensitivity, as it is able to detect very thin layers (<1 µm) of organic coating. This feature is of great importance when reflectance FT-IR is used to monitor the removal of old coatings.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
