Evaluation of Gypsum and Calcium Oxalates in Deteriorated Mural Paintings by Quantitative FTIR Spectroscopy

Abstract

This study dealt with the development of a procedure based on quantitative FTIR spectroscopy to determine the amount of some contaminants in wall paintings, mortars, and stone micro‐samples. As contaminants we selected gypsum, CaSO₄ · 2H₂O, and calcium oxalate monohydrate, CaC₂O₄ · H₂O, among the most common polluting substances present in architectural porous materials. Calibration curves of absorbance versus analyte concentration were determined by adopting both the internal and external standard methods. As internal standard we used Prussian blue, Fe₃[Fe(CN)₆]₄, that presents an unique infrared peak at 2094 cm^− 1. The correlation coefficient for a linear fit was very good for every calibration, being in each case greater than 0.9900. Furthermore, the precision of the evaluation of gypsum and calcium oxalates varied in the range 8–15% and ca. 4% respectively. In order to verify the findings, some specimens, which came from real frescoes, were analysed both by DSC calorimetry and FTIR spectroscopy: we found a satisfactory match between the quantitative analyses performed by the two techniques. For the first time, the application of this method allowed the quantitative determination of the desulphatization power of the Ferroni‐Dini method—based on the ammonium carbonate and barium hydroxide technique—in the conservation of mural paintings.

^#Paper presented at the 5th IRUG Infrared and Raman User Group Meeting, The Getty Conservation Institute, Los Angeles, March 4–8, 2002.

Keywords:

• Mural paintings
• Conservation
• Gypsum
• Calcium oxalates
• Quantitative FTIR spectroscopy

Acknowledgments

The authors express their gratitude to people working in the Cultural Heritage Ministry (Province of Florence, Pisa, Siena) who made possible the scientific cooperation in mural paintings workshops. Thanks are also due to Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase, CSGI, Florence, for financial support.

Notes

^#Paper presented at the 5th IRUG Infrared and Raman User Group Meeting, The Getty Conservation Institute, Los Angeles, March 4–8, 2002.