Hydrofluoric acid pre-treatment of matte artists’ paints for binding medium analysis by Fourier transform infrared microspectroscopy

A rapid and safe method has been developed for preparing matte artists’ paint samples for binding medium analysis by Fourier transform infrared (FTIR) microspectroscopy. Hydrofluoric acid (HF) transforms many inorganic components of matte paints that commonly interfere with medium analysis, viz. carbonate and silicate fillers and pigments, into chemical species whose infrared absorptions are shifted outside the spectral area of interest. This method provides a route for removing these interferences without relying on lengthy and sometimes unsuccessful solvent extraction of the binding medium. Microsamples of common binders, pigments, and inorganic fillers were reacted with solution or gas phase HF to determine the effectiveness of the chemical treatment in removing unwanted interferences while preserving the binding medium. A second method of chemical treatment, using in situ generation of HF gas from acidified sodium fluoride, proved equally effective while minimizing the hazards of handling and storing liquid HF solutions. The success of the technique in removing mid-infrared interferences in binding medium analysis is demonstrated on various paintings dating from the 19th and 20th centuries.