Characterization of binders and pigments using an integrated analytical approach: Application to wooden reliefs created by Vasko Lipovac in the 1970s

This contribution describes the study conducted on two painted wooden reliefs created in the early 1970s by Vasko Lipovac, a prominent Croatian contemporary artist. Due to the poor condition in which the artworks were found, it was important to establish a strategy for conservation and long-term preservation. Accordingly, preliminary knowledge about chemical composition of the paint pigments and binders was pointed out as significant information addressed in this study. Since the chemical composition of the paint determines the conservation methodology and materials, the proper treatment ensures stability of the original materials, and accordingly, long term preservation of an artwork. An integrated analytical approach involving different instrumental analytical techniques was applied to each of the microsamples collected in the paintings. For the elemental characterization of the pigments, total reflection X-ray fluorescence spectroscopy (TXRF) was used, and the obtained results were complemented with the information about mineral phases present in the pigments by X-ray diffraction (XRD) analysis. The TXRF results demonstrated that most of the pigments related with the named “skin tone” contain appreciable amounts of Cd (1.8–9%) with constant presence but variable amounts of Se (0.24–1.1%), reinforcing the interpretation that the pigment originally used is cadmium sulfoselenide (the so-called “cadmium red”), although by XRD we only observed the presence of its alteration to cadmium sulfate. The XRD results revealed a complex mixture of different inorganic compounds present in the painted wooden reliefs (i.e., barite, rutile, quartz, calcite, chlorite, etc). The presence of zinc white with wurtzite structure in almost all the samples can be considered as the main constituent of the coloring painting layer. Raman spectroscopy contributed to a more accurate characterization of the analysed paint samples, since the vibrational spectra as a result of Raman scattering proved very useful to obtain information on the presence and type of the synthetic pigments. Fourier transform infrared spectroscopy (FTIR) and pyrolytic gas chromatography coupled with mass spectrometry (Py-GC/MS) were used to determine the chemical composition of the paint samples binders. Py-GC/MS clarified the doubts about acrylic and alkyd components in paints where polyvinyl acetate (PVAc) is predominant by detecting acrylate monomers and phthalic anhydrides and provided information about their relative presence in the samples. On the other hand, FTIR spectroscopy was a necessary tool for unambiguous identification of nitrocellulose in the white and black samples from one of the paintings, as well as for the firm assumption of its presence in most of the other samples since Py-GC/MS is not suitable for its detection.