Characterization of carbon-based black pigments

EUGENIA TOMASINI; GABRIELA SIRACUSANO; MARTA MAIER

Barcelona

Conferencia; 10th Biennial International Conference of the Infrared and Raman Users Group; 2012

Carbon-based black pigments are a group of dark-coloured materials composed by some form of elemental carbon that are classified according to the starting material used and their method of manufacture. These pigments consist mainly of highly carbonized solid products obtained by burning or pyrolysis of organic materials as well as natural forms of carbon such as coal and graphite. Some of these pigments contain inorganic materials, as hydroxyapatite in ivory black or alkali metal carbonates in cokes and chars made from vegetable precursors, which may be helpful in the identification of the pigment. Very often, the presence of a carbon-based pigment is inferred from the absence of black iron and manganesium oxides. In most cases the pigment is assumed to be charcoal or simply carbon black, a term generally used to describe any pigment based on carbon. As the precise identification of pigments is important for understanding the technology employed in the manufacture of an artwork and the resources available, the unambiguous characterization of carbon-based pigments remains a big challenge. The aim of this work was the discrimination between reference carbon-based pigments by Fourier transform infrared spectroscopy (FTIR) with attenuated total reflectance (ATR) and scanning electron microscopy (SEM) coupled to an energy-dispersive X-ray spectrometer (EDS). Eight pigments provided by Zecchi (lampblack, vine black, ivory black, bistre, earth of Cassel, Van Dyck, bitumen, and graphite) and two pigments from Kremer (Van Dyck and vine black) were studied. Analytical characterization of the reference pigments, together with a sample of commercial vegetable charcoal, has been performed by FTIR-ATR and SEM-EDS. The information obtained by both techniques was organized in two flow charts in order to help in the discrimination of the pigments studied. Our methodological approach was successfully applied to the identification of two carbon-based black pigments from two South American colonial paintings dated from the early eighteenth century .and housed in two churches in Bolivia. Both black pigments showed clear differences in their FTIR spectra as well as the presence of carbon-based pigments as determined by Raman spectroscopy. The combination of FTIR and SEM-EDS allowed the identification of the pigments in the painting of Caquiaviri and the mural painting of Andamarca as ivory black and charcoal, respectively.