Identification of darkened pigments in cultural objects by graphite furnace atomic absorption spectroscopy and inductively coupled plasma-mass spectrometry

Inorganic pigments in paint have been used by artists throughout history, primarily because of their resistance to chemical change over long periods of time. Many of the historically-important pigments consist of transition metal oxides, sulfides or carbonates. The determination of metals in pigments is a powerful tool that can be used by conservators and researchers to identify pigments. In this study, pigments were sampled using a dry cotton bud that was contacted lightly with the surface of painted objects to remove a small quantity of pigment (<1 μg). This sampling approach causes no visible damage to a paint surface. Evidence of contact cannot be detected visually, even with the aid of a magnifying glass. Prior to analysis, pigment metals were extracted from the cotton with concentrated HNO3 in a 2 ml polystyrene beaker. The pigment metals were determined using graphite furnace atomic absorption spectroscopy (GFAAS) and electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS). The small sample size requirement and high sensitivity of these instruments make them very suitable for analyzing pigment metals. Solution nebulization SN-ICP-MS was also used for obtaining a mass scan for most elements in the periodic table. The application of both GFAAS and ICP-MS is described for determining the identity of the metals in different pigments of two cultural objects (a painting and a map), which had darkened over time. Raman spectroscopy was used for confirming the identity of a darkened red pigment in a map as Red Lead (Pb3O4).