Crystallographic Studies in Cultural Heritage: Solid State Behaviour of Inorganic Pigments

Most inorganic pigments generally consist of the colouring agents such as hematite (red) or glauconite (green) together with white pigments, including alumosilicates and calcium compounds. This usually leads to a wide colour range dependent on the exact percentage of the colouring agent in the solid mixture. Some inorganic pigments have been in use for thousands of years due to their easy availability and desirable attributes: ochres have been in use since prehistoric times to produce cave and rock paintings, and are still used to this day; terra verde (green earth) first made its appearance in decorations and frescos in the first century B.C. Whether these pigments are used in frescos, cave ornaments or paintings, shortcomings in their particular hue, transparency or fading character very often inspire research towards a better understanding of these physical characteristics. We present a study in which crystal engineering was applied in an attempt to tackle such problems. The solubility of Venetian red ochre and its solid state behaviour at higher temperatures were investigated and compared with similar studies on terra verde. Hot stage microscopy showed that, although the pigment retained its red colour upon heating, some crystallites lost transparency, indicating a phase change. The actual colouring agents hematite and goethite in Venetian red ochre are insoluble in most solvents and solvent mixtures. However, the solvent was found to have a significant effect on the undissolved pigment and other components. The pigment examined in this study was a mixture of microcrystalline powders with smaller quantities of larger crystallites thought to be gypsum. Multi-elemental analysis by X-ray fluorescence spectroscopy showed the most abundant elements to be sulphur, calcium, iron, magnesium, silicon and aluminium. Fourier-Transform Infrared (FTIR) spectroscopy indicated the presence of metal oxides, metal carbonates and alumosilicates. Powder X-ray diffraction experiments helped to identify and quantify the proportions of hematite and goethite in four different pigments.