ABSTRACT
Chapter 2 describes the preparation of 40 inorganic pigments covered in this book. Some of these pigments are naturally occurring minerals; most can be produced by chemical synthesis. Lazurite, a blue mineral mined in Central Asia, was extracted to yield the pigment ultramarine. The gem form of lazurite is lapis lazuli. In 1825, the French learned how to synthesize ultramarine by a complex set of reactions in which China clay, sulfur and coke were transformed into a aluminosilicate framework which enclosed blue polysulfide chromophores. Prussian blue does not occur in nature and was eagerly used to dye textiles. This pigment has unusual porous structure which was used to engulf radioactive cesium fallout from the Chernobyl accident and protect grazing sheep in Britain. White lead and verdigris are produced by a process involving the corrosion of a metal (lead or copper, respectively) caused by gases which are released by decaying organic matter. Red lead is a compound which contains both Pb2+ and Pb4+. Its synthesis from PbO must be carefully controlled because residual starting material creates a structure in which the core and shell have different colors and this is revealed when the pigment ages. The Fe (III) oxide pigment group is interesting because five different hues can be produced by synthesis. Magnetite is a member of this family which contains both Fe2+ and Fe3+. Carbon blacks are synthesized by oxidation of hydrocarbons. Airborne particles of carbon black are referred to as black carbon, i.e., soot and as air pollutants cause respiratory disease. Chrome yellow is synthesized by combining PbCrO4 and PbSO4 which co-crystallize to form solid solution. The sulfate content becomes a critical determinant in the stability of the yellow color. Many of these pigments contain heavy metals (Cr, Co, Pb, Fe, Hg, Cd and As) which are the basis of their chromophores. A smaller group of pigments do not contain heavy metals (carbon black, ultramarine, orpiment and realgar).
