Pigments

Pigments

1. OCHRES

   • iron oxides, like, e.g., Fe2O3 (haematite).

2. unsoluble coloured compounds, prepared in form of fine powder, and used for painting

   • minerals, i.e. inorganic chemical compounds found in nature (ores). They are mainly binary or ternary ionic compounds.

3. LAPISLAZULI

   • most ancient and precious blue Na3Ca(Al3Si3O12)S
     • blue stone composed of different minerals. The blue mineral is lazurite, whose colour center is an ion S-.

4. ORPIMENT

   • Auripigmentum As2S3
     • pigment, but has a bright, yellow colour. It was widely applied, especially in East

5. CINNABAR

   • HgS
     • natural ore, but as a pigment was produced since the Middle Age. Metallic mercury and sulphur were heated in a closed vessel: a reaction occured, with formation of mercury sulphide. Chemically, the natural and synthetic form are identical, although the synthetic form was called vermilion

6. MINIUM

   • Pb3O4
     • with cinnabar, was the other most used red pigment for painting on paper, parchment, easel or panel. It was mainly produced artificially by heating lead white.

7. MALACHITE

   • Cu2(CO3)(OH)2

8. AZURITE

   • Cu3(CO3)2(OH)2
     • often found together, as ores. They are both copper carbonates, with similar composition. The more stable form is malachite, and a chemical transformation of azurite into malachite is possible. The turning can take place also in the pictorial layer, and so blue areas can turn into green. Often, natural malachite is actually a mixture of azurite and malachite. Copper pigments have an active role in degradation of organic materials, like paper, pachment, textiles, and should be monitored in Cultural Heritage sites.

9. ARTIFICIAL

   • VERDIGRIS
     • green copper
       • CH₃COO⁻ e Cu2(OH)2CO3
   • LEAD WHITE
     • hydrocerussite) is the white pigment par excellence, practically present everywhere, used until the XIX c. It was used both for preparing white backgrounds and in mixture with other pigments to modulate the colour or to highlight. It was produced for chemical alteration of lead, exposing lead plates to vinegar vapours and air. The reaction was: Pb + O2 + CO2 à PbCO3

10. + BINDER

    • The colour for painting is obtained by mixing a binder with the pigment. The binder is added until you get a dense, but still fluid medium ( about 1:1)
      • From a physico-chemical point of view, the coloured mixture is a suspension and not a solution, becuase the pigment grains do not dissolve in the binders, but are dispersed. Therefore, in a pictorial layer, i.e. the dried painted layer, is possible to observe with a microscope single pigment grains
        • Until recent years (XX c.), binders were organic substances obtained from plants or animals
          • EGG TEMPERA
            • Egg yolk and white were used as a binding medium for most of paintings, until the oil replaced them (around the XIV-XV c.). This binding medium was called tempera, and white, yolk or both were used. In miniature paintings, egg tempera was used always, because oil was not suitable for the flexible leaves of manuscripts
          • DRYING OILS
            • Drying oils are oils with a high content of unsaturated fatty acids. They do no really «dry», but polymerize to give a rigid film. Among the most used ones, linseed oil, but also nut oil, poppy oil, and mixtures of them. Oil polymerization is slow, and can last for a long time, and therefore we can evaluate freshly painted layers from ancient painted layers
          • PROTEIN GLUES
            • Protein glues are all based of the protein collagen. Collagen is a fibrous protein, forming skin and other connective tissues. It can be dissolved in hot water, and, after cooling, it forms a rigid film. Mainly used for preparatory layers (background layers), e.g. with plaster (calcium sulphate)
          • NATURAL WAXES
            • In the so-called encaustum (encaustic painting), hot wax is mixed with the pigments. Encaustic painting was very popular among Greeks and Romans. Natural waxes are esters of fatty acids and long-chain alkylic alcohols.
              • CH3(CH2)29O-CO-(CH2)14CH3
          • current paints are often produced disperding mineral pigments in an artficial binder (usually, binders are polymers dissolved in a solid, o microdispersed in water)
        • INORGANIC
          • The inorganic pigment is composed by small crystals (pale blue in figure) dispersed in the binder. At each interface between crystal and air, the light ray divides in reflected and refracted rays, deviating from the original path. Part of the light is, therefore, reflected back to the observer reflected refracted reflected When we have a layer containing many of these small crystals, light rays are refracted and reflected many times, passing a long time in the layer, and being, finally, reflected back in large amount. The optical path - the path of the ray through the crystal(s) - determines the level of colour saturation: if the path is short, the color is weak and the layer is almost transparent, if the path is long, colour is strong, and the layer looks opaque.
            • completely absorbed and the colour is gray). Inorganic pigments have the following advantages with respect to organic dyes 1) They are lighfass and chemically stable (generally) 2) They do not change, usually, colour on changing the pH of the substrate 3) They do not change due to different features of the binders
              • Prussian blue Fe4[Fe(CN)6]3 Artificial pigment of the second half of 1700