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Red ochre, haematite. |
13D8 |
THERE ARE FOUR BROAD GROUPS of pigment sources: natural earths, natural minerals, natural and synthetic organic dyestuffs, and artificial metal compounds. Earth pigments are obtained from inorganic ores or clays from which (typically) a metal oxide or silicate can be extracted. Mineral pigments are obtained by crushing various inorganic crystals. Natural organic dyes are extracted mainly from vegetation sources, though a few are obtained from animals and insects; the vast majority of organic colours are nowadays synthesised from coal-tar. The fourth category consists of a variety of artificial metal oxides or salts (usually metal chromates, carbonates or sulphides). The four pigments used most consistently by cave painters (after 18000 BC) were red ochre (haematite), yellow ochre (limonite), black manganese dioxide (pyrolusite) and white china clay (kaolin). This selection of pigments, known as the tetrachrome palette, remained popular through many centuries in many ancient and tribal civilisations.
Mineral pigments known to have been used in Ancient Egypt (after 3000 BC) include arsenic compounds realgar (red) and orpiment (yellow), the copper compounds malachite (green) and azurite (blue), plus the semi-precious stone lapis lazuli (ultramarine). Vegetable dyes employed for textiles included henna (dark red) safflower (orange), anatto (orange) and saffron (yellow). Important artificial pigments include minimum (red lead), flake white (white lead, naturally occurring as cerussite) and vermilion (naturally occurring as cinnabar).
The painters' palette expanded during the Middle Ages to include, most importantly, the brown earth pigments, bole, sienna and umber, plus smalt (vitreous blue cobalt silicate) and verdigris (green of Greece, copper carbonate), often combined with a resin to obtain so-called copper arsonate. A wide range of vegetable dyestuffs were also made available, including those extracted from madder (red), brazilwood (red), weld (yellow), buckthorn (sap green), indigo (blue) and woad (blue). The main dyes obtained from animal sources were Tyrian purple (from the murex shellfish), sepia (from the cuttlefish), crimson (from the kermes insect) and carmine (from cochineal). The first important artificial pigment of the modern era was Prussian blue (Desbach 1704). Other important additions included the introduction of cobalt blue (Thenard 1802), chrome yellow (Vauquelin 1809) and French ultramarine (Guimet 1826). The most fortunate advance in the nineteenth century was the accidental discovery, by Perkin in 1856, of the first coal-tar dyestuff, and named mauvine; it was followed by the synthesis of artificial alizarin (Liebermann 1869) and indigo (Baeyer 1878).
The most important coal-tar dyes (combined with white to make lakes) introduced in the twentieth century are phthalocyanine blue, quinacridone purple and naphthol red. Probably the most important pigment to be introduced is titanium dioxide, a cheap non-toxic white considerably more opaque and permanent than either flake white or zinc white.
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