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Blue pigments
Ultramarine blue : a remarkable achievement for chemistry Among all the blue pigments, ultramarine blue has a particularly rich and eventful history. Up until the first quarter of the 19th century, the pigment was extracted at great expense from a semi-precious stone : the lapis lazuli, or azure stone. Imported from Afghanistan and requiring a long and difficult process of extraction, the pigment was mainly intended for the painting of religious scenes in the illuminations of the Middle Ages and paintings from the 16th century.
In 1824, the French Society for Encouragement of National Industry launched a competition for the synthesis of a less costly ultramarine blue, which was won by Jean-Baptiste Guimet in 1828. It was not until around 1970 that the origin of the colour was fully elucidated because, unlike other mineral pigments which owe their colour to a metallic element, it is the trisulphide radical anion that explains the colour of ultramarine blue. The sagacity of chemists has rarely been put to such a test, both to understand the origin of the colour and to perform its synthesis.
Egyptian blue : the first synthetic blue pigment Egyptian writings provide no information on the process used to prepare these pigments, blocks of which have been found. The research conducted at the Centre for Research and Restoration of the Museums of France have elucidated this process using various physico-chemical analyses.
A mixture of calcium, silica and copper compounds, heated to between 870 and 1100°C for several hours, leads to a compact and heterogeneous mass that contains blue crystals of cuprorivaite (CaO.CuO.4SiO2) as well as and silica in the form of quartz and tridymite crystals. The blue colour varies from dark blue to light blue depending on the heating conditions. Moreover, moderate grinding makes the blue colour lighter if the size of grains becomes less than 20 micrometers.
Azurite This is a basic copper carbonate 2CuCO3.Cu(OH)2 found as ore in copper mines. It was often used by painters until the 17th century.
Prussian blue : a stroke of serendipity This is a synthetic inorganic dye (ferric ferrocyanide : Fe4[Fe(CN)6]3) discovered by chance in Berlin in 1710 and often used for dyeing. Its fixation on a metal oxide such as alumina makes it a pigment for paint. However, it dose not belong to the great blue pigments.
Cobalt blue This synthetic pigment (CoO.Al2O3) has been widely used in oil paintings from the 15th century (under the name German Azur). In 1802, Louis Jacques Thénard developed a process of synthesis using cobalt phosphate. When it is ground in oil, this pigment has a faint blue colour. However, mixtures with white pigments give very beautiful colours.
Phthalo blue : pour imiter les pigments anciens This magnificent pigment of the 20th century was discovered by accident in 1938 by the Scottish company Scottish Dyes Ltd (now Imperial Chemical Industries). Research led to the marketing In 1934 of phthalo blue or monastral blue, a copper phthalocyanine. This organometallic pigment is highly resistant to light and to temperature or humidity changes. Moreover, it offers the possibility of imitating ancient pigments, particularly to produce glazes.
Klein blue : less original than it seems Just like his illustrious predecessor Pablo Picasso, the Nice-born painter Yves Klein went through a blue period, notably with his monochrome works from the 1950s and 1960s. With the help of a young chemist, Édouard Adam, he created a blue pigment made of ultramarine blue incorporated in an original liquid paste, the binder of which is neither oil nor water. The International Klein Blue was registered in 1960 at the National Institute of Industrial Property.
Laboratoire de photophysique et photochimie supramoléculaires et macromoléculaires, CNRS-ENS Cachan
- Peintures San en lumière (extrait)
Réalisation : Luc Ronat - Production : CNRS Images (2010)
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