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OCHRES

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Originally appearing in Volume V19, Page 989 of the 1911 Encyclopedia Britannica.
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OCHRES , a class of See also:

pigments varying in See also:colour from yellow to red, and consisting mainly of hydrated See also:iron See also:oxide. The Yellow Ochres are native earths coloured with hydrated ferric oxide, the brownish yellow substance that See also:colours, and is deposited from, highly ferruginous See also:water. These ochres are of two kinds—one having an argillaceous basis, while the other is a calcareous See also:earth, the argillaceous variety being in See also:general the richer and more pure in colour of the two. Both kinds are widely distributed, See also:fine qualities being found in See also:Oxfordshire, the Isle of See also:Wight, near See also:Jena and See also:Nuremberg in See also:Germany, and in See also:France in the departments of See also:Yonne, See also:Cher and See also:Nievre. The See also:original colour of these ochres can be modified and varied into browns and reds of more or less intensity by calcination. The nature of the associated earth also influences the colour assumed by an ochre under calcination, aluminous ochres developing red and See also:violet tints, while the calcareous varieties take brownish-red and dark-See also:brown hues. The well-known ochre Terra da Sienna which in its raw See also:state is a dull-coloured ochre, becomes when burnt a fine warm See also:mahogany brown See also:hue highly valued for See also:artistic purposes. Yellow ochres are also artificially prepared—See also:Mars Yellow being either pure hydrated ferric oxide or an intimate mixture of that substance with an argillaceous or calcareous earth, and such compounds by careful calcination can be transformed into Mars See also:Orange, Violet or Red, all highly important, See also:stable and reliable pigments.

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