1911 Encyclopædia Britannica/Ochres
OCHRES, a class of pigments varying in colour from yellow to red, and consisting mainly of hydrated iron oxide. The Yellow Ochres are native earths coloured with hydrated ferric oxide, the brownish yellow substance that colours, and is deposited from, highly ferruginous water. These ochres are of two kinds—one having an argillaceous basis, while the other is a calcareous earth, the argillaceous variety being in general the richer and more pure in colour of the two. Both kinds are widely distributed, fine qualities being found in Oxfordshire, the Isle of Wight, near Jena and Nuremberg in Germany, and in France in the departments of Yonne, Cher and Nièvre. The 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 violet tints, while the calcareous varieties take brownish-red and dark-brown hues. The well-known Terra da Sienna which in its raw state is a dull-coloured ochre, becomes when burnt a fine warm mahogany brown hue highly valued for artistic purposes. Yellow ochres are also artificially prepared—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 Orange, Violet or Red, all highly important, stable and reliable pigments.