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  Paint and Varnish Facts and Formulae by J. N. Hoff, 1905    

Metallic brown, natural Indian reds, Venetian reds, purple oxides, ochres, siennas and umbers come under this head, and are various combinations, consisting mainly of iron oxide and silica or clay found in nature.

Metallic brown, an iron oxide much used, is prepared after being mined by simple roasting and grinding. It contains from 50 to 75 per cent, of oxide of iron, the balance being clay and silica. In the case of the ochres, raw siennas and raw umbres, the earth is simply dried, washed, ground and floated; whereas, burnt sienna and burnt umber is produced by first roasting the raw product to the desired shade, or depth of color, and subjecting it to the further treatment above described.

Other red oxides are similarly treated. These natural earth pigments are very stable and permanent and should be preferred wherever possible, for tinting, or as bases where lead or zinc is not indicated.

Some of the strongest and best toned ochres are produced in England, but those most generally used in this country are imported from various parts of Europe. France, notably, sends us the so-called Rochelle ochres. These ochres vary in quality according to the locality from which they come, and the care given them in their preparation for the market. The difference between yellow ochre and the various red ochres, or red oxides is a chemical one. The color in every case is due to the iron they contain.

The difference being that, in the case of the yellows, the iron oxide exists in combination with water, hence, these ochres are called hydrated iron oxides; while the red ochres are anhydrous, contain little or no water. If yellow ochre be roasted, therefore, it becomes red or dark brown, as the moisture is driven off. This is the case also in the formation of burnt siennas and burnt umbers. The various tones of yellow ochre depend upon the varying amount of clay or silica present, and the greater or less percentage of combined water they contain.

Yellow ochre has been used for centuries in painting and decorating. It is, to all intents, a permanent pigment, and has no appreciable effect on other pigments, except, perhaps, a few of the very sensitive lakes, which latter are too fugitive to be used. It is seldom adulterated because of its price, and the fact that there are vast quantities of cheap ochre obtainable. Its yellowness is sometimes artificially improved with turmeric or other vegetable yellows, or by the admixture of chrome yellow, notably in producing so-called golden ochre.

By pouring ammonia water mixed with alcohol over the suspected sample, such adulterations can usually be detected. If pure, the liquid will not discolor, otherwise, it will be stained. Ochres ground in oil are largely adulterated with barytes to save linseed oil, as pure ochres are light and very absorbent. Italy has famous sienna deposits of beautiful tone and texture, and the umbers come from Turkey and southern Europe.

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