Online Encyclopedia


Online Encyclopedia
Originally appearing in Volume V21, Page 599 of the 1911 Encyclopedia Britannica.
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GROUP IV. Hydrates or Hydroxides.—Several native earths belong here, notably yellow ochre, raw umber, raw sienna and Cappagh brown. These substances owe their colours mainly to hydrates and oxides of iron and of manganese, but the presence of a colourless body such as white clay or barium sulphate is usual with the paler pigments. A false yellow ochre from Cyprus is really a basic ferric sulphate, and does not properly belong to this group. Besides the yellow and brown pigments, there is a magnificent deep green pigment in this group, known as emerald oxide of chromium or viridian. The blue copper preparation which goes under the name of bleu lumiere and mountain blue, a very unstable pigment, is also essentially a hydrate, though by no means pure. It should be stated that all the earthy or native hydrates belonging to this group contain water in two states, namely, hygroscopic or loosely-attached and constitutional. Before grinding them in oil, the reduction in the amount of the hygroscopic moisture by means of a current of dry air or a gentle warmth often improves the hue and working quality of these pigments.
End of Article: GROUP IV

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