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Ochre Oxford Ochre Stone Ochre Di Palito &c Are Native

earths, consisting chiefly of silica and alumina in combination with
iron, which latter forms the principal colouring matter. They are among
the most ancient of pigments, and their permanency is proved by the
state of the old pictures. In a box of colours found at Pompeii, and
analyzed by Count Chaptal, he discovered yellow ochre purified by
washing, which had preserved its original freshness. They may all be
produced artificially in endless variety as they exist in nature, and
are all converted by burning into reds or reddish-browns. Several ochres
are found in the natural state of so very fine a quality, that they
require no other preparation than that of being washed. Their colours
may be imitated to a certain extent by means of iron alone, uncombined
with silica and alumina; but such ferruginous preparations are not
equally durable, and as their chemical action is stronger, they are more
likely to affect those pigments which are damaged by iron. It often
happens in colours that one component of weak stability, or powerful
for evil, is strengthened and held in check by another; thus in the case
of the ochres, the silica and alumina by keeping a tight hand on the
iron, both ensure its safety, and prevent it injuring others.


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