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Bole Almagra Sil Atticum Terra Sinopica &c They Are Rather

hues and tints than definite colours, or more properly belong to the
tertiary, semi-neutral, and broken colours. As a rule they are native
pigments, found in most countries, and very abundantly and fine in our
own; but some are products of manufacture, and obtainable in the variety
of nature by art.

The colouring matter of these earths is the red oxide of iron, as that
of the yellow ochres is the yellow oxide. All the yellow ochres are more
or less reddened by being burnt, as yellow oxide of iron itself becomes
red on calcination. It was observed in the fourth chapter that time has
often the effect of fire, more or less intense; and hence it is that
yellow ochres occasionally assume a buffish-red hue, by the gradual
peroxidation of the iron. Similarly, if a yellow ochre be but partially
calcined, the red so obtained is apt to deepen or darken. Especially do
these changes take place when the iron oxides are not associated with an
earthy base; when, in fact, the so-called ochres cannot be classed as
such. In this case, too, as was lately remarked, the pigments are more
chemically active, and more likely to affect those colours to which iron
is inimical.


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