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
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
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