NATURAL IRON OXIDE PIGMENTS -3 # Ochers
Post 408 ⇒ by Gautam Shah →
Ochre is an iron oxide pigment of natural and synthetic sources. Ochers are coloured soft deposit of clays often with mixed layers or pockets harder crystalline iron ore. Some of the best yellow ocher’s are mined at Roussillon, Southern France. Here the mineral formations are naturally stained with colours to provide a wide variety of earth or natural iron oxide colours.
The Greek word ochros, for Ochers describes it to a pallid or pale yellow, but natural ochers are brilliant colours. Ochers have a colour range that varies from yellow to deep orange or brown, due to the hydrated iron oxide. This is unlike the Red oxide which is from hematite powder, a form of iron oxide (Fe2O3). An ochre containing a large amount of hematite has a reddish tint, and is known as ‘red ochre’. The dominant yellow colour of ochre is due to the mineral limonite. Ochers are of two kinds, one with an argillaceous or clayey basis has richer colours, whereas the other with a calcareous base is slightly of ‘flatter’ colours. The nature of the associated minerals affects the colour, such as calcareous varieties have brownish-red and dark-brown shades, and aluminous types offer red and violet tints.
Different colours of Ochre pigments are extracted from different veins, and then mixed to obtain specific shades. Other shades are created by roasting (‘burnt’ or calcination), and dehydrating the mineral clays.
Yellow Ochre is a very ancient pigment. It is without any trace of green. The oxide colours are called Earth colours, due to their richness, brightness and warmth. Ochers are mixed with high refractive whites like Lime (or zinc, titanium dioxide), or low refractive ‘extenders’ such as the barytes to achieve, respectively, high opacity or translucency.
In Ancient Greece, red-ocher was called miltos, (hence Miltiades red-haired or ruddy). In Athens when assembly was called, everyone was supposed to attend it, and failure to attend it incurred a fine. To prevent people loitering around slaves swept the open space of the Agora with ropes dipped in miltos . It was also known as raddle, reddle or ruddle. In Ancient Egypt, the ochre was often used in place of gold, which was considered to be eternal and indestructible. It was used for painting tomb interiors in place of toxic orpiment (an orange-yellow coloured arsenic sulfide mineral). Ochre was used for painting women’s faces. Romans used the yellow ochre to to represent gold, skin tones, and as a background colour in their paintings such as the murals of Pompeii.
A rational process for refining ochre pigment was developed by the French scientist from Roussillon province of France, Jean-Étienne Astier (1780s). He washed the clay to separate the grains of sand from the particles of ochre. The decanted and dried ochre was crushed, sifted, and ground as the pigment. Best of the qualities were used for artists’ colours.