NATURAL IRON OXIDE PIGMENTS – 1
Post 406 – by Gautam Shah
Natural Iron oxide pigments are some of the most commonly available colourants, nearly everywhere on earth. There are as many regional, as there are local colour variants, offering a vast palette of colours ranging from nearly black, to red, brown, etc. Natural Iron oxide pigments are very stable (chemical reactivity), sunlight fast (UV radiation), nontoxic, and remain unaffected by moisture or heat (up to 550C for red oxide and up to 105C for yellow oxides). Iron oxides are resistant to oils do not react readily with solvents.
Iron-oxide pigments are extracted from many different types of ores such as Hematite (reds), Goethite and Limonite (Yellow-browns), Siderite (brown-reds), and Magnetite (blacks). Red hues are attributed to hematite Fe2O3, Yellows to hydrated oxides of iron, and Browns to MnO2 or black impurities. The pigments consist of Ferrous or Ferric oxides, with impurities such as clay, silica, calcium carbonate, manganese, etc.
Iron oxide Yellow pigments are based on iron oxide hydroxides are referred to by many different names, including goethite, limonite, raw sienna, Mars yellow, yellow earth, yellow ochre, yellow iron oxide, yellow hydrated oxide and iron hydroxide pigment. Iron oxide Red pigments are based on anhydrous iron oxide, and referred to as hematite, red oxide, rust, red ochre, burnt sienna, Mars red, etc. Iron oxide brown colours are mainly based on Manganese oxide or pyrolusite are brown yellow oxide, ochre, umber, burnt umber, etc. But many colours are more known by the locations of the extraction, as seen by names such as Venetian red, Sinopia, Turkey red, Indian red, Spanish red, Pompeian red, Persian red, etc.
Oxide colours have retained their fascination mainly due to the stability and consistency. Red and Yellow oxides were two shades that have survived in caves for nearly 70000 of years. The advantage of red oxide offered was against the most commonly known red colour of the blood. But colour of the blood turns brown on drying and is highly susceptible to biological degradation. Blood colours are translucent and are not comparable to red of the oxide colours. Black was used in cave paintings, but its source the carbon (lamp) black offered very fast, intense and indestructible shade. Even the occasional (in few regions) use of Black iron oxide is no match for it. Black colour was also made by reacting Iron with an acidic substance. Another natural red colour was the Indian vermilion, made by reacting turmeric with an acid.
Oxide colours were used for body decoration. Pigments grinding equipment believed to be between 350,000 and 400,000 years old has been reported in a cave at Twin Rivers, near Lusaka, Zambia. The technology of refining by way of cleaning, washing, sieving and grinding, and calcining is very ancient. The naming of oxide colours as raw or burnt for ochre, umber or senna, was familiar to all artists for ages. By heating or calcining the oxide colours became intense or rich.
Before the Industrial Revolution, the range of colours available for art and decorative uses were limited. The range consisted earth or mineral pigments or colours of biological origin. At places colourants from unusual sources such as botanical materials, animal waste, insects, etc. were used. The colour range was sought to be enlarged by purification, mixing with other colours and chemical treatments.
It was realized by artist that the range of hues and tones offered by Iron Oxide colours is limited. During the industrial revolution period efforts began to produce very large and consistent quantity of synthetic equivalents.