Rust

Synthetic manufacture of iron oxide was known by at least the 15th century, though large scale production did not emerge until the middle of the 19th century.

Historical methods of manufacture included the simple heating of iron filings, by the dissolving of them in aqua regia (a mixture of hydrochloric and nitric acids) and roasting the resulting iron salt, or by heating iron sulphate (‘martial vitriol’) directly.

From the mid-18th century synthetic iron oxides were called Mars colours, a literal translation of ‘crocus martius’, the latin designation given by alchemists referring to artificial pigments made with iron oxide.

The real impetus for the production of Mars com-pounds came in the 18th century, when sulphuric acid became an important commercial item, especially as a bleach for the textile industry.

Modern methods of manufacturing allow the colour of the product to be tailored to order. The purest and finest oxides are produced from the precipitation and hydrolysis of iron salt solutions; hue and tinting strength are affected by hydration, particle size and by the presence of additives such as manganese.

Because the manufacturing processes for iron oxide pigments can be exactly controlled, the modern versions are usually purer, have smaller particle sizes, greater tinting strength and are much more opaque than the natural ochres with which they are chemically similar but have silica and other impurities present.