The process of creating synthetic iron oxide had been recognized since at least the 15th century, although it wasn't until the middle of the 19th century that large-scale production commenced. Early manufacturing techniques involved heating iron filings, either by dissolving them in aqua regis (a combination of hydrochloric and nitric acids) and subsequently roasting the resulting iron salt, or by directly heating martial vitriol (iron sulfate).
By the mid-18th century, these man-made pigments were collectively referred to as Mars colours.
Importantly, in the context of art authentication and conservation, the 18th-century surge in industrial chemistry ushered in an era where previously elusive materials became both economically feasible and easily obtainable. The advent of sulphuric acid, employed as a textile bleach, facilitated the commercial production of Mars pigments. In contemporary manufacturing processes, precise engineering allows for the production of colours tailored to specific hues.
The colour's hue and tinting strength are influenced by factors such as hydration, particle size, and the inclusion of additives like manganese.
As the 18th century drew to a close, apprehensions regarding the risks associated with lead white became a matter of grave concern. Produced in vast quantities, lead white was not only the predominant white pigment in artists' materials but also found widespread use as a colourant in various industrial applications.
Zinc, with its origins tracing back to fluffy deposits of zinc oxide observed by the Ancient Greeks, saw its metal isolation in India and China around the 10th century. The adoption of this process spread to Persia, where zinc was referred to as tutiya (meaning smoke), a name inspired by the appearance of white vapour during the smelting of zinc ore.
Despite its early history, zinc did not find widespread use as a pigment until 1834, when it was introduced as a watercolor pigment under the name Chinese white. In artists' paints, zinc white exhibits a cold, flat tone compared to lead white and lacks the opacity characteristic of titanium white. Consequently, it is often recommended as a mixing white, seamlessly blending with other colors without overpowering them. This transition from lead white to zinc white marked a pivotal moment in the history of pigments, and an important shift for art conservators, scientists and authenticators alike. The transition addressed concerns of toxicity and providing artists with a safer and more versatile alternative.
The dark-hued mineral ilmenite, abundant in iron and titanium, caught the attention for its potential uses as a pigment when titanium was identified in 1791. In the late 19th century, synthetic titanium dioxide first emerged, finding initial applications in ceramic glazes to enhance opacity and acid resistance.
The purification process involves treating it with sulphuric acid to create a sulphate solution, followed by hydrolysis to yield a white precipitate of hydrated titanium dioxide. Roasting this precipitate in a furnace produces the final white pigment. Despite initial resistance in the paint industry due to limited supply and higher costs compared to lead white, the slow acceptance of titanium white gained momentum in the 1920s. This shift was accelerated by laws restricting the use of toxic lead-based pigments, making titanium white an evident choice for the modern age.
In contemporary manufacturing, titanium dioxide is produced through a chloride process and boasts a diverse range of applications, including decorative paints, plastics, and printing inks. It stands as the most widely used pigment in history.
Although titanium white was first introduced in 1921, it was not until the 1940s that it became widely available for artists. From this perspective, it has been a key pigment in identifying a number of important forgeries, not least Sotheby's privesale of a Frans Hals forgery, Portrait of a Man for $11.75 million in 2011.
