White pigments hold a very significant role in the painter’s palette; they are very frequently applied as a ground layer, in many cases combining multiple white pigments together, or used plain or mixed with other pigments to produce several shades of colours. A short summary of the most common historical white pigments is given in the following paragraphs. Additionally, some basic information on the common types of binding media is provided.
Lime white, CaCO3, is probably the oldest white pigment in the world. It most commonly consists of chalk, a white pigment of limited hiding power, mainly used for painting grounds; under ordinary conditions it is a stable pigment . Chalk is a very pure limestone, formed during the cretaceous period, of fine calcite crystals consisting mainly of fossil remains of the shells and skeletons of microscopic plankton. It can also contain small amounts of other minerals, usually quartz and clay minerals. There are many deposits throughout the earth that are exploited commercially.
Kaolinite, Al4[Si4O10](OH)8, is named after the original type locality of Kao-Ling, China and occurs most commonly as compact earthy masses of microscopic crystals or as larger pseudohexagonal tabular crystals. Pure forms of the mineral are white, although the presence of impurities (such as iron and magnesium) may colour it grey or yellow. Kaolinite is common worldwide and forms as secondary deposits usually from the decomposition of feldspar group minerals. Alternatively, it can form as the final alteration product after illite and montmorillonite, if sufficient water is present .
Kaolinite is well known for its use in the manufacture of porcelain, as well as in the manufacture of paper and paint . Members of the kaolinite sub-group have been identified at rock art sites in various locations throughout the world [27,28,29].
Lead carbonates were extensively used as cosmetics by ancient Egyptians, Greeks, and Romans; this application of lead carbonates continued up to 18th c. AD . Additionally, from the Roman period onwards, lead white has been by far the most important of the white pigments used in Europe. The term can generally be used for any lead-based white pigment and can even be extended to describe lead chloride oxides, lead phosphates and lead sulfates; it usually refers to lead carbonate hydroxide, 2PbCO3.Pb(OH)2 .
Lead carbonate hydroxide is chemically equivalent to the naturally occurring hydrocerrusite, which is extremely rare and therefore rarely used as a pigment source. During the Roman period, Vitruvius in the 1st c. BC and Pliny in the 1st c. AD describe the synthesis of this mineral, called “cerussa” at the time, by steeping lead in vinegar . The processes used by the Romans are the basis of the medieval to 17th c. technologies for lead white manufacture. A thorough description of the most common production methods of lead white until the early 20th c. is given by Eastaugh et al. .
The pigment is permanent, relatively stable in all media (but particularly when used in oil). On the contrary, it was rarely used in fresco, where lime white was substituted. It was commonly adulterated with other white pigments, particularly chalk, baryte and kaolinite. The use of lead white has been widely reviewed [indicative publications: 32, 33].
The concern over lead poisoning increased during the Industrial Revolution. Although lead white continued to be marketed, alone and combined with other pigments, its presence sharply decreased in the first half of the 20th c., from nearly 100% to less than 10% by 1945 and it was widely abandoned in the period after World War II .
In the second half of the 18th c. an effort began to find viable substitutes for lead white, in response to the increasing awareness of the health hazards entailed in its manufacture and use . The search was oriented in particular towards investigating zinc oxide (ZnO) for this purpose. Although known since antiquity as a by-product of brass production and for its medicinal properties, this substance was seemingly never used as a pigment [33, 35]. The development and refinement of methods for producing a non-poisonous white pigment from zinc metals or ore, and the improvement of the material itself, lead to the production of a pigment, which was first commercialised by Winsor & Newton in 1834 under the name Chinese White and marketed as an artists’ pigment in a watercolour medium. Shortly thereafter it was improved for use in oil, when Leclaire succeeded in improving its drying and covering properties, inferior until then to the also still less costly lead white. By 1859, zinc oxide white, which had a further advantage over lead-based pigments because it did not blacken in the presence of hydrogen sulfide, was being produced on an industrial scale in Europe and the United States.
Besides zinc oxide white and kaolin, which was still being used primarily for grounds and as an additive and filler, several barium-based materials (such as barium sulfate, BaSO4) were also proposed as artists’ pigments, either in the form of natural ground barytes, or the more pure synthetically produced pigments such as blanc fixe . The reduced hiding power of these substances, however, tended to restrict their use in the paint industry as extenders for lead, zinc, and later, titanium compounds, or as a base for lakes. These pigments were introduced commercially in the early 19th c.; after 1950, although they were still available both in their natural and synthetic forms, they sharply declined in production for artistic purposes.
Another white material developed in this period was the composite pigment lithopone (BaSO4.ZnS), which was obtained by precipitating zinc sulfide with barium sulfate. It was first produced according to patents granted in France around 1850 . Initially, the most common lithopone pigments were produced using equivalent solutions of barium sulfide and zinc sulfate, which ended up producing a mixture of approximately 29.5 percent zinc sulfide and 70.5 percent barium sulfate. However, purer and higher strength varieties have since been manufactured, with varying ratios of the two ingredients; two grades of lithopone, known as gold and bronze seal, contain between 40 and 50 percent zinc sulfide, resulting in a pigment with much more hiding power and tinctorial strength .
Despite the cheapness of its manufacturing processes and good properties as pigment, lithopone had the tendency to darken when exposed to sunlight. In the late 1920s, a small amount of cobalt, varying from 0.02 to 0.5% of the zinc content, was added prior to the calcination process to prevent discoloration. Nevertheless, the usage of lithopone as an artists’ pigment was difficult to establish and it was primarily used as a cheap extender for other white pigments, like ZnO .
The elementary metal titanium, never found unbound from other elements in nature, was first discovered in England in 1791 by William Gregor in the iron titanium oxide ilmenite, and by M.H. Klaproth also in rutile ores in Germany in 1795, who confirmed it is a new element and gave it its name. Before that, the use of titaniferous minerals for the production of pigments is not attested, though in Mayapan site (Yucatan Peninsula, Mexico) a small amount of nanocrystalline TiO2 particles (in rutile form) has been consistently detected using phase mapping in red pigment samples . During the 19th c. several attempts were carried out to produce coloured pigments from natural ground titaniferous minerals, including titanium dioxide, with limited success. Most of these attempts focused on the most common crystallographic form of TiO2, rutile; of the other two forms, anatase and brookite, the former was used rarely and the latter does not seem to have ever been used directly as a pigment .
The first successful attempts to manufacture superior quality synthetic products took place practically simultaneously in Europe and the US in the early 20th c. The first commercially successful composite pigments of anatase were obtained in the 1920s, through precipitation of anatase titanium dioxide onto a barium sulfate base. Despite the fact that the rutile form was much less rare and was known to possess better hiding power and weathering properties than anatase, difficulties in its refinement continued to privilege the latter. Only in 1937 were successful methods for producing synthetic rutile developed, with the pigment becoming widely available in the market in the second half of the 20th c. .
The initially high cost of titanium dioxide whites shortly diminished, as various drawbacks typical of the early manufacturing methods were overcome. Combined with the high quality of the produced pigment, titanium white almost completely supplanted the other white pigments by the middle of the 20th c. .
A binding medium is the film-forming component of paint. A pigment should not dissolve in the binding medium nor be affected by it. However, it is well established that different binding media can strongly affect the paint, especially after ageing. Additionally, binding media, which are usually organic substances, can significantly hinder the identification of pigments, by altering their signals as determined by various analytical techniques.
Before the Middle Ages, artists used pigments in beeswax (encaustic) melted and manipulated with hot rods. Since the early Middle Ages, paintings on wood panels have traditionally been produced in egg tempera. Egg yolk is a semi-opaque medium: it can be used translucent or nearly opaque.
During the Renaissance, egg tempera was gradually supplanted in popularity by oil paint on canvas. In oil painting, the most popular oil for binding pigment, thinning paint and varnishing finished paintings is linseed oil. The linseed oil comes from the flax seed, a common fiber crop. Linseed oil tends to dry yellow and can change the hue of the colour .
Safflower, walnut and poppy seed oils are sometimes used in formulating lighter colors like white because they “yellow” less on drying than linseed oil. They have the drawback of drying more slowly and may not provide the strongest paint film . Poppy seed oil, for example, takes 5–7 days to dry, compared to 3–5 days for linseed oil.