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The amateur painter, Roger Dewhurst, recorded the use of Indian yellow in 1786. He noted, in letters to friends, that it was an organic substance made from the urine of animals fed on turmeric and suggested that it should be washed to prepare it for use as a pigment. Its source remained a mystery for many years. Mérimée, in his book on painting of 1830, didn't believe it was made from urine, in spite of its odor. George Field believed it was made from camel urine.

It was not until 1886 that the Journal of the Society of Arts in London embarked on a systematic inquiry of the pigment known as purée of India. An investigator began his search at Calcutta. He was sent to Monghyr, a city in Bengal. There, he found a small group of cattle owners who fed their cows on mango leaves and water. The cows' urine was a bright yellow. They were extremely undernourished as they only received normal fodder occasionally. Other Indian dairy cattle farmers of the same caste despised these 'colormen' and limited their production. They were reportedly producing one thousand to fifteen hundred pounds of the pigment per year but the investigator doubted the production figures when he saw the small number of cows involved (Harley 1970, 105).

In order to prepare the urine as a pigment, this genuine magnesium euxanthate (Cl9H16OllMg.5H20) was heated until sediment was formed which was strained, pressed into lumps by hand and dried. The extensive trade between India and England made it generally available in England and was considered an English specialty (Harley 1970, 55-56). The rounded lumps were brown or dirty green on the outside and a brilliant yellow-green on the inside. The cleaner, more golden yellow varieties were greater in value (Doerner 1934, 66).

Indian yellow was used in both oil and watercolor painting. It was favored for its great body and depth of tone. It had a peculiar characteristic in watercolor for it faded in artificial light and in the dark but was fairly stable in direct sunlight (Field 1885, 134). In 0il, it dried slowly, as it required one hundred percent for grinding; the addition of varnish improved its drying (Doerner 1934, 66). It could be mixed with all other pigments but in oil its lightfastness was improved when isolated between layers of varnish (Wehlte 1982, 95).

Under the microscope, Indian yellow appears as crystalline (Gettens and Stout 1966, 119). It is decomposed by hydrochloric acid. When burned, it should leave white ashes, as many organic substances do (Doerner 1934, 66).

Law prohibited the production of Indian yellow in the early years of the twentieth century. Wehlte adds that its departure may have been due to the Indians for whom the torture of sacred animals was against their religion. It also may have been due to British laws that prohibited cruelty to animals (Wehlte 1982, 95).

Scheele's green and Schweinfurt green would become virtually obsolete by the end of the nineteenth century. The preparation of zinc oxide at the end of the eighteenth century made the development of cobalt green, also known as zinc green, possible. The Swedish chemist, Rinmann is credited with developing a process for making a compound of cobalt and zinc in 1780 that he published with the Stockholm Academy of Sciences. Arthur Herbert Church published Rinmann's process in his book, The Chemistry of Paints and Painting (London, 1901) (Gettens and Stout 1966, 109). According to Church, cobalt green (CoO.nZnO) was made with the compounds of oxides of zinc and cobalt by mixing them "with an alkaline carbonate" (Harley 1970, 77-78), and then exposing the mixture to strong heat. After washing the sediment that resulted, the pigment was ready to grind. The pigment was always bluish-green in spite of the ability to widely vary the proportion of zinc to cobalt oxides in production. The compound that is formed is chemically joined (Harley 1970, 77-78).

Cobalt green was a semi-transparent, moderately bright green. Most sources cited considered it to be absolutely permanent as most pigments produced at high temperatures are (Gettens and Stout 1966, 109). However, tests made in 1847 and published in 1910 showed a browning of the color in full-strength and a fading of it when mixed with lead white. The colormaker, Blockx, added that the date of the tests bears certainty that the green was made by Rinmann's process (Blockx 1910, 72).

Cobalt green appears as fine, rounded and transparent particles under the microscope. It becomes reddish when mixed with hot acids or alkalis (Gettens and Stout 1966, 109).

Artists did not favor cobalt green although it could safely be mixed with all other pigments and was a fast drier in oil. The poor tinting strength and high cost of cobalt green kept it in limited use. Field called it, "chemically good and artistically bad" (Field 1885, 141).

The next cobalt pigment to be introduced to the artists' palette was cobalt blue. Although smalt, a pigment made from cobalt blue glass has been known at least since the Middle Ages, the cobalt blue established in the nineteenth century was a greatly improved one (Mayer 1970, 67).

The isolation of the blue color of smalt was discovered in the first half of the eighteenth century by the Swedish chemist Brandt. In 1777, Gahn and Wenzel found cobalt aluminate during research on cobalt compounds (Harley 1970, 53). Their discovery was made during experimentation with a soldering blowpipe (Wehlte 1982, 148). The color was not manufactured commercially until late in 1803 or 1804.

The Minister of the French government, Chaptal, appointed Louis Jaques Thénard and Mérimée to look into the improvement of artists' colors. Thenard developed this new cobalt blue by his observations at the Sevres porcelain factory. He experimented with roasting cobalt arsenate and cobalt phosphate with alumina in a furnace (Laurie 1926, 94). He published his results in late 1803-4 in the Journal des mines, "Sur les couleurs, suives d'un procédé pour préparer une couleur bleue aussi belle que l'outremer."

Thénard tried the blue in oil and gum media and by the time his report was published, the color had not changed after a two-month exposure test. Production began in France in 1807 (Harley 1970, 53). Most sources cited regard Thenard as the inventor of the blue. However, Leithner of Vienna is also mentioned as one who developed cobalt arsenate as early as 1775 (Wehlte 1982, 148).

Cobalt blue (Co0.A1203) was generally regarded as durable in the nineteenth century. It requires one hundred percent of oil to grind it as an oil paint otherwise its cool tone can turn greenish due to the yellowing of linseed oil. To avoid the yellowing, Laurie suggested that it be used as a glaze color or mixing it with white. It is totally stable in watercolor and fresco techniques. Field called it a "modern, improved blue" (Pavey 1984, 23). John J. Varley, author of List of Colours (London, 1816), recommended it as a good substitution for ultramarine blue for painting skies (Harley 1970, 54).

Cobalt blue has coarse particles, like azurite and ultramarine, genuine but is distinguished microscopically by their non-crystalline appearance. It is chemically insoluble and unchanged, even in strong hydrochloric acid (Laurie 1914, 49).

Forty years passed before another cobalt pigment was developed. Although little is recorded on the history of cobalt yellow (or Aureolin), all sources cited credit the discovery of this potassium and cobalt compound to N. W. Fischer in Breslau in 1848. Gettens and Stout cite J. G. Bearn (The Chemistry of Paints, Pigments and Varnishes, London, 1932) for the method of manufacturing cobalt yellow. Mixing cobalt made potassium cobaltnitrite (CoK3 (NO2) 6.H20 salt with a concentrated solution of potassium nitrite. The sediment which results must be thoroughly washed or the pigment will not be stable (Gettens and Stout 1966, 109-110). It was first introduced as a pigment for artists' use by Saint-Evre, Paris in 1852 (Mayer 1970, 49).

Laurie and Blockx consider cobalt yellow chemically illogical for a permanent pigment. Laurie, however, refers to tests made by Captain Abney and Professor Russell who showed that it was reliable in watercolor (Laurie 1926, 89-90). Blockx tested it in 1879 and by the time his book was published in 1910, he found that it withstood strong sunlight. He added that it must be carefully manufactured or it will brown in oil. He considered it the only color that could reasonably replace Indian yellow (Blockx 1910, 66-67). It was known for its good mixing quality with all other pigments and for particularly good tints in watercolor. At high magnification, tiny crystals and crystalline clusters appear (Gettens and Stout 1966, 110). It is unchanged in acids but will turn black in sodium sulfide and brown in caustic soda (Laurie 1914, 54).

The remarkable range of pigments that could be produced with cobalt included cobalt violet, known since 1859. Salvetat first described the preparation of cobalt violet, dark in Comptes Rendus des Seances de l'Academie des Sciences XLVIII in an article titled, "Matieres minerales colorantes vertes et violettes." The dark variety is anhydrous cobalt phosphate (CO3 (P04) 2) which was made by mixing soluble cobalt salt with disodium phosphate. It was washed and then heated at a high temperature. The light variety, developed in Germany in the early nineteenth century and is anhydrous cobalt arsenate (CO3 (ASO4) 2). The light variety was particularly poisonous because of its arsenic content.

Both of the cobalt violets were considered to be very permanent but the light variety could change in oil due to the yellowing of linseed oil. They were both compatible with all painting media. Their transparency, weak tinting strength and high cost limited their use but their fastness to light made them more desirable than the older organic dye violets.

Cobalt violets appear as irregular-shaped particles and particle clusters under the microscope and are largely unaffected by chemical tests (Gettens and Stout 1966, 109).

The last new pigment to be produced with cobalt in the nineteenth century was caeruleum blue. Although Höpfner introduced it as early as 1821, it was not widely available until its reintroduction in 1860 by George Rowney in England (Wehlte 1982, 149). Its name was derived from the Latin word caeruleum, meaning sky or heavens (Pavey 1984, 40). Caeruleum was used in classical times to describe various blue pigments (Gettens and Stout 1966, 103).

Like cobalt green, blue and smalt, it was made by the action of heat on cobalt oxide with other metallic bases (Field 1885, 147). Caeruleum is cobaltous stannate (CoO.nSnO2) and is made by mixing cobaltous chloride with potassium stannate. The mixture is thoroughly washed, mixed with silica and calcium sulfate and heated (Gettens and Stout 1966, 103).

This variety was a fairly true blue (not greenish or purplish) but it did not have the opacity or richness of cobalt blue (Field 1885, 147). It was not recommended for use in watercolor painting because of chalkiness in washes. In oil, it kept its color better than any other blue and was particularly valuable to landscape painters for skies (Doerner 1934, 81).

Microscopically, its particles are fine, rounded and uniform. It is not affected by strong chemicals (Gettens and Stout 1966, 103).

After the new cobalt pigments, the next major group of pigments to be developed in the nineteenth century were derived from chrome. This orange-colored mineral was first observed, although not identified, in 1770 in the Siberian Beresof gold mines (Paint and Painting[1982], 92]. In 1797 a Parisian chemist, Louis Nicholas Vauquelin isolated natural lead chromate (chrocoite) and called it chrome because of the range of hues that could be derived from it (Harley 1970, 92). He named it after the Greek word xpwua, meaning color (Gettens and Stout 1966, 142).

The gold mine was not in continuous operation so that Vauquelin's study was slow until a new source of the mineral was found in the Var region of France during the early years of the nineteenth century.

The preparation of chromates of lead, specifically chrome yellow (2PbSO4.PbCrO4) was published by Vauquelin in the Annales de Chimie IXX in 1809. A solution of soluble lead salt (acetate or nitrate) was added to potassium chromate. Varieties of yellow, orange and red could be produced with slight adjustments of the solution (Harley 1970, 93). Orange required that the solution be treated with a caustic soda (Laurie 1926, 89). Red necessitated a strong solution of potassium chromate (Gettens and Stout 1966, 106).

Chrome yellow was greenish in hue and deep chrome was a more brilliant, deeper yellow (Field 1885, 124). Chrome orange was extremely bright and Field described it as "rank" (Harley 1970, 118). Chrome red was also known as scarlet chrome and was a cold, red hue. It was the least popular of the new chrome pigments due to a tendency towards dullness when mixed with lead white (Doerner 1934, 75). It was not listed in the color catalogues of Winsor and Newton after 1842 (Harley 1970, 119).
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