synthetic inorganic pigments
Synthetic inorganic pigments are created through chemical manufacturing rather than by grinding and washing clays or minerals taken directly from the earth. The techniques for producing these substances on an industrial scale were developed after 1800, making them the first modern synthetic pigments of importance to artists.
The amazing story of these early industrial pigments is well told in Philip Ball's Bright Earth. Nearly all synthetic inorganic pigments were discovered or identified in the grand European flowering of inorganic chemistry that occurred in the century after 1750, when European industries sponsored intensive minerological and metallurgical research, and early chemists isolated and identified many new metallic elements cadmium, cobalt, chromium, zinc, manganese, magnesium, and so on. (These new puzzle pieces helped John Dalton to formulate modern atomic theory in around 1805.) Several synthetic inorganic pigments still used today, including iron blue, cobalt green, cobalt blue and zinc oxide, were discovered prior to 1800.
These manufactured pigment compounds generally have excellent chemical purity and color consistency, and are cheaper to buy and available in larger quantities than natural inorganic pigments. With very few exceptions, all inorganic pigments used in artists' paints today are industrially manufactured. (Some dry powder natural inorganic pigments are available from specialty pigment retailers.)
As an artist, your primary concern is to understand the generic attributes of these pigments across different manufacturers and different pigment hues (chemical or crystal variations) that is, to see paints as physical substances rather than as "colors". For example, the violet and blue ultramarines are typically granulating and moderately transparent; the many lemon yellow to deep red cadmiums are all powdery, permanent, opaque and quite staining; compounds made with mercury are poisonous and fugitive. The historical information can also help you to understand the rapid expansion in artists' pigments that occurred in Europe between the 18th and 19th centuries.
The following table presents the average pigment attributes for the most important synthetic organic pigments, based on all paint ratings in the guide to watercolor pigments.
Cadmium compounds. Although Franz Stromeyer recommended cadmium as an artists' pigment after he discovered the element in 1817, cadmium paints were not commercially available until significant deposits of cadmium were developed around 1840. (By 1830 the pigment was already used as a ceramics colorant in towns near German zinc mines; cadmium and zinc ores were often found together.) The first cadmium sulfide paints (with hues ranging from middle yellow to light orange) were marketed around 1842, and lighter shades of yellow produced by adding zinc sulfide were introduced a few years later. Cadmium deep orange, scarlet and red, creating by adding increasing amounts of selenium (cadmium sulfoselenide, PO20 and PR108), were described in patents of 1892 but not marketed in artists' paints until 1910. (The same hue range can be produced by adding mercury to produce cadmium mercury sulfide, PO23 and PR113, using a process invented in 1948; these pigments are highly toxic and not used in art materials.) Today the cadmium pigments range in hue from a bright, whitish lemon yellow (PY35) through middle and deep shades of yellow (PY37) and orange (PO20), to scarlet, red, and deep red (PR108). The hue shifts result because the included metals expand (selenium, mercury) or contract (zinc) the average lattice dimensions within the cadmium crystals, placing the reflectance boundary at longer ("red") or shorter ("green") light wavelengths. Pigment is manufactured by exposing a solution of cadmium chloride or sulfide, and salts of zinc, barium or selenium (to adjust the hue), to hydrogen sulfide gas, then washing, filtering and calcining the precipitate that results. Cadmiums are saturated, semiopaque to opaque (they become less opaque as they dry), lustrous, dense and powdery pigments, expensive but extremely permanent in pure form and if not mixed with lead or iron oxide paints. If the pigment contains even small quantities of impurities, it will darken (by formation of sulfides of iron or lead, especially when exposed to moisture) or lighten (especially in the yellows, by oxidizing from sulfide to sulfate). Pure cadmiums reflect no blue light at all, which makes for very luminous and saturated yellows, moderately bright oranges and scarlets, and rather dark reds that become increasingly dark and unsaturated in the deepest red shades. The reds makes very dark mixtures with other colors, especially blue greens. The hue of specific brands of paint can vary considerably from one manufacturer to another, as shown in this cadmium color key; opacity and color saturation vary as well because of extenders added to the paints in manufacture. Cadmiums are considered mildly to moderately toxic (especially the reds, which also contain selenium). Theoretically they might be eaten or inhaled (as a pigment powder, sprayed paint, pastel residue, or fumes from heated pigment), but there was not a single documented case of cadmium poisoning from artists' paints uncovered in a 1954 industry report, and I have not found any cases reported since then. Formulations that include more than 15% barium sulfate as an extender (denoted by the color index names PY35:1 or PR108:1) are less toxic, less expensive, and slightly less saturated than pure cadmiums. The so called cadmium green (PG14) is a chemically fused mixture of cadmium yellow and cobalt aluminum oxide; it is sometimes imitated in watercolors by physically mixing these separate pigments.
Chromium compounds. Chromium is a constituent of several green, yellow, orange and red pigments. The name (from the Greek chroma or "color") refers to the large color span of its compounds, noted when chromium was discovered in the mineral ore crocoite by Nicolas-Louis Vauquelin in 1797. Several warm hued pigments called "chrome" colors, first described by Vauquelin in 1804, were commonly used and highly valued during the 19th and early 20th centuries: chrome yellow (lead chromate, PY34, which in pure form has a middle yellow hue; the color is shifted toward lemon yellow by increasing admixture with lead sulfate), two shades of chrome orange (PO21 for the yellowish shade, and PO45 for the red shade), and chrome red (PR103). The many related pigments include barium chromate (PY31, too dull and opaque for use in watercolors), molybdenium chromate (molybdate orange, PO35), zinc yellow (the carcinogenic zinc potassium chromate, PY36, probably discovered in the 1820's and only occasionally used as a pigment after 1850), and strontium chromate (PY32, a bright light yellow that has been considered too opaque for use in watercolors). Hue variations within each of these pigments can be produced through differences in particle size or other added compounds. Use of chrome colors began to decline steadily in the 20th century because most shades contain lead and the light yellow shades are impermanent (they fade in light and blacken when mixed with sulfur pigments), and the introduction in the 1950's of more lightfast, silica encapsulated lead chromates did not reverse this trend. Most important, however, was the fact that the cadmium compounds were brighter, less toxic and much more lightfast (though more expensive) in the same hue range. All that remains in modern use is chrome titanium oxide (PBr24), a moderately saturated medium yellow that is both lightfast and an appealing creamy orange color. However chromium provides two important and very lightfast pigments in the greens. The hydrous (water containing) chromium oxide, commonly known as viridian (PG18, vert émeraude in France), is a moderately saturated, weakly tinting, granular, transparent and moderately staining blue green, discovered and produced in limited quantities from a secret process by the Parisian colormen Pannetier and Binet in 1838. The process was publicly disclosed and commercially applied to the manufacture of artists' colors by Charles-Édouard Guignet (Guignet's green) around 1859. The anhydrous (water free) chromium oxide, usually sold as chromium oxide green (PG17), is an unsaturated, smooth, highly opaque and staining yellow green the primary pigment in camouflage paints that was known since 1809 but produced as an artists' color by Pannetier only in 1862. Increasing the average particle size of PG17 shifts the color from a grayish yellow green toward a darker blue green. The pigment is manufactured by heating chromium salts in the presence of boric acid, soaking in water (to produce the hydrated form, viridian), then grinding and washing to remove residue salts. (The historical colors green cinnebar or chrome green were mixtures of strontium chromate or lead chromate with prussian (iron) blue.) Chromium is also used in cobalt compounds such as cobalt green deep (PG26) and the green shades cerulean blue and cobalt turquoise (PB36).
Cobalt compounds. Cobalt produces the most diverse range of pigments currently available in artists' paints: its colors range from cobalt violet (PV14 and PV49) through several shades of cobalt blue (PB72, PB73, PB74, PB28, PB35), two shades of cobalt turquoise (PB36 and PG50), several varieties of cobalt green (PG19, PG26 and PG50), and cobalt yellow (PY40) even a grayish cobalt black (PBk13). Cobalt blue has been used since antiquity in porcelain or glassware, and as a pigment (smalt) since the late Middle Ages. The first modern cobalt paints date from cobalt green (PG19), discovered around 1780 by the Swedish chemist Sven Rinmann, but not used as an artists' color until around 1835. Other hues followed shortly: cobalt blue (cobalt aluminum oxide), isolated chemically in 1777 by Gahn and Wenzel, but first synthesized in 1802 by Louis Thénard and manufactured commercially from 1804; cerulean blue (cobalt tin oxide and cobalt chromium oxide), discovered by Andreas Höpfner in 1805, but not used in an artists' paint until 1860, when it was sold as coeruleum by George Rowney in the UK; cobalt yellow (potassium cobaltnitrite), discovered in 1831 by Nikolaus Wolfgang Fischer and again independently by Émile Saint-Evre in 1851, and originally known as Fischer's yellow before it was marketed as an artists' pigment in the late 19th century under the name aureolin; and two forms of cobalt violet the deep or bluish shade (anhydrous cobalt phosphate), developed in 1859 by Jean Salvétat to replace the poisonous light or reddish shade (cobalt arsenate), which was developed by Thénard around 1803 from the mineral erythrite. All cobalt pigments are manufactured by calcining (roasting at extremely high temperatures) a mixture of cobalt oxide with an alkaline carbonate and, to produce the various colors, compounds of other metals (phosphorus, aluminum, tin, chromium, titanium, zinc or potassium); the resulting matrix is then ground into a fine powder. Smaller pigment granules increase transparency and staining. (There is also a genuinely transparent but slightly less vivid cobalt blue pigment, consisting very small, tile shaped pigment particles, which has been used by Winsor & Newton in their artists' colors.) Cobalt pigments are powdery to slightly gritty or granular, relatively nonstaining, semitransparent, moderately saturated, and very permanent (with the exception of aureolin, which is somewhat impermanent to both light and moisture). All reflect a noticeable amount of "red" light, making for warm yellows, violets and blues, but rather dull greens and turquoises; the greenish compounds with chromium become steadily duller and more opaque as the proportion of chromium is increased. Cobalt paints are considered slightly toxic because they can cause sickness if inhaled or ingested in significant quantities.
Copper compounds. Historically copper is a major source of blue or green pigments, which only recently have been available as lightfast synthetic organic compounds. The natural mineral forms chrysocolla (hydrated copper silicate), malachite and azurite were known and used since antiquity. Cyan blue copper carbonate, called blue verditer or bice, is the synthetic form of azurite. It played a minor role in artists' colors in the 18th and 19th centuries (it has a relatively low tinting strength, and was more commonly used as a housepaint). Copper is more important as a constituent of several impermanent, synthetic inorganic green pigments. Verdigris (copper acetate, PG20) is an ancient pigment manufactured by exposing copper strips to acetic acid (vinegar); it fell out of use by the 19th century (not least because it was impermanent and would eat through cellulose). Scheele's green (PG22), the first modern synthetic green pigment, was discovered around 1775 by the Swedish chemist Carl Wilhelm Scheele; the hue ranged from pale yellow green to deep middle green. Schweinfurt green (copper acetoarsenite, PG21) is an intense, light valued, blue green compound of arsenic and verdigris; it was discovered independently in 1814 by the paint manufacturer Wilhelm Sattler and the chemist Ignaz von Mitis, and commercially manufactured from public recipes after 1822 under a variety of names Vienna green, King's green, Paris green, Mitis green, Parrot green, and (in English speaking countries) emerald green. All these synthetic copper compounds fell out of use by the mid 20th century because they have very poor permanency (they turn black through the formation of copper oxide, PBk15, especially when used with sulfur pigments), and because those containing arsenic are extremely toxic (Schweinfurt green was also sold as a pesticide "the only sure exterminator of the potato bug and cotton worm" reads one 19th century ad and was possibly the poison used to kill Napoleon).
Copper is also the metallic atom in the green azomethine pigments (PG117 and PG129), and the green and blue phthalocyanines, all described in the section on synthetic organic pigments. A couple of black pigments are known, made of the spinel crystal of copper chromite (PBk22 and PBk28), but these are not currently used in artists' paints.
Iron compounds. Iron represents an extraordinarily old, widespread and versatile family of relatively dull but extremely permanent and nontoxic pigments. All the common iron oxides have been used since antiquity (see under natural inorganic pigments) as the red, yellow and brown earth colors. Synthetic manufacture was known at least by the 15th century, though large scale production (and common use in artists' paints) did not emerge until the middle 19th century. 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 synthesized versions usually are much purer, have smaller particle sizes and greater tinting strength, and are much more opaque than the natural iron oxides of the same type. They can also be mixed to match precisely any yellow, orange, red or brown hue, including near black browns. For these reasons the natural pigments have been almost entirely replaced by synthetic oxide mixtures, currently marketed in watercolors under the names venetian red, english red, indian red, or light red (PR101) and mars yellow (PY42, PY119), but are now also used in hues formerly made of natural red (PBr7) or yellow (PY43) iron oxides. Several transparent iron oxides with an extremely small particle size and a dark, relatively rich color are now also available, often used as wood or leather stains and occasionally as artists' pigments. These complexities make the earth colors a category that you must explore, trial and error across different brands of watercolors, to find the hue range and handling qualities you prefer.
A more exotic iron pigment is iron blue (hydrous ferriammonium ferrocyanide, sometimes with sodium or potassium ions substituted for the ammonia ion) known to 18th and 19th century artists as Prussian blue, Berlin blue, Paris blue, Milori blue or Chinese blue (PB27). This is the first modern synthetic inorganic pigment, discovered by chance in Berlin (hence the name "Prussian" or "Berlin" blue) in 1704 when the colormaker Heinrich Diesbach attempted to make a crimson pigment called Florentine lake from cochineal, alum, ferrous sulfate and some borrowed potash that was contaminated with animal blood. Diesbach communicated the recipe to his pupil de Pierre, who began to manufacture it in Paris. (Hence "Paris" blue. The label "Chinese" derives from the use of iron blue in the blue patterns on Meissen china, manufactured near Dresden.) Held secret for two decades, the manufacturing process was published in England by John Woodward in 1724, but by then alternative methods of production had been devised and the pigment was being manufactured throughout Europe and in America. It has been used in watercolors since around 1730 and is still sold today, although most watercolor artists now seem to prefer the more intense phthalocyanine pigments. PB27 is a dark, unsaturated, staining, semitransparent and completely nontoxic middle blue; the color is sometimes adjusted by mixing with barium sulfate or alumina (which makes antwerp blue, a lighter, greener and less lightfast pigment). Iron blue is very reliable in pure form (both the ASTM reports and my own tests give it "excellent (I)" lightfastness), but it loses permanency if mixed with impurities such as potassium ferrocyanide or with other pigments such as titanium dioxide; my lightfastness tests demonstrated significant lightfastness variations across different watercolor paint manufacturers. Some descriptions of its quirks (for example, that it fades in masstone when exposed to strong light, but returns to its original color in darkness) have been uncritically handed down from the 19th century and are, as far as I can determine, myth. PB27 is currently manufactured as a precipitate from the reaction in solution of iron salts with sodium or potassium ferrocyanide, which is aged and oxidized to create the blue color. The pigment tends to agglomerate into rather stringy clumps that resist milling, but a special manufacturing method developed by BASF in 1982 (using the anodic oxidation of iron particles in hydrogen cyanide acid) produces very fine, pure and easily dispersable pigment particles with an atypically intense reddish color, valuable in printing inks. The pigment prussian green is a fused matrix of iron blue and lead chromate; cyanine blue is a mixture of iron blue and cobalt blue.
Lead compounds. Lead compounds have been discontinued from all watermedia paints because of their well established toxicity. They are also unsatisfactory because they oxidize easily and will darken when mixed with cadmium pigments. Historically important forms of lead are primarily red (lead tetroxide or red lead, PR105), yellow (lead tin oxide, known as massicot or lead yellow, used up to the mid 18th century) and white (lead carbonate mixed with lead hydroxide and/or lead oxide, PW1; and lead sulfate, PW2) pigments that have been used in Europe and China from antiquity up to the present day. Both lead carbonate (as flake white or cremnitz white) and lead antimony (naples yellow, PY41) have marvelous pigment attributes. Lead white is probably the most important white pigment in the history of painting, and is still favored in oil paints for its warmth, opacity and buttery handling (although all lead pigments were more popular prior to the 20th century). However, in watercolor paints these handling benefits are less noticeable, and do not compensate for the pigments's high toxicity if ingested or inhaled, and for the tendency of unprotected lead pigments to turn brown or black in the presence of sulfur (urban air pollution). The color of naples yellow, which can range from a pale yellow to a light reddish brown, is usually simulated with a mixture of yellow cadmium sulfide and red iron oxide, lightened with zinc white. However these mixtures are often fugitive; Winsor & Newton has replaced their earlier naples yellow hue with chrome titanate yellow (PBr24).
Magnesium compounds. Historically, magnesium is most important as magnesium euxanthate, commonly called peoli, gaugoli or indian yellow. It was probably introduced into India in the 15th century from Persia (modern Iran), and is prominent in Indian paintings of the Mughal period (late 16th to 19th centuries). The pigment was known in Europe as early as 1780, and used infrequently throughout the 19th century, mostly in watermedia paints. In daylight the fresh color fluoresces in "yellow green" wavelengths; this combines with a deep yellow pigment color to produce a unique luminescent, duotoned golden yellow hue. This synthetic inorganic pigment was made from the urine of cows fed on mango leaves; crystals of the concentrated dried urine were formed into balls and covered with mud for shipment to England, where some still reside in the Winsor & Newton pigment archives. The manufacturing process was not accurately described until 1886, after laws against cruelty to animals had been passed by the British regime in India (the mango leaf diet is inadequate nutrition for the cows). It's commonly reported that this process was banned entirely in 1908, but no modern source has actually found a copy of this edict; instead the pigment likely disappeared due to enforcement of statutes against animal cruelty passed in 1890. More to the point, demand for the pigment had by then largely disappeared, displaced by the new cadmium yellows, cobalt yellow (which often inherited the name "indian yellow") and gamboge. Winsor & Newton discontinued the pigment in watercolor paints in the 1920's, but their substitute indian yellow (listed under PY153) provides a close color match. Modern tests show that original indian yellow has good lightfastness in a gum arabic vehicle, though the fluorescing yellow color fades after moderate exposure to light. In modern pigments magnesium is important primarily in the formation of spinel (magnesium aluminum oxide, MgAl2O4), an octahedral crystal lattice in which the atoms of magnesium or aluminum (or both) can be replaced by other metal ions (chiefly titanium, iron, zinc, manganese, copper, cobalt, nickel or chromium) to produce a wide range of whitish, highly durable turquoise, green, yellow or red pigments.
Manganese compounds. An important pigment mineral, currently found mostly as a secondary component of blue, green and iron oxide pigments. The most important modern pigment is manganese violet (manganese ammonium pyrophosphate, PV16), which was developed by E. Leykauf in 1868 as a replacement for the more expensive cobalt violet, but not offered as an artists' pigment until 1890. It is a moderately saturated, granulating, semitransparent and nonstaining purple, not as intense as the cobalts or modern synthetic organic violets, but very lightfast, and available in several brands of watercolor paints. Manganese blue (barium manganate, PB33) is a weakly tinting, moderately saturated, granular, semitransparent and nonstaining greenish blue, a near perfect cyan hue for the subtractive "primary" mixtures. The date of its origin and first use as an artists' pigment has been difficult for me to confirm: some sources claim it was discovered in 1907, and Mayer states that "barium-manganate compounds of this type have been known to chemists and color makers since the nineteenth century," but a patent on economical methods of manufacture was not granted until 1935 (this synthetic form was used primarily to tint concrete and ceramics), and it was apparently not widely available as an artists' pigment until the 1950's. It is much less popular as a pigment today than it was decades ago (when it was hardly popular at all), not least because it is relatively polluting to manufacture; the principal pigment sources now operate in Asia. Currently only two paint manufacturers (Lukas and Old Holland) still offer manganese blue in watercolors, though many offer a "manganese blue hue" made from a cyan phthalocyanine. Manganese carbonate mixed with calcium carbonate (PW18) is chalk white, used in cheap watercolor paints and sometimes as a whitener for gouache. Manganese also forms some black or dark gray pigments: "bog manganese" or magnesium black (manganese dioxide, PBk14), and the magnetically flocculating manganese ferrite (PBk26).
Mercury compounds. The earliest mercury pigment is natural mercuric sulfide, found in the mineral cinnebar, the primary ore of mercury, and used as a pigment since Roman times. Synthetic mercuric sulfide or vermilion (PR106) was widely used in European oil painting up until 1850's. The color is a bright, opaque scarlet red. It is not a satisfactory watercolor pigment by modern standards because it is highly toxic (mercury will cause severe metal poisoning) and fugitive (it quickly withers to a dull brown in the presence of light). Modern substitutes ("vermilion hue") have been formulated of cadmium scarlet (PR108). Iodine scarlet (mercury iodide), discovered by Nicolas-Louis Vauquelin in 1814 and sold in England under the name scarlet lake, is a brilliant orangish red unfortunately very fugitive, extremely toxic, and worthless as a pigment. (Interestingly, the name "scarlet lake" was subsequently applied to other scarlet pigments, all of them also relatively impermanent.)
Sulfur compounds. Sulfur is a constituent of many pigments, including arsenic, cadmium, mercury, chromium and lead. It has always been a troublesome chemical, because of its tendency to blacken other metallic pigments it is mixed with.
The most poisonous of these pigments arsenic sulfide or orpiment, mercury sulfide or vermilion, and lead sulfate are no longer used in artists' materials, as they are both exceedingly toxic and unacceptably fugitive.
For watercolorists, the most important sulfur pigment is sodium aluminum sulfosilicate or ultramarine blue (PB29), which is chemically identical to the principal pigment in natural lazurite. The artificial form was first noticed by J.W. von Goethe in 1787 as a residue in Italian lime kilns near Palermo. A method to produce it artificially (and at less than a tenth of the cost of natural lazurite) was independently devised in 1828 by Jean-Baptiste Guimet (France) in response to a prize of 6000 French francs offered for its development and Christian Gmelin (Germany). By 1830, it was being produced on a commercial scale and immediately adopted in artists' colors. (Winsor & Newton began selling it in 1832, and J.M.W. Turner was an early adopter.) It is a dark, moderately intense, semitransparent and staining pigment varying in hue from a greenish blue to a blue violet. According to Andrew White at Holliday Pigments (UK), ultramarine is manufactured by heating a finely powdered mixture of china clay (kaolin), soda ash (sodium sulfate and/or carbonate), charcoal, silica and sulfur in closed crucibles for about 21 days, up to a temperature of about 750°C after 7 days; this results in a green, glassy matrix that turns deep blue if it is exposed to oxygen as it cools. The matrix is then crushed, washed, dried and ground. The final hue depends both on the exact proportion of ingredients and the amount of heating, and all ingredients must be free of iron. All hues are very lightfast. Ultramarine violet, developed in around 1878 by Johann Zeltner, comes in two shades, an unsaturated red violet and slightly more saturated blue violet (both are indexed PV15). These are made by mixing ultramarine blue with sal ammoniac (blue shade) and heating for several hours at 150°C. There is also an ultramarine green (PG55, no longer produced commercially and different from ultramarine blue GS), and a rarely used delicate pink hue (PR259), produced by heating the ultramarine pigment with dry hydrochloric acid.
Titanium compounds. Titanium dioxide is the supreme white pigment: no other compound matches its scattering properties (opaque whiteness), chemical stability, and lack of toxicity. Nearly 5 million tons were manufactured in 2000, mostly by factories in Asia and Australia; because of its extremely high refractive index it is the principal opacifying and color brightening ingredient in a wide range of architectural and crafts paints. Two manufacturing processes are used: both involve sifting or grinding natural titanium sands or recycled titanium slag, soaking the raw materials in a heated, reducing acid or chloride bath, separating out impurities through a sequence of chemical reactions followed by centrifuging or filtering, and grinding to finished particle size. Available in several oxide crystal forms, of which rutile and anatase are the purest. Whiteness and opacity increases in smaller particle sizes, which are also more costly to manufacture. In watercolors, titanium oxide (PW6) comes in a brilliant white and (from Daniel Smith) an off white or buff formulation. A limited range of colored pigments are created by substituting different metal atoms for the titanium atoms in rutile crystals: these include nickel (PY53) and chromium (PBr24). A beautiful range of light valued cobalt greens and turquoises has been created by incorporating cobalt and titanium atoms into crystals of magnesium aluminum oxide (or spinel, PG50); these pigments have a characteristic whitish hue that is integral to the pigment crystal and highly permanent.
Zinc compounds. Zinc sulfide was developed in 1850 as a white pigment used in oil based paints, especially as a compound with barium sulfate (lithopone). In artists' paints lithopone has been almost entirely displaced by the titanium oxides. Zinc oxide (PW4), a byproduct of copper smelting, was known to the Romans (who called it cadmia), but they used in primarily as a skin ointment (it is still used today for sunblock and sunburn). Around 1750, the German chemist Cramer discovered how to produce it by burning metallic zinc, and the French colorist Guyton de Morveau suggested it as a pigment in 1782. However, it was not widely adopted by artists until 1834, when Winsor & Newton marketed an especially dense form (manufactured by burning the metal at much higher temperatures) as "chinese white." Victorian watercolorists used it extensively both as a brightening foundation coating on watercolor papers, and as an opacifying additive to transparent watercolors. It has entirely replaced lead white in watercolors and acrylics, although it tends to dry to a brittle film in oils. Zinc also combines with cobalt to produce a lovely deep blue cobalt color (PB72). Finally, zinc green, PG16, consists of zinc chromate with ferrous ferrocyanide (iron blue).
Synthetic inorganic pigments are among the most lightfast watercolor pigments available. Some of them are also the most expensive pigments you can buy, which means that manufacturers often add fillers and brighteners to reduce the amount of pigment used in artists' paints.
Buy a small tube of a specific synthetic inorganic pigment from several manufacturers, to learn the variations in quality across different brands. Use a tinting test to determine the quantity of pigment in the paint. Cobalt blue, cadmium red, ultramarine blue and viridian are excellent selections to find out whether a manufacturer is really committed to making a high quality product.
The authoritative source on synthetic inorganic pigments is Industrial inorganic pigments, edited by Gunter Buxbaum (Wiley, 1998). A summary of the information (by most of the same authors) is available in Ullmann's Encyclopedia of Industrial Chemistry (Wiley, 2000), available at any good chemistry library. Historical pigment information for natural inorganic pigments is scattered across several sources. A good starting point is the four volume Artists' Pigments: A Handbook of Their History and Characteristics edited by Robert Feller (v.1), Roy Ashok (v.2), Elisabeth West Fitzhugh (v.3) and Barbara Berrie (v.4) (Oxford University Press, 1994-2001). (Ullmann's also has a chapter on "Artists' Colors.") If you can manage to read German, then the pages on Alte Pigmente (up to c.1780) and Moderne Pigmente at Volkert Emrath provide an interesting, gallery style overview (with pigment microphotographs).