Chemistry of Synthetic Organic Pigments

Synthetic organic pigments are carbon based molecules manufactured from petroleum compounds, acids, and other chemicals, usually under intense heat or pressure. The techniques for producing these substances on an industrial scale were invented after 1860, which created the modern era of consumer color. Chemical and industrial innovations increased at an astonishing pace through the end of the 19th century and have continued up to the present.

The amazing story of these early pigments is well told in Philip Ball’s Bright Earth. Aniline, an oily, poisonous liquid chemical extracted from the distillation of coal tar compounds (themselves byproducts of coke and coal gas production), was the jumping off point for a wide range of synthetic organic pigments and early pharmaceuticals such as aspirin and synthetic quinine. The first of these colorants was mauve, a beautiful purple dye developed by Sir William Perkin in 1856.

Many other dyes created from coal tar (and using ingredients other than aniline) soon followed, including the first artificial production of a natural dye — alizarin, one of two colorants found in natural madder — synthesized in 1868 by the German chemists Carl Gräbe and Carl Lieberman, and still sold today as alizarin crimson (PR83) and red violet mixtures made with it.

Unfortunately, the aniline pigments as a group tend to be very fugitive (they have poor lightfastness), and by the end of the 19th century paint manufacturers were labeling paints "permanent" to assure artists that their pigments were not aniline derived. To the extent aniline based paints were adopted by 19th century painters such as Vincent Van Gogh or Edgar Degas, their art has suffered drastically as a result. Nevertheless, from the research into these early dyes evolved modern organic chemistry and economic methods of colorant synthesis, which provide all the synthetic organic pigments in use today.

Why are so many modern pigments synthetic organics? Organic simply means "containing carbon atoms," and the key to organic pigment chemistry lies in carbon’s amazing ability to combine with itself in a great variety of atomic structures — rings, chains and branches — including the most important and basic, the benzene ring of six interconnected carbon atoms. These structures in turn can attach to each other or to a variety of other atoms or chemical compounds (especially of nitrogen and hydrogen), to produce almost limitless molecular variations. Out of this chemical diversity comes a large number of molecules with intense color attributes: of these, the least toxic, most permanent and most economically manufactured are used as colorants.

Many synthetic organic pigments, especially the azo pigments, are derived from water soluble dyes. Dyes by themselves bind chemically to materials in a way that prevents them from being edited after they are applied. In addition, cellulose fibers, the primary ingredient of paper and canvas (as well as cotton clothes), rarely accept a dye without the action of a third chemical, called a mordant, to mediate the bond between dye and cellulose fiber. These properties make dyes impractical as an artists’ material.

So how can they be used in paints? The dyes are laked — bonded chemically to a colorless, transparent, insoluble salt that often acts as its own mordant — which turns the dye into an insoluble pigment. The gum arabic in watercolors then binds this complex but chemically stable pigment to the paper. The most common base for modern laked pigments is aluminum hydrate (aluminum hydroxide, also called transparent white), also commonly used as an extender in oil paints; more opaque lakes are made with barium sulfate (the natural mineral barytes whose synthetic forms are called barium white, permanent white or blanc fixe) or kaolin (hydrated aluminum silicate, also known as china clay or pipe clay); modern synthetic organic dyes can also be laked onto other relatively inert pigments, especially oxides of chromium, iron, tin or other metals, to increase the color of the finished product. Pigments from the Romans through the 18th century were laked on calcium carbonate (chalk) or aluminum potassium sulfate (alum).

There several important groups of water insoluble, crystal pigments that do not require laking, including the phthalocyanines, quinacridones and vat dyes such as the perylenes and anthraquinones. (Vat dyes are normally applied to fabrics presoaked in vats of a mordant solution.) Many of these are important as printing inks because of their very small particle size.

Synthetic organic pigments are manufactured to be very potent and are often the most saturated and strongest tinting colorants available for any specific hue. However, they can be expensive to manufacture, and their lightfastness can change dramatically depending on particle size, crystal form, or the type of substrate used in laking. Many are pigments where lightfastness ratings from different manufacturers and the ASTM most disagree, perhaps a sign of this fundamental variability.

Many synthetic organic pigments form agglomerations or clumps during manufacturing. In some cases this clumping can be broken down by intensive milling before the pigments are made into paints, but more often clumping or particle aggregating is controlled through the methods of manufacture (using acids or mechanical grinding) and by additives in the pigment solution. The ease with which pigments can be milled or kept as individual particles after manufacture is called its dispersability. Many pigments have such low dispersability that agglomerates cannot be broken down by milling, so they must be treated immediately after they are made to prevent the pigment from clumping, or kept in a dispersing solution until the pigment is actually used in manufacture.

Synthetic organic pigments can also take on a variety of crystal modifications (especially in the phthalocyanines and quinacridones), and these can have very different color and lightfastness characteristics, although all are grouped under the same color index name.

It’s worth noting that, for at least the last three centuries, artists were not of any economic significance in the discovery and development of pigments. About half the total volume of synthetic organic dyes and pigments produced around the world is used in printing inks; another quarter is used in architectural paints; and the rest for coloring textiles, plastics, automotive finishes, ceramics, enamels, papers, cements, candles, foods, cosmetics and pharmaceuticals. Art materials manufacturers buy what they can use from the scraps and remnants of the industrial pigment consumption driven by mass consumer products.

Synthetic organic pigments are fabricated from a limited number of elements. I’ve illustrated some pigment groups with a schematic molecular image (structure image) so that you can see the essential molecular form of the dye. In these schematics, the atoms are represented by these symbols:

The color creating aspects of the molecule depend on the chromophore, a pairing or grouping of atoms that create a complex and shifting cloud of electrons across the electron shells of two or more atoms. These electron transitions permit efficient absorption of specific light wavelengths, which creates a color that is the visual complement to the absorbed light (a compound absorbing in the "blue" and "violet" or short wavelengths appears to have a yellow color; a compound absorbing in the "green" wavelengths appears to have a purple color, etc.). Other groups of atoms called auxochromes influence the pigment color by altering the light absorbing capacity of the chromophores, usually in the long wavelengths. The most important chromophore groupings are shown in the figure below.

chromophore groupings important in synthetic organic pigments
in all cases the carbon, nitrogen or oxygen atom pairs are joined by a double valent bond (share two electrons)

Azo pigments form the largest, most diverse and most important group of synthetic organic colorants: of 336 currently manufactured synthetic organics pigments, 60% are in the azo family. All are created using the process of diazotization discovered by Peter Gries in 1862: an aromatic amine (an ammonia derivative that is linked to carbon and additional hydrogen atoms) is dissolved in a near freezing acid, then mixed with a solution of sodium nitrite. The explosively reactive products of this mixture are coupled with a wide range of other hydrocarbons to form the specific type of azo molecule. The process binds the carbon atoms into rings of six carbon atoms (benzine rings) and links these rings into complex chains with nitrogen and oxygen (hydrogen is present throughout to complete the structure). With the exception of the metal complex pigments, the "family skeleton" of azo compounds always contains one pair of nitrogen atoms joined by a double valent bond.

From this noxious brewery comes some of the brightest and most beautiful pigments ever discovered. Azo pigments can be made in almost any hue, but in practice the range is limited to the warm side of the color wheel: yellow, orange, red and brown. Better and cheaper blue and green pigments are available in the phthalocyanines, while the few violet azo pigments are impermanent.

As an artist, your major concern is to understand the average lightfastness and generic handling attributes of these pigments across different manufacturers and different pigment hues (chemical variations) — that is, to see paints as physical substances rather than as "colors". For example, both the blue and green phthalocyanines and the orange to magenta quinacridones are among the most transparent synthetic organic pigments available, although they can be quite staining; the phthalocyanines are also among the most lightfast. In contrast, cool indanthrone and the warm pyrroles are typically opaque and heavily staining too. (Note that lightfastness is strongly affected by the specific molecular form of a dye, by the finishing and laking process in manufacture, and by the particle size of the pigment: the average lightfastness ratings may lump together specific pigments with excellent or poor lightfastness, especially in large pigment families.) 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.

synthetic organic pigments in watercolors listed in order of decreasing lightfastness averaged across all pigment forms and hues
	Tr	St	Gr	Bl	Df	Lf
disazo condensation	2.5	3.5	0.8	1.5	2.3	4.0
pyrazoloquinazolone	1.0	2.5	0.0	2.5	1.0	4.0
metal azomethine	2.8	2.5	1.5	3.0	2.0	3.9
anthrapyrimidine	2.5	2.5	0.5	3.5	1.5	3.8
phthalocyanine	3.5	3.6	0.2	2.5	1.6	3.7
perylene	2.6	3.7	0.3	3.3	1.5	3.7
quinacridone	3.1	3.3	0.3	2.7	2.1	3.6
indanthrone	2.1	3.7	0.6	2.6	1.9	3.6
pyrrole	1.7	3.6	0.2	2.7	2.5	3.6
dioxazine	2.7	3.3	0.0	2.5	2.0	3.5
pyranthrone	4.0	2.0	1.0	2.0	1.0	3.5
nickel dioxine	3.3	0.8	0.0	3.3	1.5	3.5
quinophthalone	3.0	3.0	0.0	1.0	3.0	3.5
benzimidazolone	2.8	3.4	0.1	2.2	1.8	3.5
perinone	2.7	2.0	0.3	2.7	2.0	3.5
arylide/diarylide	2.4	3.0	0.0	2.5	1.6	3.5
isoindolinone	2.0	2.3	0.0	3.3	2.0	3.5
naphthol	2.6	3.3	0.3	2.2	1.4	3.4
thioindigo	3.0	4.0	0.3	2.7	2.0	3.3
anthraquinone	3.0	2.9	0.0	2.6	2.4	3.2
Key: Tr = transparency, St = staining, Gr = granulation, Bl = blossoming, Df = diffusion, Lf = lightfastness. For explanation of the pigment numerical ratings, see What the Ratings Mean.

The major chemical groups in the synthetic organic category are listed below, in the approximate chronological order of their introduction as commercial pigments.

Monoazo (arylide). A family of about 30 azo pigments, identified by the term arylide, providing almost exclusively yellow hues. Many (PY3, PY65, PY73, PY74, PY97 and PY98) are commonly marketed under the trademark name Hansa Yellow, first introduced in 1909. The two orange (PO1 and PO6) and red (PR211) monoazos are not used as artists’ pigments.

The structure of PY3 is characteristic: a pair of carbon rings, joined by nitrogen to a central cluster of four carbon atoms (note the double nitrogen atoms just left of center). Variations in the hue arise from additional atoms hung asymmetrically off the outer carbon rings. About 5 different monoazo pigments are commercially available in watercolor paints. In general the monoazos are serviceable and relatively inexpensive colorants, semitransparent with good tinting strength. Unfortunately they have only moderately good lightfastness in artists’ materials. They work very well as student paints, but always should be tested carefully for lightfastness before using in a major work. (The naphthol pigments are also sometimes classified as monoazo pigments, for example by the Colour Index International.)

Disazo (diarylide). Another family of about 30 azo pigments developed around 1940, identified by the term diarylide, and (like the monoazos) providing mostly yellow hues of special significance to the printing industries. The three orange hue diarylide pigments are relatively impermanent.

The structure of PY83 is representative: two identical arylide molecules, in opposing orientation, joined at the same carbon ring. Variations in the hue arise from differences in the atoms arranged around the outer (end) carbon rings. Diarylide pigments are very important in printing inks, but only one (PY83) is currently offered in watercolor paints. Although the diarylides are often more saturated and have higher tinting strength than the arylides, the doubling of the molecule unfortunately also significantly reduces the lightfastness. For that reason these pigments are generally not suitable in artists’ colors, especially watercolors.

Disazo Condensation. A small group of 17 azo pigments formed, like the diarylides, of two coupled arylide molecules: but these are joined in condensation with a bifunctional hydrocarbon molecule — hence the name. An industrially economical process to do this was discovered in 1951 by M. Schmid at CIBA, but use in artists’ paints has been very limited. Available hues range from yellow (PY93, PY95, PY128, PY166), orange (PO31), red (PR144, PR166, PR214, PR220, PR221, PR242, PR248, PR262), and brown (PBr23, PBr41, PBr42). The few available as watercolor paints are semitransparent, have high tinting strength, and are typically very lightfast (more lightfast than analogous monoazo pigments, though also more expensive).

Benzimidazolone. An important group of about 20 azo pigments with a broad range of hues, from yellow (PY120, PY151, PY154, PY175, PY180, PY181, PY194), through orange (PO36, PO37, PO60, PO62, PO72) and red (PR171, PR175, PR176, PR185, PR208) to a maroon of fair lightfastness (PV32), and a delicious brown (PBr25). Developed and patented by Hoechst in 1960, the benzimidazolones were first used as watercolor pigments in the late 1970’s. They are relatively expensive, but are also among the most durable pigments used in artists’ paints. (Winsor & Newton, for example, has chosen a benzimidazolone pigment for their "winsor yellow" and "winsor orange.")

The structure of PY151 is representative: a base structure very similar to the arylides, but with a charactertistic triangle of two nitrogen atoms and a carbon atom attached to the righthand carbon ring. Variations in hue arise from different arrangements of atoms attached to the lefthand (and sometimes the righthand) benzine rings. As a group the benzimidazolones are nontoxic, saturated, semitransparent and nonstaining, provide beautifully clear if somewhat bland colors (on all counts, cadmium pigments provide a revealing standard for comparison). For lightfastness alone the benzimidazolones should usually be preferred to comparable arylide or diarylide pigments in artists’ colors (although there are a few benzimidazolones with only good lightfastness, such as PY120). The widest range of benzimidazolone colors are available in watercolor paints made by Winsor & Newton, Rembrandt and Daniel Smith. Curiously, "benzimidazolone" is also the pigment name most often replaced by "permanent," "winsor," "azo" or some other marketing moniker (I especially like DaVinci’s "benzimida" as a nickname). To my ear, "benzimidazolone" is no harder to say (or remember) than "Benji, my dad’s alone!" and everyone seems to be getting along just fine with the label "quinacridone" ... but marketing prejudices about artists die hard — very hard.

Beta Naphthol. A relatively small group of azo pigments, among the oldest synthetic organic pigments, providing primarily red (toluidine red PR3, PR49, PR53, PR68) and a few orange (dinitraline orange PO5, PO17, PO46) hues. First produced around 1870, only a few of these pigments are still used today, and primarily for inexpensive applications because the pigments are cheap to manufacture and only moderately lightfast. (The 16 BON arylide pigments, with few exceptions all middle red to bluish red hues, are also acceptably lightfast when laked to manganese salts.) Most beta naphthols not sufficiently lightfast for use in watercolor paints and are now found only in student paints (Blockx stopped using them in their "professional" paints in 2006).

Naphthol. (Naphtol is a registered trademark of Hoechst AG; the generic label for the same compounds manufactured by other companies is naphthol, with a second h. The word is from the Greek for "mineral oil", and salutes the origin of these pigments in petroleum.) Developed and patented in 1911, the naphthol compounds represent the single largest group of azo dyes and pigments. (In fact, about 20% of all synthetic organics available, over 50 in the red category alone, are naphthol pigments.) Originally used as cotton dyes, they were soon laked as pigments and were first used in artists’ paints in the 1920’s. The most important group for artists is the naphthol AS pigments. The color range is concentrated in the long wavelength end of the spectrum, including warm orange (PO24, PO38), scarlet (PO5, PR188, PR261), many reds (PR2, PR3, PR5, PR7, PR8, PR9, PR17, PR22, PR112, PR150, etc.), carmines (PR23, PR146, the many pigments listed under PR170), maroon violet (PV13, PV25, PV44), and brown (PBr1).

The structure for PR112 (for artists, one of the most important naphthol AS pigments) is typical: two carbon rings linked by nitrogen to a central structure of two overlapping carbon rings. Variations in the hue arise from a different arrangement of atoms attached to both the lefthand and/or righthand carbon rings (in PR112, the three chlorine atoms on the left). Naphthols are nontoxic, often extremely saturated, and in watercolors are semitransparent and strongly staining pigments. The middle reds are especially brilliant: they are traditionally used to make lipstick. Lightfastness in watercolors varies from poor to very good, so it matters which specific pigments you choose.

Reputable watercolor manufacturers seem to accept PR112, PR170 and PR188 as "lightfast enough" pigments; PR112 and PR188 have held up well in my own lightfastness tests, but PR170 seems to me marginal and better left in the tube. Admittedly these are splendidly vibrant and sexy pigments, but if you worry whether your hot date has the permanency your mom requires, always do your own lightfastness tests.

Metal Complex. A small group of about a dozen azo pigments of marginal industrial significance, and (it seems) with an uncertain future in the world of artists’ pigments (production of PG10 and PO65 has been recently discontinued). All combine a symmetrical pair of carbon (organic) compounds with a metal atom (usually nickel or copper). Included in this group are the azomethine metal complexes. Colors range from green (PG8) to green gold (PG10, PY117, PY129), yellow (PY150, PY153, PY177, PY179), orange (PO59, PO65), and red (PR257, PR271). Most shades are rather dull or dark in masstone, but brighten significantly in tints.

The schematic for PY153 shows the basic organization: two hydrocarbons symmetrically attached to a single metallic atom (in this case, nickel). First developed around 1920, many metal complex pigments did not come on the market until the late 1940’s, but these compounds have proven to be unique, highly durable and reasonably lightfast artists’ paints. Most azomethines reflect a significant amount of green light, making them wonderful choices to mix muted, natural greens with the phthalocyanines (PG7, PG36 and PB15) and iron blue (PB27). They are nontoxic but may (depending on the metal atom used) irritate the skin, particularly after prolonged contact with the raw pigment powder.

Isoindoline & Isoindolinone. These are specialized forms of disazomethine pigments, in the azo group, first described in 1946 and first offered commercially in the 1960’s. Less than a dozen are available, in colors that range from yellow (PY109, PY110, PY139, PY173, PY185) to orange (PO61, PO66, PO69) and red (PR260).

The structure of PY110 is representative: a central carbon ring joined by nitrogen to two benzine rings, with attached chlorine and carbon atoms. Variations in hue arise from differences in the auxochrome atoms symmetrically attached to the end rings. These are extremely lightfast pigments, even in very pale shades and in thin applications. Although they are not widely used in artists’ paints at present, it’s likely that continued refinements on these compounds will create important new lightfast pigments for future artistic use.

Phthalocyanine. An extremely important group of modern dyes, chemically similar to the natural organic structure porphyrin (the basis of hemoglobin and chlorophyll). Independently discovered three times between 1907 and 929, in 1933 R. Patrick Linstead analyzed and named the blue molecule (from the Greek words naphtha and cyanine for "dark blue from mineral oil"), developed methods to manufacture it efficiently, and pointed out its excellent pigment potential. It was commercially introduced in 1935 under the name monastral blue; the green shades were introduced in 1938. (In France these pigments are sometimes called bleu anglais and vert anglais.) The phthalos form complexes with nearly every heavy metal atom (66 different metal complexes are known), but the compounds that matter to artists are made with a central copper or nickel atom, which form square, flat dye molecules.

The structure of alpha phthalocyanine blue (PB15:3) is representative: four carbon rings linked into a flat plate by carbon and nitrogen; the metal atom (in this case, copper) bonds to two of the four inner nitrogen atoms. The green shades, which are chemically less stable, form by replacing 15 of the hydrogen atoms on the outer carbon rings with chlorine (PG7) or chlorine and bromine (PG36) atoms. The individual dye plates can form chains or polymers by linking the copper atoms to each other through intermediate oxygen atoms; these form the pigment particles. Phthalo blues and greens have been available in artists’ paints since the 1950’s, but have only recently gained wide use among watercolorists. (The strongly staining character of these early phthalo blue paints was discouraging.) The colors used in artists’ paints range in hue from a reddish blue (PB15:1 or PB15:6) to greenish blue (PB15:3), cyan (PB17), turquoise (PB16), bluish green (PG7), and yellowish green (PG13, PG36); only the metal free form (PB16, a dull greenish blue) is a true synthetic organic pigment. All shades (but especially the greens) increase in chroma and tinting strength as average particle size goes below 0.15µm, which is achieved by finishing with acids or mechanical grinding. Phthalocyanines are indispensable pigments in the green part of the color circle: PG7 or PG36 are base ingredients for a wide range of convenience green mixtures. The natural scarcity of blue and green pigments is illustrated by the fact that phthalo blue is the most important blue pigment discovered since cobalt blue (1804) or ultramarine blue (1828); phthalo green is the most important green pigment since emerald green (1814) or viridian (1838).

Quinacridone. A large family of modern, moderately saturated and highly colorful pigments, repeatedly noticed in chemical research since 1896, but not recognized as useful pigments until 1955 (by W. Struve at DuPont, who also developed economic methods of manufacture). The first quinacridones were marketed in 1958 as automobile colorants and artists’ paints, which were promptly adopted by New York abstract expressionist painters. The available hues range from golden yellow (PO49), through reddish orange (PO48), middle red (PR209), coral (PR207), red (PV19), rose (PV19 and PV42), magenta (PR122, PR202), maroon (PR206), and a dark reddish violet (PV19). Nearly all quinacridones have excellent lightfastness ratings in watercolors.

The structure of beta quinacridone PV19 is characteristic: two pairs of oxygen and nitrogen atoms set in five (hence the "quin," for five) interlinked rings of carbon. Chemical variations arise from groups of atoms hung symmetrically from both sides of the molecule, which act both as auxochromes to modify the color and as complementary chemical bonds that link the quinacridone molecules into chemically more stable crystal chains. (The diagram shows the nitrogen/hydrogen and oxygen components of PV19 in these auxochrome locations.) For example, substitution of these auxochromes by methyl (CH₃) gives PR122 and by chlorine (Cl) gives PR202. Interestingly, a solution of quinacridone molecules typically is a pale yellow to orange color: the pigment color is actually determined by the particle size, the crystal modification, variations in the auxochromes, or by crystallizing together different quinicridone molecules, including quinacridone quinone or even other pigments (a proprietary pigment by Ciba-Geigy, PR N/A, is a mixed crystallized form of beta quinacridone with a diketo-pyrrolo pyrrole). Much of the color variation arises from differences in the way the quinacridone molecules combine into crystals, which can be altered through grinding with salts or heating in solvents. All quinacridones are nontoxic, mid valued, transparent and moderately staining pigments, and in watercolors become slightly more saturated in tints. They handle in washes and mix with other colors extremely well.

Among the miracles of modern industrial chemistry, such as the discovery of artificial ultramarine, aniline dyes and the phthalocyanines, one must include the discovery and development of the quinacridones, which provide pigments of superior lightfastness in a region of the hue circle plagued and poxed by a long history of fugitive organic pigments — in particular, natural organic pigments such as rose madder, alizarin crimson, and genuine carmine. This alone makes them a tremendous boon to artists. They also provide wonderful additions or substitutes to the yellow and orange range of "earth" colors, but unfortunately lack of demand for gold quinacridone (PO49) in the automotive industry has caused that pigment to disappear.

Perinone. A handful of important vat dyes that have been known since the 1920’s but have been available as pigments only since the 1960’s. Hues cover a relatively limited range, including perinone orange (PO43) and the somewhat dull perinone red deep (PR194). Perinone orange has good lightfastness in watercolor paints, and (when combined with aluminum flake pigments) makes a fine copper metallic paint.

Perylene. Described and used since around 1912 as vat dyes that are chemically related to the perinones, the perylenes were first manufactured and sold commercially as pigments in 1957. Available colors are limited to moderately saturated scarlets (PR123, PR149, PR190), reds (PR178), dark maroons (PR190, PR179, PR224, PV29), and a very dark green (PBk31).

The structure of PR149 is characteristic: a mesh of seven interlocking carbon rings, linked to two outer carbon rings by nitrogen atoms. Differences in color arise from modifications to these two outer rings. All the perylenes are nontoxic, mid valued, transparent and strongly staining pigments with very good to excellent lightfastness in watercolors (many are also used as automotive colors). Use of the perylenes by artists has been relatively infrequent so far, apparently because more saturated pigments are available in the same hue range.

Anthraquinone. A small group of about 10 pigments, most of them with a long history as textile vat dyes. They made dull, weak pigments until methods of purification, careful precipitation and grinding were discovered that retained most of the dye’s color brilliance. The group includes anthrapyrimidine yellow (PY108), anthraquinoid red (PR177) and my favorite, indanthrone blue (PB60).

Diketo-pyrrolo pyrrole. A small but very important group of new synthetic organic pigments, discovered in the early 1980’s and systematically developed into pigments with very good lightfastness. About six are currently offered, in the shades orange (PO71, PO73), scarlet (PR255), red (PR254) and carmine (PR264, PR274). Pyrroles have also been crystallized with quinacridones to produce hybrid pigments (PR N/A).

The structure of PR254 is typical: two carbon rings joined by a complex bridge of carbon, nitrogen and oxygen atoms. Variations in the color of DPP pigments arise from differences in the atoms hanging symmetrically off both ends of the molecule. All the pyrroles are nontoxic, extremely lightfast, semitransparent to semiopaque, and staining. They are very attractive substitutes for cadmium pigments in the same hues, as the mixing range of the DPP pigments is much broader than the cadmiums — though for the orange and scarlet hues, the saturation is less. (For a similar color range with slightly less saturation but with much more transparency, see the quinacridones.)

Dioxazine. A small group of chloranil derived colorants including one very important pigment, dioxazine violet (PV23 and PV37), developed in 1952 by Hoechst AG as a dye, and now used in plastics and automotive finishes to warm the color of phthalo blue. The pigment is obtained by dissolving the dye in a very hot acid, then washing and salt grinding the precipitate that results. The pigment exists in two crystal modifications, a red and a blue shade, whose hue can be modified by different methods of manufacture or grinding: both have the same color index name, are poorly distinguished by manufacturers, and are apparently confused in the lightfastness testing literature (see the comments under PV23 in the guide to watercolor pigments). Watercolor manufacturers consistently offer the blue shade, which is described "extremely lightfast" in the authoritative industry documentation but "fugitive" in many watercolor testing references. (I found "very good" to "excellent" lightfastness in my own watercolor tests.)

Triarylcarbonium. Two groups of triphenylmethane pigments, obtained by laking basic dyes. The useful colors are green (PG1, PG2, PG4, PG45), blue (PB1, PB2, PB9, PB10, PB14, PB18, PB19, PB56, PB61, PB62), red (PR81, PR169) or violet (PV1, PV2, PV3, PV27, PV39). Some shades are mildly fluorescing and are still used for their brilliance, especially as unlaked basic dyes (for example the rhodamine B, BV10, used in Holbein’s opera). Their lightfastness ranges from poor to worthless and no paint containing these pigments or any unlaked basic dye should be used in professional quality artworks. (Some paint companies, such as Schmincke and Holbein, offer these pigments in paints labeled "brilliant" that are intended for printing or photoreproduction work — that is, artwork that does not have to last very long.)

In addition to these important families of synthetic organic pigments there are several unique pigments in almost every color category. Most are not used in watercolor paints because they lack permanency, are costly, or do not perform better than a more common alternative pigment. However, the creative pace of modern industrial organic chemistry is unrelenting, so we’re likely to see more of these new compounds in paint lines of the future.

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