Anilines, or chemicals extracted from the distillation of coal tars (themselves byproducts of coke and coal gas production), were the original base for synthetic organic pigments. These new compounds produced some stunningly brilliant colors. The first of these was the dye mauve, developed by Sir William Perkin in 1856. Many others soon followed, including the first artificial production of a natural dye -- alizarin, one of two colorants found in natural madder -- synthesized in 1868 by C. Gräbe and C. Lieberman, and still sold today as alizarin crimson (PR83) or mixtures with it. Unfortunately, the aniline pigments as a group tend to be very fugitive (have poor lightfastness): all have been superseded by the more modern petroleum compounds.
Many if not most synthetic organic pigments are derived from water soluble dyes. Dyes by themselves bind 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), will not accept a dye without the action of a third chemical, called a mordant, to mediate the bond between dye and fiber. These properties make dyes impractical as an artists' colorant.
So how can they be used in paints? Lake pigments are dyes that have bonded to a colorless, insoluble salt that acts as its own mordant; the gum arabic then binds this complex but chemically stable pigment to the paper. Among the more common mordants used in laked pigments are alum (aluminum potassium sulfate, used to lake pigments since the Roman era) and transparent white (aluminum hydrate). Vat pigments are laked from dyes that are normally applied in vats to fabrics pretreated with the mordant.
Synthetic organic pigments are typically saturated and are often expensive. Their lightfastness varies, however, and several lake pigments are those where lightfastness ratings from different manufacturers and the ASTM most disagree, a sign that the quality of the laking process, or pigment particle size, can affect the pigment durability.
Most synthetic organic pigments form agglomerations or clumps during manufacturing. In many cases this clumping can be controlled through the methods of manufacture, otherwise the clumps must be broken down (by milling) before the pigments are made into paints. The ease with which this can be done is related to the pigment's 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.
Some synthetic organic pigments (such as the quinacridones) can also take on a variety of crystal modifications, and these can have different color and lightfastness characteristics, although they are all grouped under the same color index name.
About half the total volume of synthetic organic pigments produced around the world is used in printing inks; another quarter is used in paints; and the rest for coloring plastics, textiles, papers, cosmetics, and office products.
Synthetic organic pigments are all 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 pigment form. In these schematics, the atoms are represented by these symbols:
Azo pigments form the largest, most diverse and most important group of synthetic organic pigments: of 336 currently manufactured synthetic organics, 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.
The major pigments in the synthetic organic category include:
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 on the left). 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. 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' colors 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 artists' colors 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, 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' colors. (Winsor & Newton, for example, has choosen 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 pair of nitrogen atoms with 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) 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 monniker. 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. (I especially like DaVinci's "benzimida" as a catchy nickname.)
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, today only a few of these colors are still used, for inexpensive applications, primarily because they are cheap to manufacture and 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.) In artists' colors however most beta naphthols not sufficiently lightfast, and should be used only in student paints. (They are used in some artists' colors by companies such as Blockx that seem committed to mediocre quality products.)
Naphthol AS. (Naphtol AS is a registered trademark of Hoechst AG; the generic label for the same compounds manufactured by other companies is naphthol AS, with a second h.) Developed and patented in 1911, the naphthol AS formulations represent the single largest group of azo pigments (about 20% of all synthetic organic pigments available, over 50 pigments in the red category alone). Originally used as cotton dyes, the pigments were first used in artists' colors in the 1920's. The color range is concentrated in the long wavelength end of the spectrum, including warm orange (PO24, PO38), scarlet (PR188, PR261), many reds (PR5, PR8, PR17, PR22, PR112, PR150), carmines (PR23, PR146, the many pigments listed under PR170), maroon violet (PV13, PV25, PV44), and brown (PBr1).
The structure for PR112 (for artists, probably the most important naphthol AS pigment) 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 left and righthand carbon rings (in PR112, the three chlorine atoms on the left). Naphthols are nontoxic, often extremely saturated, semitransparent and strongly staining pigments. The middle reds are especially brilliant: appropriately, they are also used to make lipstick. Lightfastness in watercolors varies from very good to poor, so it matters which specific pigments you choose. Reputable manufacturers seem to accept PR112, PR188 and PR170 as lightfast pigments, and they've held up very well in my own lightfastness tests; if flash is what you're after, these are splendidly vibrant and sexy colors. (If you worry whether your hot date has the permanency your mom expects, 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' colors. 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.
Isoindolinone and isoindoline. 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 joining by nitrogen atoms two complex carbon rings, with attached chlorine atoms. Variations in hue arise from differences in atoms symmetrically attached to both rings. These are extremely lightfast pigments, even in very pale shades and in thin applications. Although they are not widely used in artists' colors 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 pigments, chemically similar to the natural organic structure porphyrin (the basis of hemoglobin and chlorophyll). Independently discovered three times between 1907 and 1929, R. Linstead analyzed and named the molecule in 1933, developed methods to manufacture it efficiently, and pointed out its excellent pigment attributes. It was commercially introduced in 1935 under the name monastral blue; the green shades were introduced in 1938. The phthalos form complexes with nearly every metal atom (66 different metal complexes are known), but the compounds that matter to artists are made with a central copper atom.
The structure of alpha phthalocyanine blue (PB15:3) is representative: four carbon rings linked into a flat disk 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. These molecule disks can form long chains by linking the copper atoms to each other through intermediate oxygen atoms. Phthalo blues and greens have been available in artists' paints since the 1950's, but have only recently gained wide use among watercolorists. (The tendency of the phthalo blues to stain intensely may have been the discouraging factor.) The colors used in artists' paints range in hue from a reddish blue (PB15:1) to greenish blue (PB15:3), pure 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 are affected by particle size, which is reduced by finishing with acids or mechanical grinding. Phthalocyanine is an indispensable pigment 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 ultramarine blue (1828) or prussian blue (1704); 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 but not used in artist's paints until the late 1980's. 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 ends of the molecule. Interestingly, a solution of quinacridone molecules typically is a pale yellow to orange color: most of the color variations we see in artists' pigments arise from differences in the way these molecules are combined into crystals, which form the actual pigment particles. The crystal form (and the color) can be adjusted by mixing together different quinicridone molecules, including quinacridone quinone or even other chemicals (a proprietary pigment by Ciba-Geigy, PR N/A, is actually a mixed crystallized form of beta quinacridone with a diketo-pyrrolo pyrrole), or by changing the crystal structure through grinding the pigment with salts or heating in solvents. All quinacridones are nontoxic, mid valued, transparent and moderately staining pigments, and peculiarly become slightly more saturated in tints. They handle in washes and mix with other colors extremely well, and very effectively replace many fugitive colors in the red and crimson part of the spectrum -- in particular, historical organic pigments such as rose madder, alizarin crimson, and genuine carmine. This alone makes them a tremendous boon to artists. But they also provide wonderful additions or substitutes to the yellow and orange range of colors.
Perinone. A handful of important vat dyes that have been known since the 1920's, but were laked to create pigments only in the 1960's. Hues cover a relatively limited range, including perinone orange (PO43) and the somewhat dull perinone red deep (PR194). Perinone orange has very good lightfastness in watercolor paints, and (when combined with aluminum flake pigments) makes a fine copper metallic paint.
Perylene. Described since around 1912, but not laked into pigments and sold commercially until 1957, the perylenes were originally vat dyes that are chemically related to the perinones. Available colors are limited to moderately saturated scarlets (PR123, PR149, PR190), reds (PR178) and dark maroons (PR190, PR179, PR224, PV29).
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 long used 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 indanthrone blue (PB60).
Diketo-pyrrolo pyrrole (DPP). 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 dull orange (PO71), orange (PO73), scarlet (PR255), red (PR254) and carmine (PR264, PR274).
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 with one very important color, 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 paints). 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 as salts of basic dyes. All useful varieties are green, blue or violet; some violet shades are still used for their brilliance. They are not especially lightfast and should not be used in professional quality artworks. (Some paint companies, such as Schmincke and Holbein, offer them in paints labeled "brilliant" that are intended for artwork used in printing or photoreproductions -- 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.
Last revised 07.25.2001 • ©
2001 handprint media
Synthetic organic pigments are carbon based molecules manufactured from petroleum compounds, acids, and other chemicals under intense heat or pressure. The techniques for producing these substances on an industrial scale were invented around 1860, and the pace of innovation has increased sporadically since then.
