The amazing story of these early pigments is well told in 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.
Unfortunately, the aniline pigments as a group tend to be very fugitive (they have ), 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.
. ( is a registered trademark of Hoechst AG; the generic label for the same compounds manufactured by other companies is 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 (, , PR261), many reds (PR2, , PR5, PR7, PR8, , PR17, PR22, , PR150, etc.), carmines (, PR146, the many pigments listed under ), maroon violet (PV13, PV25, PV44), and brown (PBr1).
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 ( or ) to greenish blue (), cyan (), turquoise (), bluish green (), and yellowish green (PG13, ); 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 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 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).
. 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 ( stopped using them in their "professional" paints in 2006).
While consumers are happy that aspirin works so well in so many areas, scientists are excited in understanding how it works and finding ways to make it work better. They have come a long way since the 1970s and realize that many more secrets await discovery. Aspirin itself is a small chemical molecule, the properties of which have been known for more than a century. However, the living body with which it must interact as a medicinal agent is most complex and not well understood despite scientific advancement. Aspirin research involves many approaches that will be discussed in more details later in the book: cut and try, educated guess, breakthroughs and setbacks, laboratory experiments, theories and controversies, synthesis of knowledge from many disciplines, clinical trials with definitive or inconclusive results, and judgments based on incomplete knowledge.
The major chemical groups in the synthetic organic category are listed below, in the approximate chronological order of their introduction as commercial pigments.
As an artist, your major concern is to understand the average lightfastness and generic handling attributes of these pigments across different manufacturers different pigment hues (chemical variations) that is, to see paints as rather than as For example, both the blue and green and the orange to magenta 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 and the warm 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 .
To discover what a thing is good requires knowledge about relevant situations, which are often subtle and difficult. Lack of such knowledge partly explains why many chemicals sat on the shelf for decades before their therapeutic values were realized. This happened to aspirin’s rival Tylenol. Its active ingredient was synthesized in 1878, but had to wait until 1955 before being developed into a popular drug. Ever more revealing are the stories of antibacterial drugs. Sulfanilamide was synthesized in 1908, but it was the discovery of its therapeutic effectiveness in 1932 that won a Nobel Prize. Penicillin was discovered in 1928 and its therapeutic properties in 1939, and both discoveries were cited in the Nobel Prize. In Nobel Prizes such as these, the scientific community acknowledges the equal scientific importance of discovering and developing a drug. Unfortunately, this point is often overlooked in science studies, so that Hoffmann is often accorded with the credit for aspirin, to the neglect of Eichengrün and others in Bayer.
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.
. 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 (, ), scarlet (), red () and carmine (, PR274). Pyrroles have also been crystallized with quinacridones to produce hybrid pigments ().