Humanity, from antiquity to modernity, has carried a wish to use dyes as a way to symbolize or beautify. From the ancient Egyptian use of the madder and saffron plants to dye cloth for burial wrappings to the modern employment of dies in the clothing industry (1,2), dyes have maintained their status as one of the most prevalent and widespread examples of applied chemical technology. Although some of the most ancient pigments were natural minerals, such as red lead oxide made by the Egyptians from the heating of lead with basic lead carbonate (3), this paper will deal mainly with organic dyes. A brief discussion of the chemistry behind the colors ensues and will proceed to an overview of the development of dye chemistry throughout the ages.

The visible spectrum of light consists of radiation of wavelengths between approximately 350 to 780 nm. Absorptions of light by a molecule for these wavelengths require the high energy of electronic transitions in a molecule. While sigma bonds are generally not effected by light with wavelengths in the visible region, electrons in pi bonds are more susceptible to promotion into a higher-energy orbital. The typical case of promotion in the visible spectrum is from a p to p* orbital. Certain chromophores, or functional groups involved in absorption, such as conjugated bond systems, tend to have greater absorption in the visible region. The Woodward-Fieser rules summarize these tendencies and explain why dyes tend to be large conjugated molecules. With this very limited introduction to the chemistry of dyes, we are now ready to examine the rich history of dyes and dying. As was mentioned above, the ancient Egyptians used madder to dye wrappings for mummies. We now know that the dye from this plant is alizarin, which was synthesized in 1869 both by William Perkin and Heinrich Caro in independent laboratories. (5) Until Perkin's breakthrough synthesis of the "coal-tar" dye mauve, which will be discussed in length below, most dyes came from natural sources. Some exceptions, though, are Runge's synthesis of aurine from phenol in 1834 (6), and Laurent's successful conversion of phenol into picric acid in 1842 (7). These natural dyes often utilized a mordant to improve color fastness. A mordant can simply be described as a "metallic salt with an affinity for both fibers and dyestuffs." (8) The first conclusive evidence relating to the use of mordants suggests that they were used in Egypt's Middle Kingdom sometime between 2200 and 1500 B.C. At this same time, indigo was being grown for the purpose of dying not only in Egypt, but in Persia and Indochina as well. By this period in history, the Chinese had a dying tradition nearly 1000 years old. (9)

The next great civilizations of Greece and Rome undoubtedly had dye chemistry intertwined in their culture, including their literature and mythology. Homer, the great Hellenic poet, tells in the Odyssey of using "colored rugs and coverlets,"(10) to honor a guest. The great Roman epic poet Virgil also writes of the importance of color as he describes "purple robes..." as a "hero's shrouding." (11) It is fabled that Alexander the great deceived the Persians by using a red dye, possibly madder juice, to make it appear his army was wounded. Another Roman myth proposes that Hercules discovered Tyrian purple, the plum-colored dye which comes from the snail Murex. He supposedly noticed the dark purplish stain upon his dog's jaws after the animal had bitten a snail. (12,13) Tyrian purple was popular for many centuries being worn only by royalty or those with high religious esteem. (14) Some other dyes of antiquity include the "lac" dyes used in India or Cochineal dye in Mexico which came from the rather unique source of

insects. (15)

The next great leap forward for the dye chemistry industry occurred in the latter part of the 12^th century when Norman dyers developed a Guild of Dyers in London. Although, dye chemistry remained a type of informal artisan's trade until 1429 when the first European book on dyeing was published in Italy. Later, in 1662, the first English text concerning this topic was published. (16) Dyeing was slowly becoming an academic discipline.

It was not until the 19^th century that the pace of advancement in this industry quickened to a gallop. With the advances of organic chemistry inevitably came the first widespread marketing of a synthetic dye. In 1856, 18 year-old William Perkin was studying beneath A.W. Hofmann at the Royal College of Chemistry in London. While on a break from his studies he tried to synthesize quinine after a lecture in which Hofmann described the need for a synthesis of this anti-malarial drug. Perkin attempted to oxidize allyl toluidine with potassium dichromate to achieve the product. With discouraging results, Perkin once again tried this synthesis by using aniline instead of toluidine. By luck, Perkin's aniline contained some ortho- and para-toluidine impurities. This reaction led to the production of a black sludge which he noticed contained a striking purple dye when he washed his glassware with ethanol or water. Perkin quickly devised a way of extracting the new dye and with the help of his father started the first synthetic dye works in 1858. The new dye was called mauve or mauveine by the French. Interestingly enough, though, it had also been called Tyrian purple.(17) Thus, Perkin successfully ushered the history of dyeing from its "pre-aniline" period into the new "post-aniline" period. (18)

The field of synthetic dye chemistry was now exploding as the increased textile production of the late 19^th century industrial revolution increased the demand for inexpensive dyes. Soon Germany would become the leader in the synthetic dye industry as chemists educated by the likes of Liebig and Wohler dominated the synthesis of "coal-tar" dyes. (19) These chemists discovered dozens of new dyes a year which were processed at and shipped from large-scale factories. These improvements in the making of dyes quickly eliminated the production of natural dyes due to their expense and inefficiency as compared to the new synthetic colors. (20)

The synthetic dye industry today is vast and contains many groups of dying processes and dyes. From the synthesis of biological stains used in the preparation of microscope slides to the production of acetate rayon dyes and nylon dyes used in the preparation of commercial textiles, the industry continues to develop new processes and dyes to serve the needs and wants of humanity. (21) One area of early synthetic dye chemistry though, azo dyes, remains one of the largest and most important to the industry. The birth of azo dyes came in 1858, the same year Perkin started his factory for the production of mauve (22), although their value was not appreciated until Bottiger produced congo red, the first direct cotton dye, in 1884 (23). Johann Peter Griess had made the original discovery that a diazo compound could be derived from the reaction of nitrous acid with aromatic amines. Upon experimentation, he further concluded that this diazo compound could couple to another aromatic amine resulting in the formation of a dye. (24) This area of chemistry has been greatly expanded and refined and now includes trisazo, tetrakisazo and polyazo dyes. The arenediazonium ion, containing the -N=N- chromophore, serves as a weak electrophile which may perform an electrophilic aromatic substitution on an aromatic ring to produce a vast and diverse array of different dyes. (26) Upon referral to the above discussion of the chemistry behind the colors, one can see how these dyes with their great amounts of conjugated p bonds serve as excellent dyes.

The future of the synthetic dye chemistry appears certain. As the global market continues to expand and western culture proceeds to penetrate even the world's most isolated regions, the demand for inexpensive dyestuffs will continue to rise. It is promising that this demand will be well met as "the possibilities of further synthesis [of dyes] are unlimited." (27) With these prospects in sight for the synthetic dye chemistry, one might say that this industry certainly promises a bright and colorful future.

Literature Cited

1. Robertson, Seonaid M.; Dyes from Plants; Van Nostrand Rheinhold Company; New York; 1973; 72.

2. DYES Online; Available: http://www.dyesonline.com/intro.htm#hist (Sunday, October 4, 1998.)

3. Hudson, John; The History of Chemistry; Chapman & Hall; New York; 1992; 4.

4. Wade, L.G. Jr.; Organic Chemistry, Third edition; Prentice Hall, Inc., Upper Saddle River, New Jersey; 1995; 695, 907.

5. Hudson, John; The History of Chemistry; Chapman & Hall; New York; 1992; 253.

6. Decelles, Corinne; J. Chem. Ed.; 1949; 26; 583.

7. DYES Online; Available: http://www.dyesonline.com/intro.htm#hist (Sunday, October 4, 1998.)

8. Robertson, Seonaid M.; Dyes from Plants; Van Nostrand Rheinhold Company; New York; 1973; 24.

9. Robertson, Seonaid M.; Dyes from Plants; Van Nostrand Rheinhold Company; New York; 1973; 24.

10. Homer; The Odyssey; Random House, Inc.; New York; 1989; 363.

11. Virgil; The Aeneid; Random House, Inc.; New York; 1983; 167.

12. Robertson, Seonaid M.; Dyes from Plants; Van Nostrand Rheinhold Company; New York; 1973; 24.

13. DYES Online; Available: http://www.dyesonline.com/intro.htm#hist (Sunday, October 4, 1998.)

14. Robertson, Seonaid M.; Dyes from Plants; Van Nostrand Rheinhold Company; New York; 1973; 72.

15. Decelles, Corinne; J. Chem. Ed.; 1949; 26; 583.

16. Robertson, Seonaid M.; Dyes from Plants; Van Nostrand Rheinhold Company; New York; 1973; 72-3.

17. Roberts, Royston M; Serendipity: Accidental Discoveries in Science; John Wiley & Sons, Inc.; New York; 1989; 65-8.

18. Decelles, Corinne; J. Chem. Ed.; 1949; 26; 583.

19. Hudson, John; The History of Chemistry; Chapman & Hall; New York; 1992; 253-4.

20. Webb, Hanor A.; J. Chem. Ed.; 1942; 19; 461.

21. DYES Online; Available: http://www.dyesonline.com/intro.htm#hist (Sunday, October 4, 1998.)

22. Hudson, John; The History of Chemistry; Chapman & Hall; New York; 1992; 253.

23. Decelles, Corinne; J. Chem. Ed.; 1949; 26; 583.

24. Hudson, John; The History of Chemistry; Chapman & Hall; New York; 1992; 253.

25. DYES Online; Available: http://www.dyesonline.com/intro.htm#hist (Sunday, October 4, 1998.)

26. Wade, L.G., Jr.; Organic Chemistry, Third edition; Prentice Hall, Inc., Upper Saddle River, New Jersey; 1995; 906.

27. DYES Online; Available: http://www.dyesonline.com/intro.htm#hist (Sunday, October 4, 1998.)