(b. Poznan, Poland, 13 17?] February 1834; d. Dresden, Germany, 11 October 1910),
Caro’s career mirrors the dynamic expansion of the synthetic dye industry and illustrates the process by which science entered into close and permanent cooperation with industry. The son of a successful Jewish grain dealer who moved to Berlin in 1842, Caro was given a vocational education, proceeding in 1852 from the Gymnasium to the Gewerbeinstitut, where he was trained as a dyer. Simultaneously he attended chemistry lectures at the University of Berlin. In 1855 he was hired as a colorist by a calico printing firm in Mülheim an der Ruhr, where natural dyes and secret recipes were still in use. After demonstrating the power of scientific solutions to certain production problems, Caro, like many a technician of his generation, was sent to England in 1857 (the year following William Perkin’s discovery of mauve from aniline) to learn the most up-to-date dyeing techniques.
Two years later he was back in England, having resigned his Mülheim job, eager to plunge into the exciting new field of synthetic dyes. As analytical chemist for the Manchester chemical firm Roberts, Dale & Co., he soon earned admission into partnership by discovering a more efficient synthesis of Perkin’s mauve. Through engineering experience gained in commercializing this process and the simultaneous study of A. W. Hofmann’s researches on the synthesis and constitution of the new aniline dyes, Caro rounded out his training as an industrial organic chemist. While maintaining close touch with other budding dye chemists—all of them at one time Hofmann’s students or assistants, and including among them several Germans destined for future leadership in the dye industry—Caro now launched on several lines of research that ultimately led to his major scientific contributions. Following up Peter Griess’s pioneering studies on diazo compounds, he discovered induline, a useful aniline dye; and in 1864–1865, together with C. A. Martius, found and developed a commercial synthesis for Bismarck brown and Martius yellow. Meanwhile, Caro also began a decade of intermittent studies that contributed significantly to elucidating the triphenylmethane structure of rosaniline dyes.
Personal success and marriage to a British subject, Edith Eaton, were not enough to hold Caro in England. Like most of his German chemical colleagues in England, he saw greater opportunity at home. In 1866 he settled in Heidelberg to pursue fundamental research in Bunsen’s laboratory. Two years later he accepted the directorship of what was probably the first true industrial research organization, the Badische Anilin- & Soda-Fabrik in Ludwigshafen.
Continuing his previous practice of working closely with fundamental researchers in aromatic chemistry at universities and elsewhere, Caro and his organization hurried from one commercial triumph to the next. When, in 1869, Graebe and Liebermann showed him their new laboratory synthesis of alizarin, he found a much cheaper process to make this coveted dye commercially. In 1874 he brominated Baeyer’s fluorescein to produce eosin, a fluorescent, red dyestuff. Further collaboration with Baeyer in azotizing tertiary amines (e.g., dimethylaniline) yielded methylene blue and related dyes. Substantial mastery over azo reactions achieved during the mid-1870’s by Griess, Otto Witt, and others yielded a spate of valuable azo dyes. Of these Caro codiscovered and brought into production chrysoidine, orange, and fast red. Besides numerous lesser discoveries he also made fruitful suggestions to colleagues and associates. An intensive collaboration, begun in 1880 with Baeyer and the Farbwerke Hoechst to effect a commercial synthesis of indigo, led Caro to a technically but not commercially feasible solution in which indigo was to be made from cinnamic acid, which in turn had been derived from benzal chloride. After 1889, when Caro resigned as director of research, the indigo project was continued by BASF chemists under the leadership of his successor, Heinrich Brunck, until success came at last in 1897. Among Caro’s last major finds were naphthol yellow (1879) and persulfuric acid, H2SO5, (1898). The latter, also known as Caro’s acid, is a combination of sulfuric acid and hydrogen peroxide and is a strong oxidizing agent.
By 1883 Caro had become a leading spokesman for the German chemical industry. He made outstanding contributions to the formulation of effective patent laws and practices for the protection of chemical inventions. In several articles and personal tributes he also recorded for history the spectacular growth of the dye industry during his own lifetime.
I. Original Works. All of Caro’s technical publications are in the form of patents and articles. His published speeches were gathered into a book by his daughter Amalie Caro, Gesammelte Reden and Vorträge von Heinrich Caro, (Leipzig, 1913). The Deutsches Museum in Munich holds much of his correspondence with Adolf von Baeyer, Graebe, Liebermann, and other chemists. Three of his historical articles are of great value: his obituary of Peter Griess, in Berichte der Deutschen chemischen Gesellschaft, 24 (1891), i -xxxviii; “Über die Entwicklung der chemischen Industrie von Mannheim-Ludwigshafen a. Rh.”, in Zeit-schrift für angewandte Chemie, 17 (1904), 1343–1362; and “Über die Entwicklung der Teerfarben-Industrie”, in Berichte der Deutschen chemischen Gesellschaft, 25 , no. 3 (1892), 955–1105.
II. Secondary Literature. Obituaries and tributes are the main biographical sources regarding Caro’s life. The most important and lengthy of these is the obituary by A. Bernthsen, in Berichte der Deutschen chemischen Gesellschaft, 45 , no. 2 (1912), 1987–2042. In addition see A. Bernthsen, “Zum 70. Geburtstage von H. Caro”, in Zeitschrift für angewandte Chemie, 24 (1911), 1059–1064; and Carl Duisberg, “Heinrich Caro”, ibid., 1057–1058. Also useful is E. Darmstaedter, “Heinrich Caro (1834–1910),” in Gunther Bugge, ed., Das Buch der grossen Chemiker, 2nd ed., II (Weinheim, 1955), 298–309, which draws on new sources and provides a good bibliography.
A brief description of Caro’s correspondence at the Deutsches Museum is to be found in Kurt Schuster’s biographical essay “Heinrich Caro”, in KurtOberdorffer, ed., Ludwigshafener Chemiker, II (Düsseldorf, 1960), 45–83. Caro’s contribution to German patent legislation and practices is expertly reviewed in Paul A. Zimmermann Patentwesen in der Chemie (Ludwigshafen, 1965). For background information and additional bibliography see John J. Beer, The Emergence of the German Dye Industry (Urbana, Ill., 1959).
John J. Beer