Early advances related to synthetic and semi-synthetic dyes were made by Barth (indigo carmine, 1743) [1], Woulfe (picric acid, 1771) [2], Scheele (murexide, 1776) [3], and Runge (aurin, 1834) [2, 4]; however, a commercial industry was not significant until William Perkin created mauveine in 1856. This was followed by a period of accelerated developments of new products, which peaked near the end of the 19th century. Early synthetic dyes are generally categorised according to chemical structure. The most abundant synthetic dyes belong to the azo class. However, other chemical classes, including triphenylmethane, azine, xanthene, nitro, oxazine, indigoid and anthraquinone have each in turn had an important impact on the industry. As colourants were introduced to the market, many were used to colour materials other than textiles: e.g. waxes and varnishes, writing and printing inks, paint pigments, and early plastics. As a result, large quantities of synthetic colourants are present throughout museum collections around the world. Detailed reviews of the early industry [2, 3, 5,6,7,8,9] and respective dye chemistry [10,11,12,13,14] are plentiful in the literature, and provide valuable context for current studies of heritage materials.
For many heritage objects, efforts are made to identify colourant materials for historical study, authentication and dating, and fastness evaluation. The large number of possible compounds makes it an overwhelming task to characterise them all, and in many cases reference materials are scarce. Analysis of these colourants is further complicated by the varied nomenclature used by manufacturers, where similar names were used for different dyes. The confusing letter codes (markings) after dye names, practice of mixing products, and mischaracterization of the product composition create additional layers of complexity. Norton [15] offers insight into some aspects of this confusing issue in a section titled “The Marks of Coal-Tar Colors”. As just one example of the complexity, Norton indicated that 300,000 lb of a dye named Cotton Black with various markings (unclassified by Schultz #) was imported into the United States in 1914. The first edition of the Colour Index (CI) contains several listings that relate to this product name [16]: Cotton Black from Wülfing, Dahl & Co. (CI# 994); Cotton Black B, 3B, BG BGN BGNX, BN, C from BASF (azo direct dyes with no CI#); Cotton Black E extra from BASF (CI# 581); Cotton Black G, 2G, 3G, PF extra, R, RN from BASF (azo direct dyes with no CI#); and Cotton Black RW extra from BASF (CI# 582). In some cases, a classified dye may also have a spectrum of possible compositions depending on the production method. An example is given by Crace-Calvert [17] when discussing the methyl and ethyl-rosanilines, and the production of various forms of Hofmann’s Violet:
…by varying the circumstances of experiment, instead of three of the hydrogen being replaced by ethyl, dyes may be obtained having two, or only one, replaced by ethyl; moreover, by substituting methyl iodide for ethyl iodide, corresponding methyl compounds may be prepared. In this way Hofmann violets are obtained of different shades, varying from RRR, the very red, which is principally a salt of monomethylated rosaniline C28H18(CH3)N3, to BBB, the bluest shade.
In the 1980s, Schweppe [18, 19] aimed to simplify the problem by providing a shortlist of 65 early synthetic organic dyes with notes to assist with their identification, while also highlighting a subset of 22 stated as the most common [20]. In the field of heritage science, this list has become a common reference despite the ambiguous selection criteria. In a review of the early synthetic dyes, Barnett [20] remarks that areas for further work include identifying the most common materials using 19th century trade literature, and compiling respective fastness data. The challenge, of course, is finding quantitative data regarding dye production or use. The Colour Index is an encyclopaedic resource of colourant data; however, information related to the degree of use is limited, and modern lightfastness data is unavailable for many of the earliest materials. Similarly, most trade books of the period simply outline the vast range of products available. It is likely that a significant number of the catalogued products were rarely used, and analysis could be prioritized to specific materials given the appropriate information.
This study investigates several trends in the synthetic dye industry using early 20th century literature: multiple editions of the CI [16, 21, 22], Norton’s census [15], and the annual Census of Dyes and Coal Tar Chemicals from the US Tariff Commission [23, 24]. The goal of the work is to provide a framework to better understand the most prominent early synthetic dyes, optimise methods for identification, and further develop lightfastness data for risk assessment tools. Findings are presented as summary statistics due to the large number of materials, while tabulated values are provided in a complementary dataset for further research work [25].