Asian blues: natural indigo sources in Asia

Before the invention of synthetic indigo, the main source for dyeing textiles blue for thousands of years was natural indigo. Indigo was appreciated for the long lasting, stable quality of its colour. The sophisticated knowledge of gaining the dye material from plants and dyeing in fermentation vats was developed all over the world by discovering and using local species from at least eight plant families. As Asia has the highest diversity of species used for gaining indigo, our research focuses on field sites in China, India, Indonesia and Laos. Literature provides a solid basis for understanding principle processing steps and species used in history. What is lacking is the detailed documentation of contemporary use of indigo yielding plants that includes all steps from cultivation to the final product. This refers especially to plants that did not gain a worldwide reputation in the past, but were widely used on a local scale. Nevertheless, we have evidence from recent local ethnobotanical inventories and our own scoping visits that these plants are still in use in remote areas. We focus on the following research questions: (1) which species yielding indigo are used today by whom and in what form?; (2) which cultivation, collection, processing and dyeing methods are applied?; and (3) which similarities and differences of the applied processing and dyeing techniques can be documented by measuring process relevant parameters? The second focus of our research is on quality. The aspect of quality is not dealt with in recent ethnobotanical research about natural indigo use. It was, however, a key factor in indigo production throughout history, is regaining significance again and will be essential for future production. Key questions are: (1) how is quality defined, measured or categorized along the indigo supply chain?; (2) how can quality be influenced throughout the supply chain?; and (3) which qualities of natural indigo are being achieved today? Hypotheses will be generated about quality influencing factors throughout the supply chain. An interdisciplinary set of methods will be applied: (1) analysis of historical documents from colonial research on indigo; (2) ethnobotanical field research in China, India, Indonesia and Laos; (3) measuring of process relevant parameters such as pH, dissolved oxygen and temperature; and (4) analysis of 100 indigo samples concerning the content of indigotin and additional components which could be relevant for quality. The reference material database of these samples is an additional outcome of the project and will be the basis for follow-up research.

For 6000 years, natural indigo has been used to dye textiles blue. This plant extract, which contains the blue pigment indigotin as the main valuable component, became a globally traded product from the 16th century onwards. Production reached its peak during colonial times. With the late 19th/early 20th century, natural indigo was almost completely replaced by synthetic indigo. In a few remote areas in Asia, however, natural indigo is still traditionally used. Providing the only long-lasting blue colour in the palette of natural dyes, it is also important for new initiatives who foster local production of high quality, artisanal textiles. With the growing interest in sustainable textile production, natural indigo is once again gaining attention. The international research team of the FWF funded project Asian blues: natural indigo sources in Asia was interested in indigo-yielding plant species and how they are used today, including indigo quality parameters and potential quality influencing factors. The project followed an innovative inter- and transdisciplinary approach: ethnobotanical methods, including the participation of possibly the last generation of traditional indigo producers, were combined with quantitative measurements of process parameters, standardised indigo extractions, and chemical analyses of indigo samples from verified species. Over 40 detailed interviews, more than 70 standardised extractions and 145 indigo samples provided the data basis for this research, as well as trade records and literature from the 17th till early 20th centuries. The study documents the use of five plant species (comprising cultivation, indigo extraction, and dyeing) in four villages in China. The investigation of quality indicators and factors influencing quality as perceived by local producers enabled a deeper understanding of potential causal links in the traditional indigo use system. As travel restrictions during the pandemic made research in other Asian countries impossible, indigo-yielding species, including two additional, historically important species, were cultivated in Vienna and extracted with standardised procedures. That enabled a species-specific comparison of indigo qualities and extraction yields. In total, a reference collection of chemical profiles from indigo of seven species was established (Indigofera suffructicosa, Indigofera tinctoria, Indigofera arrecta, Strobilanthes cusia, Wrightia laevis, Persicaria tinctoria and Isatis tinctoria). The colour of the indigo extract has been an important quality indicator in historic and traditional indigo use. Assessed with modern colour measurement technology, it can be used to estimate the content of indigotin. The project had access to a unique historic indigo sample from a trade vessel that was wrecked in 1697 in Cape Town harbour en route from Mughal India to the Netherlands. Chemical analyses of this sample and the investigation of historic trade documents of that time showed how quality factors of concern in the 21st century were also of concern over 320 years ago.