Blue to black: Hypotheses on plant use complexity in traditional dyeing processes in Southeast Asia and China

Modern indigo dyeing is achieved using chemical dye vats with toxic reducing agents that have an impact on the environment and human health. Consequently, there has been interest in traditional indigo dyeing processes and their potential for more environmentally friendly industrial production. Traditional indigo dyeing was studied by conducting a literature review (China, India, Indonesia, the Philippines, and Vietnam) and field surveys (Timor Leste, Indonesia, Laos, and China) in Southeast Asia and China (SAC). Traditional SAC blue and black dyeing processes can be a combination of separate dyeing steps. Here, we documented plant species and ingredients in the blue to black dyeing processes used in addition to indigo yielding species. We recorded 80 plant species belonging to 39 families and 67 genera used in the “blue to black” dye processes in SAC. Owing to local use and phytochemicals or microbial substances of these species and their function in the dyeing processes, eight hypotheses for added species, including lime or ash water, microorganisms, food for microorganisms, electron donors, electron mediators, reducing sugars, metallic mordants, and tannins were suggested herein. The combination of hypotheses was supported by the findings and theories of previous studies and clarifies why these particular plant species are likely added to dye vats. The hypotheses and theories derived from this study pave the way for insights into indigo dyeing processes that reduce inorganic chemical additives using additional plant products, which consequently may provide a green route for cleaner production strategies. This research identifies gaps in knowledge and highlights where further work is needed to verify the hypotheses proposed for adding products to dye vats in the future.