In terms of dyeing, natural indigo has many advantages over synthetic indigo. In contrast with synthetic indigo, natural indigo is considered a green dye that has a harmonious and sustainable relationship with the environment, as it is biodegradable. Therefore, in recent years, consumer interest in natural indigo has been on the rise, which has attracted the attention of dyeing enterprises. However, the disadvantages of using synthetic dyes include the high costs of the raw materials, their toxicity, and environmental pollution. Therefore, the production and use of natural indigo has declined and gradually disappeared such that it currently occurs only in a few remote areas in China, India, and a few other countries. However, with the rise of industrialization, synthetic indigo is now used in almost the entire dyeing industry, owing to advantages such as high purity, low price, and better producibility. Further, the color of indigo has been endowed with a unique significance and is commonly used as a symbol of independence and individualism, whereby it is known as the king of colors and the color of kings. Over thousands of years, natural indigo has been integrated into the cultures of various ethnic groups and used in ethnically characteristic ways, including tie-dyeing of the Bai people, batik of the Miao people, and in the production of the bright cloth of the Dong people. Indigo paste is traditionally used in China. Indigo-yielding plant species are processed using a variety of methods developed in different regions, such as sukumo ( Persicaria tinctoria Spach) in Japan and woad balls ( Isatis tinctoria L.) in Europe. To date, 31 plant species across eight different families have been used as sources of indigo (e.g., Indigofera tinctoria L. It is extracted from indigo-yielding plant species. Natural indigo is considered one of the oldest dyestuffs known to humanity, and one of the most commonly used natural dyestuffs worldwide, particularly in Egypt, India, China, and Africa. Amid rapid modernization, traditional knowledge remains invaluable as a world heritage of humanity that warrants preservation. These findings differ from existing quality standards for synthetic indigo. By documenting the various folk quality criteria and conducting quantitative analyses, this study revealed the importance of indirubin and pH for assessing the quality of indigo paste. The ancient methods used by local people for identifying natural indigo are comprehensive and unique. There was no significant relationship between particle size and quality. Within the pH range of 9–12, high-quality indigo-paste was more acidic. Additionally, the higher the contents of indigo and indirubin-especially indirubin-the better is the quality of the indigo paste. The best quality indigo paste was judged according to the following folk criteria: dark blue in color with a purple-red luster smooth and difficult to wipe off having a sweet, bitter or spicy taste and easy cloth dyeing. Based on this knowledge accumulated over thousands of years, four criteria (color, taste, touch, and dyeing ability) were chosen by local people, and using these criteria, nature indigo was divided into five quality grades. Local people possess unique knowledge to identify natural indigo. To explore the characteristics and material basis of the traditional knowledge, we analyzed 21 indigo-paste samples using high-performance liquid chromatography with diode-array detection (HPLC-DAD), pH, and particle size analyses. The frequency of citation, mention index, and fidelity level of each indigo-paste quality criterion were calculated to determine the most commonly used, recognized, and important quality criteria. A total of 283 traditional indigo-paste artisans were interviewed in Guizhou, Yunnan, and Fujian Provinces. Three field surveys were conducted between 20. Therefore, the aims of this study were to document and assess the traditional knowledge used by local people when identifying natural indigo paste as well as quantitatively explore the characteristics and material basis of such traditional knowledge. However, such traditional knowledge is rarely documented and explained.
Local people have unique knowledge about indigo identification, which is crucial for indigo quality control and determining the dyeing effects.
As one of the oldest traditional dyes, people worldwide have used natural indigo for centuries.
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