Quality Blues: Indigenous Knowledge of Natural Indigo Identification in Southern China

Abstract

Background: As one of the oldest traditional dyes, natural indigo is commonly used for centuries by the people worldwide. In the process of indigo production, indigenous people have formed unique knowledge of indigo identification because the indigo identification is crucial for indigo quality control and the dyeing effects. However, such indigenous knowledge is rarely documented and explained. Therefore, the aims of this study were to i) document and assess the indigenous knowledge of local people identifying the natural indigo paste and ii) to explore the characteristics and material basis of such indigenous knowledge.Method: Three ethnobotanical studies were conducted in 2019 and 2020. A total of 283 traditional indigo-paste artisans were interviewed in Guizhou, Yunnan, and Fujian Provinces. Frequency of citation, Mention index, and Fidelity level of each indigo-paste quality criterion were calculated to determine the most commonly used, most recognized, and most important quality criterion. To explore the characteristics and material basis of such traditional knowledge, we analyzed 21 samples we collected by using HPLC, pH and particle size analysis methods.Results: Local people possess unique knowledge to identify natural indigo. After thousands of years, they finally chose four criteria (color, taste, touch, and dyeing ability) and based on this, the nature indigo was divided into five quality grades. The best folk criterion was as following: dark blue with purple-red luster, smooth and difficult to wipe off; it should have a "sweet" or "spicy" taste, and dye cloth easily. It found that the higher the indigo and indirubin contents, especially indirubin content, the better the quality of the indigo paste. Within the 9-12 range, pH of high quality indigo-paste was lower. However, there was no significance difference between particle size and quality.Conclusion: The ancient methods used by the local people for identifying natural indigo are comprehensive and unique. This study revealed the importance of indirubin and pH for assessing the quality of indigo paste by documenting the various folk quality criteria and quantitative experiments. These findings differ from existing synthetic indigo-quality standards. Amid rapid modernization, traditional knowledge remains invaluable in world heritage of humanity that warrants preservation.