Sociocultural characteristics of indigo paste artisans
Here, we documented the traditional knowledge and experience of 283 people to analyze how they assess the quality of indigo paste, and then conducted a quantitative analysis to explore the characteristics and material basis of such traditional knowledge. Indigo paste played different roles in different regions. Thus, in Zhenfeng County of Guizhou Province and Xianyou County of Fujian Province, indigo paste is the main source of income for local Han farmers. On the other hand, in Congjiang County of Guizhou Province, indigo paste serves as a traditional model of national self-sufficiency. Both these models exist in Yuanyang County and Jinping County of Yunnan Province.
The different social roles of indigo paste lead to different social divisions of labor. Firstly, with regard to the sex of indigo paste artisans, there were more men engaged in indigo paste production than women in Zhenfeng County of Guizhou Province (81.3% versus 18.7%) and in Xianyou County of Fujian Province (71.4% versus 28.6%). However, in the traditional self-sufficiency model of Congjiang County in Guizhou Province, only women were engaged in the production of indigo paste (Table 2). Although both production models exist in Yunnan, the scope and quantity of the trade was relatively small, which was mainly the traditional model of national self-sufficiency; therefore, the number of women (88.6%) engaged in indigo paste production was almost eight times the number of men (11.4%). These results are consistent with previous findings, namely, traditional dyeing knowledge is reportedly transmitted matrilineally, and it is mainly mastered and used by women [43, 44]. Furthermore, activities such as dyeing are considered inappropriate for men [45]. However, when indigo paste becomes a tradable commodity and generates economic benefits, men readily become involved in this work [44]; indeed, they may even participate as the main labor force.
Secondly, as for the age of the indigo-paste artisans, in the Congjiang County, mainly young women aged 30–49 years (50.4%) were engaged in the production and use of indigo paste, 40.3% were 50–69 years old, and only 9.3% were over 70 years old. In the local area, making indigo paste and dyeing cloth seemed to be the daily work of minority women. In other regions, it was mainly old women aged 50–69 years, namely, 68.8% in Zhenfeng 54.3% in Yunnan, and 76.2% in Fujian, while young women aged 30–49 years were relatively rare, i.e., 28.1% in Zhenfeng, 25.7% in Yunnan, and 9.5% in Fujian. Overall, young people aged 30–49 years (35.7%) and middle-aged and elderly people aged 50–69 years (52.3%) showed extensive knowledge and artisanship of indigo-paste production. However, previous reports indicate that most traditional knowledge was usually held by the elderly, whereas young folk did not prove very knowledgeable of the trade [9, 46].
Traditional quality criteria of indigo paste
Generally, the blue hue in indigo paste derives from indigo content, while the purple-red luster derives from indirubin content [47]. Thus, the ratio of the indigo and indirubin content determines the color of the indigo paste. However, indirubin, which is an isomer of indigo, has always been regarded as a by-product in the production of indigo paste [48, 49]. Interestingly, in our survey, people preferred purple-red indigo paste to pure blue indigo paste. In other words, the quality criteria used revealed that indirubin was judged to be more important than indigo. Although these findings differ from existing synthetic indigo-quality standards, they agree with reports of indigo paste quality assessment in ancient Chinese books. For example, there is a document in “Liping Fuzhi”(Guizhou): “投入生石灰,则满地颜色皆收入灰内,以带紫色者为上”, which means that the purple indigo paste is the best. Similarly, there is a description in “Dyeing Sutra”: “明兰宝翠,兹浆鲜红,至次年春夏可变成熟红如天青缎” [50], which means that the best quality indigo-paste should have dark blue and red luster. In addition, the Hainan Li and Miao people think that dark blue and reddish indigo paste is of better quality [46]. Therefore, the indirubin in indigo paste is not considered as a by-product but as a critical determinant of the quality of indigo paste.
In the process of indigo-paste production, many factors affect the final quality [8, 22, 51], one of them being lime. The first factor is the way in which lime is added. Before adding lime, some folk would put lime into a cloth or gauze bag. The cloth or gauze bag is then rubbed in the soaking liquid to produce a fine lime slurry that flows out of the bag. This method produces very delicate lime particles and greatly reduces the number of impurities in the lime. However, other folk would place the lime in a water scoop or bucket, and a small amount of soaking liquid is added and mixed, and then poured directly into the soaking solution (Fig. 5), thus, ignoring large lime particles and impurities. This might be why folk rub their fingers together to check the smoothness of the indigo paste obtained. The second factor is the amount of lime added. Indeed, adding too much or too little lime results in a low-quality indigo paste. In addition to indigo-yielding plants, the taste of indigo paste is mainly related to the amount of lime added during the production process and people judge whether lime is added properly based on the taste. However, during the interviews, the descriptions of taste provided by informants differed and were even contradictory. There are two possible reasons for this difference: one is that taste description is mainly influenced by personal subjectivity and the other is that informants have different perceptions and descriptions of taste due to cultural divergence. This phenomenon also occurred with respect to the use of the color criterion. Thus, for example, some people might describe high-quality indigo paste as being red in color, despite an apparent purplish-red (Fig. 6).
Verification of indigenous knowledge by modern scientific methods
The folk in the study area used indigo and indirubin together to determine the quality of the indigo paste. Compared to a pure dark-blue color, people preferred indigo paste with purple-red luster. The HPLC-DAD quantitative analysis confirmed that the quality of indigo paste is related to the content of indigo and indirubin. Thus, the higher the content of indigo and indirubin -especially indirubin -the better the quality of the indigo paste. This confirmed that the color of the indigo paste is an adequate quality criterion. However, due to the limited number of the experimental samples, the range of indigo and indirubin contents and the color distribution range of different quality grades of indigo paste could not be identified in this study and certainly warrants further research.
Informants in Guizhou Province offered inconsistent descriptions of taste criterion. Through pH quantitative experiments, we found that within a certain range (9≤ pH ≤12), the pH of high-quality indigo paste was lower, while that of low-quality indigo paste was higher. The level of alkalinity stimulates taste buds differentially among people. Local people judge the quality of indigo paste based on simple taste sensation.
By drying the samples in the laboratory, we observed that there were no significant differences in color or gloss of indigo paste in its wet state, except in the very high and the very low-quality samples. However, the color difference was obvious after drying (Fig. 7). This indicates a degree of rationality associated with the simultaneous observation of “water color” or “dry color” in Zhenfeng County, and the importance of color in assessing indigo-paste quality. Similarly, after drying different indigo pastes, the inner part of the block had a different appearance. Some indigo-paste blocks had a uniform internal color; no obvious lime particles or impurities were observed. However, there were varying amounts of white or other colored particles in other indigo paste blocks (Fig. 7). Thus, from an experimental perspective, the touch criterion is also necessary for the assessment of indigo-paste quality.