The Asian Organic Colorants project developed a strategy for analysis of traditional Chinese organic colorants used as textile dyes and organic pigments in Chinese wall paintings, and for analysis of organic colorants used as textile dyes and organic pigments in Asia.
The biological sources used to produce organic colorants in Asia, specifically China, were determined through an extensive literature search (available online as a searchable bibliography). These biological sources were acquired and used to prepare reference samples. Reference bulk samples were prepared as dyed wool and silk and as pigments. Paints were prepared from the organic pigments and were applied to wall paintings mock-ups.
From these analyses, the project created analytical databases and disseminated this information to the conservation community. Various analytical techniques were evaluated on the wall paintings mock-ups to determine their diagnostic value for the detection and identification of Asian organic colorants. An analytical strategy was developed to identify Asian organic colorants and applied to historic samples selected from the Mogao Grottoes.
The components of the project included:
- a literature review to identify biological sources used to produce organic colorants in China
- acquisition of biological sources
- preparation of references samples from biological sources
- development of analysis strategy
- analysis of historical samples
Detection and identification of traditional Chinese organic colorants presents a challenge not only because many of the biological sources used to create them have not been well studied but, in the case of organic paints, concentrations of these colorants are relatively low compared with inorganic pigments and binding media. Much less is known of these colorants than of the dye and organic pigment sources used in Europe and the Americas.
Previous work by the GCI at the Mogao Grottoes in Dunhuang, China, resulted in a wealth of information on the mineral pigments and binding media used in these caves, particularly in Cave 85. Conservators working at the site also identified the use of organic colorants and washes in the caves.
Determining specific organic colorants was a greater challenge because the biological sources used to produce the dyes and pigments were frequently unique to the geographic region, and the reference samples required for identification were not available.
The project reviewed published literature to determine the biological sources used to produce organic colorants in the Mogao Grottoes wall paintings. One hundred fifteen biological sources were identified. Based on the frequency of independent citations, twenty-four biological sources were selected for study. Most of these twenty-four sources were identified at the species level. For those cited only at the genus level, species were selected that were most likely to be available in the region based on botanical references and on discussions with a botanist at Beijing University. In addition to biological sources and recipe data, painting techniques and historical context were collected. This core literature is available as an online searchable bibliography.
Plants and resins identified for analysis were purchased at pharmacies in Dunhuang and Beijing, and at Chinese pharmacies the Los Angeles area. Items not available at pharmacies were acquired from botanical gardens in the United States and China. Some sources were purchased at the chemical market in Old Delhi, India. Validation of the species was necessary to ensure accuracy; this was done using Liquid Chromatography-Photodiode Array-Mass Spectrometry (LC-PDA-MSn). A database was designed to manage the acquisition, horticultural, and reference information for each biological source.
Bulk reference samples were prepared from each biological source, and historic recipes were used when available. When unavailable, modern laboratory practices, were used to prepare the samples. Dyeing of wool and silk were performed with or without preliminary mordanting with alum. Pigments were prepared on a hydrated aluminum oxide base. Paints were prepared by suspending pigments in a diluted animal glue medium or by concentrating the dye extract and adding glue. All preparations and reference samples were cataloged in a database. The Dunhuang Academy prepared one hundred fifty plaster wall painting mock-ups using clay and other materials available on site. Each coupon has a traditional ground layer applied to half of its surface and seven strips of common inorganic paints used in the grottoes.
The development of a strategy for analysis of organic dyes on yarns and organic pigments in paint required consideration of the following parameters: level of destructiveness to both the object and the sample, diagnostic value, sensitivity, and reproducibility. Liquid Chromatography-Photodiode Array-Mass Spectrometry is routinely used for analysis of organic dyes and pigments present in artifacts produced in Europe and the Americas. Its use in an Asian or, more specifically, Chinese context cannot necessarily be extrapolated because the majority of biological sources identified in Chinese artifacts were different from those found in the European and American artifacts. Liquid Chromatography-Photodiode Array-Mass Spectrometry, however, remained the core analytical technique because of its high diagnostic value, sensitivity, and reproducibility.
The method used by the GCI was optimized for the analysis of Asian organic dyestuffs and pigments and resulted in:
- a library of Ultraviolet-Visual (UV-Vis) spectra for color-contributing components and,
- an Electro-Spray Ionization–Negative Ion Mode (ESI-NIM) mass spectra database of the same diagnostic components of organic colorants including both MS and MS-MS spectra.
Once the analytical strategy was verified, historic samples from wall paintings at the Mogao Grottes were taken and analyzed. The strategy is also applicable to Asian dyestuffs used in textiles and Asian organic pigments used in paintings.
Last updated: January 2011