Department of Scientific Research, Metropolitan Museum of Art
Independent Scholar, NY
Senior Textile Conservator, Head of Conservation
Smithsonian's Cooper-Hewitt, National Design Museum
Until the middle of the nineteenth century, when the first synthetic dye was prepared from coal tar, natural dyes from plants and animals were used to create color on textiles. One of the major groups of yellow natural dyes is from plants containing flavonoids. Flavonoids, which lately have become well known for their antioxidant role in human health (Harborne, 2000), are a type of natural product. They are secondary metabolites found in plants, universal in vascular plants (Harborne, 1986).
Plant dyes containing flavonoids (flavonoid-dyes) are used as mordant-dyes, in other words, those dyes that require the presence of a mordant in the dyeing process. Mordants are substances that combine with colorants—in this case, flavonoids—to set colorants on fibers. An example of a mordant often used with flavonoids is a soluble aluminum salt such as alum. The color green was usually produced by a combination of the flavonoid-dye and indigo.
In this project, a sixteenth- to seventeenth-century Latin American Colonial tapestry in the collection of the Smithsonian's Cooper-Hewitt, National Design Museum underwent examination. The tapestry, previously catalogued as "Peruvian," was being considered for inclusion in the exhibition The Colonial Andes, Tapestry and Silverwork, 1530–1830, held at The Metropolitan Museum of Art in 2004 (Phipps et al., 2004). However, during examination, the tapestry showed characteristics different from those of other Peruvian textiles. Because of this observation, there was further investigation of the materials and techniques of the tapestry, and our department undertook analysis of the dyes.
Identifying flavonoid-dyes is challenging. There are a considerable number of flavonoids. In 1986, approximately 3,000 flavonoids were reported (Harborne, 1986); in 1999, 6,467 known flavonoids were listed in a handbook for flavonoids (Harborne and Baxter, 1999). Flavonoids compose one of the largest groups of plant pigments, and some are commonly found in many kinds of plants. A plant's flavonoid profile may vary in its parts, may vary at different stages of its growth, or may vary because of the environment in which it has grown (Aherne et al., 2002; Harborne, 1986, Ishikawa, N., 1988). Additionally, these flavonoids may experience changes caused by aging long after they were dyed on the yarns of historic textiles. At the same time, flavonoids, which are secondary metabolites, have been the most successfully used chemosystematics markers (Crawford et al. 1982; Fang, et al. 2001; Harborne, 1977). Plant chemosystematics or chemical taxonomy is the approach of classifying plants based on their phytochemical constituents, including the primary metabolites, the secondary metabolites, and the semantides, such as DNA or proteins. By using the chemosystematics approach as well as historic records and knowledge of natural dyes, flavonoid-dyes used on historic textiles have been identified by the flavonoids and their composition detected in the yarns.
One of the techniques currently used to analyze dyes is high-performance liquid chromatography with a diode array detector (HPLC-PDA). Ideally, when art objects are investigated, nondestructive methods should be employed. If, however, samples need to be taken, the amount of sample must be minute so that it does not destroy the integrity of the object. With HPLC-PDA, a small amount of yarn is taken from a textile, and colorants are extracted from the sample yarn for analysis. In the chromatographic process, colorants or other materials extracted from a sample yarn are separated and, at the PDA detector, the UV-visible spectrum of the separated compound is measured. The UV-spectrum characteristic of each type of compound is compared with those of reference compounds. When the colorant's spectra and time eluted are matched to those of a reference compound, the colorant is identified.
Plant dye used on the yellow yarn on the Latin American Colonial tapestry was analyzed by HPLC-PDA. Based on the characteristics of the UV spectra, the main colorants in the extract appear to be flavonoids. Among reference samples analyzed in this study, "zacatlaxcalli" 1 and "parasita from Mexico" 2 showed a chromatographic profile similar to that of the yellow sample from the tapestry. These reference samples are dyed with species of Cuscuta, one of the natural yellow dyes recorded as having been used in Mexico. Another yellow dye that is recorded as predominantly used in Mexico is xochipalli, Cosmos sulfureas (Roquero, 2006; Wallert, 1995).
The sample was further analyzed by liquid chromatography/mass spectrometry (LC/MS) at the National Gallery of Art in Washington, D.C. Although the chromatographic profiles and UV spectra of the main flavonoids in the tapestry sample and those of the reference samples were similar, the types of the flavonoids could not be identified by HPLC-PDA. There were also some differences in the chromatographic profiles. In order to confirm the flavonoid types and to identify or characterize those flavonoids, LC/MS was used. MS provides information related to molecular weight by measuring mass-to-charge ratio of ionized particles, or ions. Hence it is more selective as a detector and also more sensitive than a PDA detector.
The main flavonoids that were detected from the yellow yarn were identified or proposed as: quercetin O-galactoside (hyperoside) (see image above), quercetin O-glucoside (isoquercetin), quercetin O-apioside, kaempferol O-galactoside, isorhamnetin O-glucoside or galactoside and kaempferol O-glucoside (astragalin). Those flavonoids in the yellow yarn sample matched with those in the Cuscuta reference samples (Shibayama et al, in-press).
The Cuscuta plant (dodder) is a parasitic plant, which, in order to survive, obtains nutrients from the host plant.
Cuscuta chinensis seeds (Tu-Si-Zi) are used medicinally in China (Ye et al, 2005; Ye et al, 2002). In ancient times, herbal/medicinal plants were often used as dye plants. The Cuscuta plant is widespread throughout almost all the world's temperate regions. Most of the species, however, are found primarily in the Americas, with Mexico as its center of diversity (Lanini et al, 2005; Garcia, et al, 2007; Stefanovic et al, 2007). Cuscuta americana is one of the species used for zacatlaxcalli-dye; the species grows in Florida, Mexico, and other parts of Central America (Roquero, 2006). The two reference samples, "zacatlaxcalli" and "parasita from Mexico," which showed very similar flavonoid constituents to those in the yellow sample, are from Mexico. This result supports the conclusion that the yellow yarn of the tapestry is dyed with a Cuscuta species from Mexico. However, because there are many Cuscuta species, in order to specify the particular species used on the yellow sample, further study is necessary. This may be the earliest textile, to date, in which this dye—widely reported in historic literature—has been actually identified.
The goal of the project is to understand the origin of the tapestry. Thus far, results of the study suggest that the tapestry is most likely of Mexican, not Peruvian, origin. Also investigated were the tapestry's weave structure, design, fibers (identified as rabbit hair and cotton spun with goose feathers), fiber-processing techniques and, using radiocarbon dating, its date (Phipps et al, 2007). Other dyes on the tapestry were analyzed as well; they appear to be cochineal and a soluble redwood dye such as brazilwood dye, which are also recorded as having been used in Mexico. An additional unknown flavonoid-dye used on the tapestry's light green yarn is currently under study in order to further understand the geographical distribution of flavonoid dyes and refine the origins of this fascinating textile.
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 The dyed reference yarn was received from Lorrena Romen Torres, Escuela Nationale de Conservation, Mexico City.
 The dyed reference yarn was from a sample book: Ferro, P., Giraldo, S.G., Gómez de Padilla, M.C., and Villegas de Villamizar, M., 1996. Manual de Tintes Naturales: siguéndole el hilo al color, Editorial Superior Ltds., Santafé de Bogotá, Columbia.