Investigating Natural Dyes with New York’s Metropolitan Museum of Art

When the Metropolitan Museum of Art in New York wanted to run an exhibition of tapestry and silverwork from the Colonial Andes, they had to be certain of the exhibits’ provenance. One particular tapestry described as Peruvian caused some alarm as it appeared slightly different from the other Peruvian textiles. How could the provenance be checked? Luckily chromatography could help — and it is the story of a dye.

Flavonoids — Natural Dyes

Most dyes we now use are synthetic dyes, first developed in the mid-nineteenth century from coal tar, and modern chemistry has given us cheap and colourfast dyes that are easy to use. But man has been using dyes for thousands of years — natural dyes — which come from plants, insects and animals.

Flavonoids are a major source of plant dyes — usually yellow dyes. In plants, flavonoids are the most important pigments for flower colouration, and are also used in UV filtration and nitrogen fixation. Flavonoid dyes are mordant dyes which means they need additional chemical help to “fix” the dye to the fabric. And it was to the yellow dyes on the Peruvian tapestry that researchers turned as they tried to confirm its provenance.

The team had chosen a challenging component to work on because of the large numbers of flavonoids known, how a plant’s flavonoid profile might change at different growth stages, and because the environment it grows in can change the flavonoid. However, flavonoids have been used for many years as markers by which plants can be classified according to their type. Therefore there is a large database of flavonoids, so the team had plenty of reference material to compare their samples against.

Chromatography finds the answer

Due to the nature of the sample — an historic tapestry — it is preferably to use only a small sample size. Fortunately, high performance liquid chromatography (HPLC) only uses small samples. The researchers took a sample of yellow yarn from the tapestry and extracted the dye from it.

The resulting extract was analysed using HPLC with an ultraviolet detector, and the retention time and UV spectrum of the eluents recorded. The dye’s spectra could then be compared against reference spectra and retention times — identifying the flavonoids.

Unfortunately, although the main flavonoids could be identified, further analysis using LC/MS was needed to fully characterize the flavonoids. MS is a more sensitive detector than UV — but it is a specialist instrument not available in all situations. A close look at the development of LC-MS can be found in this article: The fascinating history of the development of LC-MS - a personal perspective.

Peruvian?

The results from the analysis suggest the flavonoids come from a plant type known as Cuscuta, which grows in many of the world’s temperate regions. However, the reference samples that most closely matched the tapestry’s samples are from Mexico. Other dyes from the tapestry were also investigated — supporting the findings from the flavonoid’s analysis.

So in this case, although a definitive provenance cannot be given, the most likely origin of the dyes used in the tapestry is Mexico — not Peru.

Image: Metropolitan Museum of Art