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September 8, 2011

Silk Artworks Reveal Their Age

Textile Conservation: A minimally invasive technique can date silk textiles

Sarah Everts

The Textile Museum
TIMING TEXTILES This Turkish tapestry from the 1500s helped calibrate the new minimally invasive dating technique.
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A new mass-spectrometry method will allow museum scientists to date ancient silks from just the fluff that falls off these priceless textiles (Anal. Chem., DOI: 10.1021/ac201746u).

The technique requires a silk sample that is far smaller than what is consumed during carbon-14 dating, the only other scientific method that can date silk, explains Mehdi Moini, a research scientist at the Smithsonian Institution's Museum Conservation Institute. He developed the technique in collaboration with colleagues Kathryn Klauenberg and Mary Ballard.

Silk, which is composed of intertwining strands of protein extruded by silk worms, has been used as a textile for some 2,500 years. "Museums spend a lot of money for valuable silk textiles," Moini says. They can include war flags, tapestries, carpets, banners, and paintings. Curators are often unwilling to sacrifice pieces for carbon dating, he adds.

To develop the new technique, the Smithsonian team took a close look at aspartic acid residues in the silk protein. Over decades, these aspartic acid residues can turn into their mirror images, switching from the L-form to the D-form in a process called racemization. Moini's team used mass spectrometry to calculate the ratio of D- to L-forms, and thereby to determine the age of silk textiles. They could perform the analysis on samples of 100 μg and smaller.

The researchers evaluated their technique on a handful of samples that included fresh silk from 2010; American civil war flags; a man's silk suit from the 1700s; Turkish tapestries from the late 1500s; Egyptian silk yarns from 993 A.D.; and silk artifacts from China's Warring States Period, which occurred between 475 and 221 B.C.

Because carbon-14 dating methods require so much material–a few milligrams--the new technique is a "promising" solution for dating silk, comments Maarten van Bommel, a senior scientist at the Cultural Heritage Agency of the Netherlands.

Although the new technique was tested on a "nice range of objects, only one object was selected for each date period," van Bommel says. He would like to see the new method validated on more silk samples.

Marei Hacke, a scientist specializing in silk at the British Museum in London is also cautiously enthusiastic about the new technique. "The possibility of dating silk in this way does indeed appear exciting and it will be fascinating to determine the accuracy of the technique as more analyses are undertaken over the next years," she says.

In particular, she wonders whether textile contaminants or environmental conditions experienced by the silk affect racemization rates in a way that would undermine the accuracy of the dating technique.

Chemical & Engineering News
ISSN 0009-2347
Copyright © 2011 American Chemical Society
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