A Handy Way to Date Silk
Of all the precious and fragile artifacts in museums, silk clothing and artwork can pose the most puzzles for historians.
They frequently have to rely on circumstantial evidence and guesswork to assign a date of manufacture for ancient silk clothing and tapestries, as traditional carbon dating requires samples so large that it visibly damages the fabrics. A new ultrasensitive chemical technique now promises to solve those silky mysteries.
The new dating method came about when chemist Mehdi Moini joined the Smithsonian Museum Conservation Institute in Suitland, Maryland, last year and gained easy access to the Smithsonian's famous collection of silk treasures. Moini had been working for years on a technique for measuring the natural decay of materials made of protein. It seemed to be sensitive enough to measure the exact amount of decay that occurs on an hourly basis- and that amounts to a molecular clock for dating proteinaceous materials like silk. The problem, until his new job, was that he had no rigorously dated silk artifacts to calibrate his clock.
Moini's technique relies on the tendency of amino acids in protein to flip from what is known as a left-handed form to a mirror-image right-handed version. That shift in the orientation of the molecule's carbon backbone is a rare event, but it does happen at a steady rate. The more time that has passed since a silkworm spun the thread, the greater the ratio between the two types of amino acids. By combining two chemical techniques- narrow capillary electrophoresis and mass spectrometry- Moini figured out how to measure that ratio in just 20 minutes using microscopic samples of silk.
Reassured by his nondestructive technique, curators at the Smithsonian and several other museums agreed to let a team led by Moini sample silk objects spanning over 2000 years of history, from ancient Chinese silks and French Renaissance tapestries to a U.S. Civil War flag and silk spun last year. Historical records provided clear dates of manufacture for the museum specimens, and the new technique proved accurate enough to nail down their ages within a 50- to 100-year window, the team reports in the current issue of Analytical Chemistry. The calibration curve "fit very nicely," Moini says, noting that no silk artwork was harmed.
Moini's study has "implications beyond identifying forgeries," says Todd Blackledge, a biologist at the University of Akron in Ohio who studies the evolution and mechanics of silk. Both silkworm and spider silk, for example, have potential uses in biomedical devices, he notes. So "we need to understand how the silk will change through time," he says, and Moini's technique appears to do that at "a very fine scale."