Dating Silk With Some Fluffy (But Good) Science
Scientists at the Smithsonian have come up with a new way to figure out the age of ancient silk artifacts, such as flags, clothing and tapestries, using just a bit of fluff that's fallen off the valuable textiles.The only other scientific way to date silk is by carbon-14 dating, which requires about 100 times more sample than the new technique. (There's another out-dated "stress-strain measurement" test, which as the name suggests, can put precious silk artifacts through some major mechanical procedures to do the dating. Sounds like just the perfect technique for getting on a textile curator's black list.) Anyway, the new technique monitors a component of silk called aspartic acid. Silk is essentially a bunch of intertwined proteins extruded from a silk worm, and aspartic acid is found within these proteins. Aspartic acid can exist in two forms, the L- and D- forms, which have the same chemical formula but are mirror images of each other. When a silk worm extrudes the silk protein, the aspartic acid is only in the L-form, but over time it transforms into the D-form.
The Smithsonian researchers measured the ratio of the D- and L-forms in a range of silk samples that date back as far as 2500 years ago, when silk was first used as a textile. Older silk samples have more of the D-form, and the scientists have figured out a simple mathematical formula that delivers the age of the silk from the ratio.As an aside, measuring the D- to L- ratio has been helpful for decades in dating a huge range of objects, from ancient ostrich eggs to the bones of human ancestors. And speaking of humans, when doctors take a close look at eye cataracts, low and behold, the aggregates covering the eye lens have a high ratio of D-aspartic acid, which has converted from the L-aspartic acid that is normally found in healthy, young eyes. But back to silk. Although L- to D- dating has been around a long time, the new technique is impressive for the small sample it consumes. Other museums researchers that I contacted for my news story on the topic were enthusiastic about the technique. However they wanted to see the technique validated on more samples, and they wanted the Smithsonian scientists to make sure the L- to D- transformation wasn't accelerated by environmental exposure--such as a silk suit or tapestry spending years in the sun--which could decrease the accuracy of the dating.