Dating Silk With Some Fluffy (But Good) Science

Just a little fluff can reveal the age of silk artifacts, such as the age of this Egyptian textile from 900 A.D. Courtesy: The Textile Museum

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.

A Turkish silk tapestry from the late 1500s. Credit: The Textile Museum

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.

Author: Sarah Everts

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3 Comments

  1. That last sentence was my first thought early on in the article. The beauty/curse of radioactivity is that is is insensitive to temperature, which makes it great for dating materials, but causes us immense problems with nuclear waste disposal – we can’t just heat it up to make it decay faster.

    But the change from L- to D- involves a transition state and kinetics subject to Arrhenius behavior and other variables. I really question how useful this technique will prove to be over time.

    • This effect of environmental factors is already discussed in the paper. Another problem with carbon dating is that it does not provide compound identification nor purity information, so you never know if the sample analyzed was the real thing or if it contained impurities. CE-MS provides both these information as well as dating.

  2. I was told by several conservators that currently very little C-14 dating is being done on silk because it simply requires too much sample, so in practice that means there’s no scientific way to date silk. (Museum staff often do it by association… for example, the age of the other artifacts found with silk object.) So perfect or not, this technique is currently the best thing yet.

    I discussed the effects of the environment with the first author and he told me that he figures most of the silk currently in museums was probably well-cared for over the artifact’s ovrall history (therefore it survived to modern day) and that this suggests it was stored well out of the sun and heat for most of its lifetime. There’s no proof this is the case, but it’s not a crazy assumption.

    As for the more technical details, you might want to take that up with the first author.