A fashionable silk coat from the 1700s.
(images from the Museum of the City of New York - http://www.mcny.org;
http://s481.photobucket.com/user/PiafEdith/media/Habitdeceremonie.jpg.html)
Pair
of Cranes on a Branch, from the Freer|Sackler
Gallery at the Smithsonian Institution. This Japanese painting was done on silk.
(image from http://www.asia.si.edu/collections/singleObject.cfm?ObjectNumber=F2004.16)
The
ribbon of this Order of the World War medal is made of silk.
(Image
from
http://smithsonianscience.si.edu/2011/09/smithsonian-scientists-devise-new-technique-for-dating-silk/)
A common method of dating an object is radioactive carbon-14 dating, where the ratio of carbon-14 / carbon-12 from the object of interest is compared to the ratio found in the atmosphere. Knowing that radioactive decay follows first-order kinetics and that the half-life of carbon-14 is 5730 years enables us to determine the age of the object up to about 60,000 years.A good primer on the origin of carbon-14 and how it is used to determine the age of something can be seen here:
The problem with using radiocarbon dating is that the sample size for a reasonable analysis should be ~20-50 mg. For a rare and small object, that can be considered too much material to sacrifice to a destructive process.
To address this concern, Dr. Mehdi Moini (currently an Associate Professor at the Dept. of Forensic Sciences at George Washington University) along with Mary Ballard (Smithsonian Senior Textile Conservator) and Kathryn Klauenberg (Smithsonian Museum Conservation Institute intern) developed a highly sensitive and reliable way to date ancient silk artifacts using capillary electrophoresis mass spectrometry.
Mehdi
Moini and
Kathryn Klauenberg from
The Museum Conservation Institute at the Smithsonian Institution. Between them
is the Beckman-Coulter capillary electrophoresis instrumentation interfaced
with an LCQ Duo Finnigan mass
spectrometer.
With
this instrumentation, they are able to detect compounds on the attomole
scale
(1 attomole = 10-18 mole)
(image from http://www.si.edu/mci/English/research/technical_studies/ProteomicsResearch.html)
While the concept of using an electric field to separate charged species started in the 60's, automated instrumentation became available in the late 1990's. A nice explanation of the technique is provided by Dr. David Kreller from Georgia Southern University.
(image from https://www.youtube.com/watch?v=CXenfe4lMxQ)
What did the research team look for in order to determine the age of a rare silk artifact? It turns out that silk is a long protein molecule composed of many amino acids, and one amino acid in particular - aspartic acid - has the ability to racemize at a fast enough rate to make the analysis reasonable for objects that are less than 2500 years old.
Nonsuperimposable mirror image molecules (enantiomers) of aspartic acid. The chiral carbon and acidic R group are highlighted.
Where do you find aspartic acid in silk?
Bombyx mori and a silkworm cocoon
(image from http://www.textiletoday.com.bd/oldsite/magazine/574)
It turns out that when you look at an individual fiber of silk, it is composed of 2 kinds of proteins. One is called fibroin which makes up silk's structural center, and the other is sericin which is a gummy substance that coats the outside of silk fibers and enables them to stick together. Aspartic acid is one of three main components of sericin.
(image from http://www.seiren.com/english/products/medical/sericin/)
L-aspartic acid becomes D-aspartic acid over a long time, and by measuring the ratios of D/L isomers in silk samples of known age, Dr. Moini and his colleagues were able to determine the half-life of the racemization to be about 2500 years.
(image from http://pubs.acs.org/doi/abs/10.1021/ac201746u)
Electropherograms of known silk samples showing a greater presence of D-aspartic acid in the oldest samples.
(image from Anal. Chem., 2011, 83 (19), pp 7579; DOI: 10.1021/ac201746u)
With a 20 minute analysis time using no more than 100 micrograms of silk fibers, the research team developed a method of determining the age of rare silk artifacts without involving significant amounts of material. Because such objects are usually carefully stored in museums and galleries, factors like temperature, humidity, and amount of UV radiation exposure are less problematic.
For further reading:
*Dating Silk By Capillary Electrophoresis Mass Spectrometry; by M. Moini, K. Klauenberg, and M. Ballard; Anal. Chem., 2011, 83 (19), pp 7577–7581;
