By turning to Raman spectroscopy, a non-destructive tool, a researcher at Indian Institute of Technology (BHU) Varanasi has found an easy way to check if the gold foil used in Thanjavur paintings is genuine or not. Currently, there is no regulatory mechanism in place to check the quality of gold used in Thanjavur paintings despite the fact that the paintings have Geographical Indication (GI) tags.

People have long known that not every Thanjavur painting that glitters is gold. There was, however, no way to find out if the gold foil and gemstones used in these traditional crafts were authentic or fake — not without ruining the painting. Now there is: Raman spectroscopy.

The gold foil used liberally in Thanjavur paintings serves twin purposes — it adds glitter to the painting and makes it more attractive and also protects and prolongs the life of the paintings. Foils made of fake material look almost similar to genuine gold thus making it difficult for consumers to know the difference.

By using Raman spectroscopy, Ramanathan Venkatnarayan from the Department of Chemistry at Indian Institute of Technology (BHU) Varanasi and his team of researchers from SASTRA University, Thanjavur, Tamil Nadu have found an easy way to identify if the foil used in the paintings is made of gold or other cheaper material.

Of the 10 gold foils tested, only three were found to be genuine. In the case of paintings, only one or two out of 10 turned out to be genuine gold foil. The gold foils and paintings (mostly made recently) used for the study were borrowed from artisans from in and around Thanjavur. Currently, there is no regulatory mechanism in place to check the quality of gold used in Thanjavur paintings despite the fact that Thanjavur paintings have Geographical Indication (GI) tags, which puts a premium on their authenticity.

Raman spectroscopy, which helps in identifying the molecules, becomes particularly attractive as it is a non-destructive tool and so the foil and painting remain intact even after testing.

The researchers found that laser light at 532 nanometer wavelength used for exciting the foil does not get absorbed if the foil used is made of gold. If the gold used in the foil is in nanometer size, it absorbs the light and has a very broad emission at higher wavelength.

“We have not been able to find out the composition of the fake material that is used in place of gold. We are still working on this,” says Dr. Venkatnarayan. “When we do not exactly know the nature of the fake material, it is impossible to predict the absorption/emission properties [of the material used in the foil]. When the foil is not made of gold, in the Raman you would see some peaks if and only if the fake material is not a metal.”

The researchers used energy dispersive X-ray (EDX) analysis to validate the findings. “Energy dispersive X-ray can also be used for finding out if the foil is made of gold. But unlike in the case of EDX, when Raman spectroscopy is used, the analysis can be done without removing the frame and glass,” says Dr. Venkatnarayan, who is the corresponding author of a paper published in the journal Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy.

Published in The Hindu on June 4, 2018