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Scientists improve technique of scanning layers of paintings

Posted April 16, 2015

Researchers from Nottingham Trent University’s School of Science and Technology have partnered with the National Gallery in London to develop an instrument capable of non-invasively capturing subsurface details from artwork at a high resolution. This is going to help restoration of invaluable art pieces and might replace invasive methods in the future. New technique provides high-resolution images that will help understand the structure of classic paintings better.

During time varnish on paintings like this degrades and restoration is hard and long process. It is crucial to know what layers are beneath the surface. Image credit: Annibale Carracci via Wikimedia, Public Domain.

During time varnish on paintings like this degrades and restoration is hard and long process. It is crucial to know what layers are beneath the surface. Image credit: Annibale Carracci via Wikimedia, Public Domain.

Sometimes old paintings hanging in an art gallery look dark and dim. Usually it is because varnish, which was originally applied to protect the paint underneath and make the colours appear more vivid, degrades over the centuries.  Restoring former glory of the invaluable paintings is a hard task for conservators as they have to carefully clean off the old varnish and replace it with new. To do it safely they must know structure of the layers underneath and this is where new technology is very useful.

Traditionally, conservation scientists used to take a very small physical sample — usually around a quarter of a millimetre across — to view under a microscope. This technique provided opportunity to look at a cross-section of the painting’s layers, which can be imaged at high resolution and analysed to gain detailed information on the chemical composition of the paint. However, it is an invasive method, even if only a very tiny amount of paint is taken.

More recently scientists started using a method called Optical Coherence Tomography (OCT). Originally intended for medical use, OCT is non-invasive method to look under the layers of the paint and varnish. It uses a beam of light to scan the intact painting without removing physical samples. However, until now resolution of commercially-available OCT setups was not high enough to fully map the fine layers of paint and varnish of these priceless artworks.

OCT works by splitting beam of light – half is directed towards the sample, and the other half is sent to a reference mirror. By measuring the combined signal, which effectively compares the returned light from the sample versus the reference, the technology can determine how far into the sample the light penetrated. By repeating this procedure a number of times, conservation scientists can create an entire image of painting’s layers.

Now scientists used a broadband laser-like light source. It is a concentrated beam of light containing a wide range of frequencies. This allows for more precise data collection. Together with other modifications this allows OCT to scan the painting at a higher resolution. It was tested on a late 16th-century copy of a Raphael painting and performed as well as traditional invasive imaging techniques. Furthermore, advanced OCT setup identified varnish layers that were almost indistinguishable from each other under the microscope.

Now scientists plan to make this technology available for other institutions as well. It would be very beneficial for analysing historical manuscripts, because they cannot be physically sampled in the same way that paintings can. They also want to make apparatus that would combine advantages of longer and shorter wavelength light sources. This new technique in the future will help restore a lot of art pieces that faded in the course of history.



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