Science is certainly marvelous, and every so often or so it seems, we are given a new and less obtrusive way to get something done. Such is the case in the realm of conservators and conservation scientists who work with the world’s great masterpieces.
Researchers at Nottingham Trent University’s School of Science and Technology have partnered with the National Gallery in London in the development of an apparatus that will allow conservators to examine a painting in detail, without using invasive techniques. The apparatus developed by the research team allows for non-invasive ‘virtual’ cross-section images of a painting.
The world’s great masterpieces were often layered with varnish, sometimes several coats over the years, to bring out the vivid colors. But, of course, the varnish degrades over the years, and conservators have to carefully remove the varnish and replace it with a coat of new varnish.
But to do this, a conservator needs to know the makeup of the underlying coats of paint used. In order to do this, a minute piece of the painting must be removed for analysis. This involves taking a piece of the painting, perhaps only a quarter of a millimeter across, and looking at it under the microscope. This requires using an area of the painting already damaged and requires a steady hand and the know-how to do it.
New technique using Optical Coherence Tomography (OCT)
OCT was originally developed for use in medical imaging. It uses light to capture three-dimensional images within living tissue. Using near-infrared light with a long wavelength, the device gives us a look deep inside the tissue. One of the main advantages of using this light beam is that it is non-invasive and does not damage the tissue.
Over the past 10 years, OCT has been used successfully in art conservation, history, and archeology. Because OCT uses a beam of light to scan the whole painting without removing any samples, researchers can actually examine the painting in even greater detail. But researchers found that the commercially available OCTs didn’t have the needed spatial resolution for in-depth mapping of the fine layers of paint and varnish.
A new and improved OCT was developed for conservators
Haida Liang, who led the project said, “We’re trying to see how far we can go with non-invasive techniques. We wanted to reach the kind of resolution that conventional destructive techniques have reached.” Liang and her colleagues used a broadband laser-like light source, one that had a concentrated beam of light with a number of frequencies.
The wider frequency range of the light source allowed for much more precise data gathering, and until very recently, was not commercially available. After a few minor modifications, the apparatus was tested on a Raphael at the National Gallery. It performed just as well as the invasive technique.
“We are able to not only match the resolution but also to see some of the layer structures with better contrast. That’s because OCT is particularly sensitive to changes in refractive index,” said Liang. The researchers plan to make the apparatus available to other art institutions. It can also be used to analyze historical documents which cannot be sampled in the way paintings are analyzed.
The paper describing the setup was published in the journal Optics Express on April 13, 2015 under the title: “Ultra-high resolution Fourier domain optical coherence tomography for old master paintings.”
