Technique allows ‘peek’ beneath surface of works of Old Masters

Non-invasive surgery is often embraced by patients – especially the squeamish – as potential benefits include minimal discomfort and trauma, reduced recovery time and no scars or post-operative complications.

Now researchers in England are applying the same non-invasive concept to the examination of the works of Old Masters.

Officials from Nottingham Trent University’s School of Science and Technology and The National Gallery in London have developed an instrument capable of capturing high-resolution details from beneath the surface of works by such luminaries as Vermeer, Rembrandt and Van Eyck.

The instrument, detailed in a paper in Optics Express, will allow conservators and conservation scientists to more effectively peek beneath the surface of paintings to learn not only how the artist built up the original composition, but also what coatings have been applied to it over the years.

The latter is important because many great works of western art are covered with several coats of varnish, applied at different times over the centuries. Varnish was applied to protect the paint and make colors appear more vivid but over time it can break down.

The goal is to carefully clean off the old varnish and replace it with new, but to do this safely it helps to understand the materials and structure of the painting beneath the surface. Analyzing the hidden layers of paint and varnish can aid conservation scientists in gathering this information.

Until recently, analyzing the layers of a painting required taking a very small physical sample – usually around a quarter of a millimeter across – for viewing under a microscope. Doing so enables researchers to see a cross-section of the painting’s layers, which can be imaged at high resolution and analyzed to gain detailed information on the chemical composition of the paint.

Because it requires removing some of the original paint, conservation scientists had to operate very carefully, usually only taking minute samples from an already-damaged area of the work.

However, non-invasive imaging techniques researchers such as Optical Coherence Tomography (OCT), originally developed for medical imaging, have proven useful in art conservation.

OCT uses a beam of light to scan the intact painting without removing physical samples and allows researchers to analyze the painting more extensively.

“In OCT, a beam of light is split: half is directed towards the sample, and the other half is sent to a reference mirror,” according to a press release by The Optical Society. “The light scatters off both of these surfaces. By measuring the combined signal, which effectively compares the returned light from the sample versus the reference, the apparatus can determine how far into the sample the light penetrated. By repeating this procedure many times across an area, researchers can build up a cross-sectional map of the painting.”

Researchers used the technique on an early 16th century work titled “The Madonna and Child,” believed to have been painted by Raphael, which is on loan to The National Gallery in London.

“We are able to not only match the resolution but also to see some of the layer structures with better contrast,” explained Haida Liang, who led the project for Nottingham Trent University. “That’s because OCT is particularly sensitive to changes in refractive index.”

In some places, the ultra-high resolution Optical Coherence Tomography setup identified varnish layers that were almost indistinguishable from each other under the microscope, The Optical Society added.

Eventually, researchers plan to make their instrument available to other art institutions. It could also be useful for analyzing historical manuscripts, which cannot be physically sampled in the same manner as paintings.

(Top: “The Madonna and Child,” believed painted by Raphael around 1508.)