Excimer Laser Restores Paintings
Brent D. Johnson
With funding from the European Commission, Art Innovation BV of Hengelo, the Netherlands, and its partners have developed a laser workstation for cleaning artwork. It removes years of dirt, oil and wax from a painting, revealing the image as the artist originally rendered it.
An excimer laser system removes the varnish from an oil painting by H.H. Neuman. Courtesy of W. Hesterman.
When conservators took on the task of restoring Michelangelo's frescoes in the Sistine Chapel, there was fear that they might alter these works, which epitomize the High Italian Renaissance. As it turned out, there was a different twist: The dark frescoes that had been formalized in textbooks exploded with color when they were liberated from their years under soot and oxidation. The great master, it seemed, was not sullen and subdued, sulking in drab grays and browns, but impassioned, with an intensity that explored the entire palette of color.
The lesson was significant for the art conservation community. It was not enough to periodically apply a coat of varnish to a canvas. The brilliant lights of the Renaissance were being overlaid with soot and submerged in a sea of resin. Conservators had to find a better way to preserve these icons.
Monique van de Vorle of Art Innovation explained that conventional techniques, such as using solvents to remove the old layers of varnish or soot, were unsatisfactory, either because they did not remove the dark layer or because the solvent penetrated too deeply, damaging the paint underneath. In contrast, the laser technique cleans the surface with submicron-depth accuracy.
Using a Lambda Physik Compex 205 excimer laser at a wavelength of 248 nm, the system strips away the surface deposits layer by layer. A single pulse of the KrF laser removes a 50- to 80-µm-thick layer of varnish.
To ensure that this system does not ablate the paint underneath, a Thermo Oriel MS260i imaging spectrograph and InstaSpec CCD detector monitor the depth of ablation using laser-induced breakdown spectroscopy. The spectrum of light from the painting is recorded with an intensified multichannel analyzer. Depending on the response, the laser continues its work or is moved to another location on the artwork.
Van de Vorle cautioned that laser cleaning should be preceded by a thorough study of the materials and that it may produce color changes and surface charring. Nevertheless, the system has demonstrated sufficient precision that it is being used with regularity by some of Europe's most esteemed conservators.
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