PROTECTION & REMEDIAL TREATMENT 4.1 115 CATHEDRAL COMMUNICATIONS THE BUILDING CONSERVATION DIRECTORY 2024 DEVELOPMENTS IN LASER CLEANING MARTIN COOPER IT IS now more than 50 years since the first laser cleaning trials on stonework were carried out, when a pulsed ruby laser was used successfully to remove hard black pollution encrustations from weathered marble sculpture in Venice. It is therefore timely to review some recent site applications of laser cleaning, to demonstrate how approaches have evolved. When commercially available flashlamp-pumped NdYAG laser cleaning systems first appeared in the late 1980s and early 1990s, interest in the application of laser cleaning in conservation increased. These early systems tended to be relatively low average power (3 to 20W) and were suited to removing dirt from sculpture and architectural building detail. For example, in 1989–90 a NdYAG laser was used to clean the marble sculptures of the façade of Cremona Cathedral. At Amiens Cathedral, between 1993 and 1996, NdYAG lasers were used on much of the limestone south portal of the west front. This was the first large-scale laser cleaning project on a historic building in France, and indeed one of the first major projects in Europe. The conservators found the NdYAG laser, which delivered up to 10W average power, to be ‘an incomparable tool for cleaning very fragile, sulphated stone, allowing the effective treatment of surfaces hitherto impossible to clean satisfactorily… on comparable, badly degraded surfaces, cleaning by laser is faster than that effected with traditional techniques of micro air-abrasion, chemical poulticing, or nebulised water spraying.’ More importantly, they noted ‘the vastly improved quality of finish’ they could achieve. An important proviso was that at Amiens all areas which retained polychromy were cleaned using other methods, since laser radiation was already known to potentially discolour or degrade certain common pigments and media (a copy of Pouli and Emmony’s useful summary of 2000, in the journal of Cultural Heritage, can be downloaded from bc-url.com/BCD241). As the 1990s continued, similar laser systems were used to clean carved stone on historic buildings around Europe, including major projects at other cathedrals in France, St Stephan’s Cathedral in Vienna, the Palazzo Pubblico in Siena, and the Church of the Maddalena in Venice. Figure 1: Laser cleaning a stone friezes of the Église Notre-Dame de l’Assomption using a 100W fibre laser in 2020 (Photo: Mescla Patrimoine) Laser cleaning systems used on historic buildings tend to emit pulses of laser radiation at a wavelength of 1064nm. The average power of a laser (which is given in watts) is the energy in a single pulse (measured in joules), multiplied by the number of pulses delivered to the surface in one second (measured in hertz). In this article ‘high-power’ systems are defined as any system with an average power of 100W or more. In general, the higher the power of a laser cleaning system, the faster its cleaning rate, and the better suited it is to large-scale cleaning. The systems used in museum conservation workshops and for small-scale outdoor work are generally low power (typically 3–20W) with a beam size of usually 5–10mm diameter. Low-power flashlamp-pumped NdYAG systems generally emit pulses of a few hundred millijoules at a rate of tens of pulses per second. By contrast, the high-power systems used to clean historic buildings tend to emit pulses of laser radiation with energies of only a few millijoules; but this energy is focused to a spot of around 100 microns and delivered at a rate anywhere between tens of thousands to hundreds of thousands of pulses each second. Fastmoving mirrors inside the handpieces rapidly scan the beam across the surface, to a pre-selected shape (generally either a line or a circle) several centimetres across. The fluence of a laser beam, given in joules per square centimetre (J/cm2), is a measure of the concentration of radiation incident on the surface and is calculated by dividing the energy in one pulse by the area of the beam. Ideally, cleaning is carried out using fluences high enough to remove the unwanted material, but low enough not to risk damaging the surface.
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