14 BCD SPECIAL REPORT ON HISTORIC CHURCHES 32nd ANNUAL EDITION under low pressure to carefully remove the layers of dirt, with steam cleaning to remove lichens and other organisms that had colonised the masonry. The air-abrasive process relies on the skill of the operator to clean the dirt layers from the stone without damaging the substrate itself. However, in this case the client’s demand was for a level of clean that was significantly greater than would usually be acceptable from a conservation perspective alone. The aim was to blend old work with new masonry, but without leaving tell-tale abrasive marks. The key here was control: restorers adjusted the pressure and particle size according to the surface being cleaned. In the interior where masonry surfaces were heavily contaminated, a latex-based cleaning system was found to be highly effective at removing surface deposits of soot and dirt. The technique involves applying a latex paste to the stonework; after a few days it can be peeled off, bringing accumulated dust and dirt with it. The process was developed in the 1990s and was used in the UK at St Paul’s Cathedral, as outlined by David Odgers in the 2003 edition of Historic Churches. The proprietary cleaning system used then 20 major cathedral sculptural schemes across France, including those of Paris, Chartres, Bourges, and Poitiers, have been restored using laser cleaning in combination with other methods. Where new stone was required to repair damaged vaults and walls, the original Lutetian limestone used to build Notre-Dame had been quarried from beneath the city and, since these quarries are no longer active, replacement stone was excavated from the Oise region in northern France and cut in the Parisian suburb of Gennevilliers. The combined result of the various cleaning processes is particularly obvious in the interior of Notre-Dame. Philippe Villeneuve, the chief architect of French national historic monuments, is quoted as saying that, once completed, the cathedral would be ‘luminous’, and the interior certainly now glows with a clarity it has not had for centuries, a brightness that has surprised even those who worked within it throughout the restoration. Whether this luminosity is seen as a triumph of conservation or a loss of layered historical patina is a debate that will run for generations. What is beyond dispute is that there is a limited New and original stonework blend seamlessly together in the cleaned and repaired pinnacle (right), contrasting with uncleaned fabric lower down the facade (left) was Arte Mundit which, controversially, contained the chelating agent EDTA and ammonia. At Notre-Dame the active ingredients have not been disclosed. Where contamination had penetrated more deeply into the porous limestone, poultices were applied – custom mixes of kaolin and fine clay with mild cleaning agents designed to draw contaminants out of the stone by capillary action as the poultice dries. This method was particularly effective on deeply ingrained soot and smoke staining, where surface cleaning alone could not reach the full depth of the deposit. For the most intricate and delicate elements such as the carved capitals and sculptural decoration, laser cleaning was used. This works by directing short pulses of light at a contaminated surface; the energy is absorbed by the darker soiling material and converts it to vapour or fine particles, while the lighter, cleaner stone beneath reflects the beam and is largely unaffected. The technique had already been used on French cathedrals for three decades – the first laser cleaning on French monuments was carried out between 1993 and 1995 on the south portal of the western façade of Amiens Cathedral, and since then some
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