The Building Conservation Directory 2024

118 THE BUILDING CONSERVATION DIRECTORY 2024 CATHEDRAL COMMUNICATIONS ÉGLISE NOTRE-DAME DE L’ASSOMPTION, ROUFFACH IN ALSACE, FRANCE, COMPLETED 2020 (with thanks to Martin Labouré, Mescla Patrimoine) Rouffach Church in southern Alsace was mainly built between the 11th and 14th centuries, using the local yellow sandstone. Exposure to atmospheric pollution had caused the western portal and sculptures on both sides of the nave to be covered by a black sulphation crust, and after testing the decision was taken to use a fibre laser (1064nm; 100ns; 100W) to clean over 100 square metres of carved stonework, including the entire western portal, the side portals, and the nave friezes. This enabled the conservators to achieve the required sensitivity on fragile areas of the stone surface, and also to avoid the large amount of powder generated during air-abrasive cleaning (giving clear visibility of the surface being cleaned, and minimising clean-up time in the work area). Air abrasives were used to clean flat areas of stonework (excluding the western portal) where the pollution crust was relatively thin as it was faster than laser cleaning in these areas. The western portal, where the crust was thicker (approximately 1mm), proved the most difficult and slowest to clean, but lasers gave the necessary control and selectivity on both sound and degraded areas of carved stonework (Figure 6). Moreover, when the time required to incorporate containment of abrasive powder on the scaffold and to clean up the work area afterwards was taken into account, for the carved surfaces of the frieze, laser cleaning was faster than airabrasive cleaning. from laser radiation. The final treatment method therefore used a combination of lasers (1064nm; 100W and 500W systems) and low-pressure steam (superheated water at 160°C, pressurised to 75 bar for ashlar but just 25 bar on decorative stone). The complete cleaning process involved a first pass with steam. After this a 50:50 mixture of denatured alcohol and water was sprayed onto the surface, immediately followed by laser cleaning. The liquid helped to disperse the energy in the laser beam and provided some additional chemical action to address the biological growth. Cleaning was then completed with a second pass of steam. LINCOLN CATHEDRAL WEST FRONT FRIEZE AND GALLERY OF KINGS, COMPLETED 2021 (with thanks to Jane Cowan, Lincoln Cathedral Works Department) In a major conservation of the medieval sculpture of the west front of Lincoln Cathedral, conservators used two lowpower NdYAG (1064nm; 5ns; 4.5W) laser cleaning systems to remove pollution crusts both from the nine relief panels of the south section of the Romanesque frieze (Figure 8) and from the eleven 14thcentury life-sized sculptures of the Gallery of Kings. Laser cleaning was selected by the conservation team as the main method of cleaning due to its ‘unequalled controllability and the finish it provides… one of the laser’s great advantages is its ability to retain the gypsum-rich patina of the stone located just beneath the crust of dark particulates’. As in Amiens 25 years previously, and for the same reasons, areas with surviving polychromy were cleaned using techniques other than laser (including selective micro air-abrasion and gel poultices). Figure 7: Laser cleaning on the Jefferson Memorial (Washington DC, USA) using 1064nm; 100W and 500W systems (Photos: Evergreene Architectural Arts Inc) Figure 8: Limestone relief panel from the Romanesque frieze on the west front of Lincoln Cathedral during laser cleaning (right side after cleaning) (Photos: Lincoln Cathedral) THE JEFFERSON MEMORIAL (WASHINGTON D.C., USA), COMPLETED 2021 The Jefferson Memorial (Figure 7), first opened in 1943, is veneered in bright white Vermont marble. In 2006 it was noticed that this was being colonised by a dark grey biological film (a microbial colony of algae, fungi and bacteria). At first this film was restricted to discrete areas, but by 2019 it had become much more pronounced, and had disfigured a significant area of the surface (including the dome, pediment, entablature, column capitals, stylobate, stairs, and retaining walls). For the cleaning project which took place between 2020 and 2021, the use of chemical cleaning methods was dismissed because of the potential environmental impacts to the neighbouring tidal basin, and so thoughts turned to laser and steam removal. A range of preliminary treatment options were tested under the direction of the US National Parks Service and their contractors. Because the marble’s surface had weathered, creating small pits between the granules of the stone, laser cleaning alone proved unable to completely remove the biological growth: biological material within these pits was physically protected Figure 6: Western portal of the Église Notre-Dame de l’Assomption (Rouffach, France); left side before laser cleaning with a 100W laser, and right side afterwards (Photo: Mescla Patrimoine)