PROTECTION & REMEDIAL TREATMENT 4.1 119 CATHEDRAL COMMUNICATIONS THE BUILDING CONSERVATION DIRECTORY 2024 ST STEPHEN’S CHAPEL, WEST NORWOOD CEMETERY, LONDON, COMPLETED 2022 In 2020, mechanical, poultice and laser cleaning were tested on the 19th-century terracotta and stoneware statues of the pediment of St Stephen’s Chapel at West Norwood Cemetery, London. These statues had localised surface deterioration (such as crumbling and flaking) and were covered with a thin black layer of pollution (Figure 9). In the trials, laser cleaning was found to be the most successful method of removing the pollution layer, the conservators (Cliveden Conservation) noting ‘this technique was not only the most efficient but also the safest for the surface of the stoneware and terracotta statues; it was possible to fully control the level of cleaning and removal of the black patina without damaging the surface’. In 2022, as part of their overall conservation, the nine statues were cleaned by conservators at Sally Strachey Historic Conservation over a period of four weeks, using a single low-power NdYAG laser (1064nm; 5ns; 4.5W). CHRIST CHURCH GATE, CANTERBURY CATHEDRAL, COMPLETED 2022 Christ Church Gate serves as the main entrance into the precinct of Canterbury Cathedral (Figure 10). The initial construction, completed circa 1520, is made up of Caen and Kentish Rag stone; but there are many important later repairs in a variety of materials, not least from the 19th and 20th centuries. The gate is highly decorated with heraldry and intricate carvings and mouldings, much of it in delicate condition. One of the primary causes of deterioration was black sulphate deposits resulting from atmospheric pollution, particularly prevalent within the carved detail. Jennifer Jordan and Kieran Batchelor, stonemason and conservators of Canterbury Cathedral, report that their initial cleaning trials, carried out with a low-power NdYAG system (wavelength: 1064nm; pulse length: 5ns; maximum average power: 4.5W), gave good results with minimal impact to the fragile surface of the stone, allowing the fine detail of the carvings to be retained. Following this successful testing, a low power NdYAG laser was used to remove the black sulphation layer from the original Caen stone frieze, spanning the length of the building, and the arch around the main gateway, a process that took some three months. Although slower than some other cleaning methods, the consistent and precise nature of the method meant hidden details and traces of surface finishes were revealed. These could easily have been lost with more aggressive cleaning methods. The technology of lasers for conservation has come a long way since those initial tests in Venice in the early 1970s. As the examples here demonstrate, Figure 9: Terracotta and stoneware figures on the pediment of St Stephen’s Chapel, West Norwood Cemetery, before laser cleaning and (below) afterwards (Photos: Sally Strachey Historic Conservation Ltd) Figure 10: Christ Church Gate, Canterbury during conservation, looking through the gate into the cathedral precinct (Photo: Canterbury Cathedral)
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