The Building Conservation Directory 2024

PROTECTION & REMEDIAL TREATMENT 4.1 117 CATHEDRAL COMMUNICATIONS THE BUILDING CONSERVATION DIRECTORY 2024 Figure 4: West doorway of Tutbury St Mary, after conservation. (Photo: Hirst Conservation) Figure 5: Detail (left) showing laser cleaning tests on some of the badly weathered alabaster carvings lining the archway, and (right) three of the alabaster beakhead mouldings after cleaning and consolidation. (Photos: Hirst Conservation) for construction sites. Using the correct parameters, they are also highly controllable and the operators were able to sensitively remove pollution layers while leaving patina intact (Figure 3), reducing the risks of overcleaning associated with other methods. It is worth noting that although lasers were used as the primary cleaning tool for the WBRP, they did not completely replace other cleaning technologies. The action of the laser is surface-level only, and so could not penetrate the surface to deal with deeply embedded staining. It was also found to be less effective than some other methods on accretions such as bitumen and paint splatters, and organic material such as lichens and algae. As a result of the success of the WBRP, high-power diode-pumped NdYAG and fibre lasers have since been used to clean other historic buildings and structures managed by the Canadian Government, including The Wellington Wall, Canada’s Four Corners Building, and certain façades of Parliament Hill’s East Block. ST MARY’S CHURCH, TUTBURY, COMPLETED 2018 The Grade I-listed St Mary’s Church in Tutbury, Staffordshire, has a highly significant 12th-century west doorway consisting of finely carved sculptures, including 30 carved stones of local alabaster: a unique example of external alabaster in the UK. In January 2013, significant loss of the carved alabaster was reported, and the subsequent condition survey identified multiple types of deterioration mainly linked to materialspecific qualities, exposure, and historic damage. A dark grey pollution crust covered large sections of the alabaster surface, and this was found to be composed of gypsum, particles of oxidised iron, and organic components (mostly from the combustion of fossil fuels). This crust was not only contributing significantly to deterioration, but was inhibiting the effectiveness of conservation treatments. Surface cleaning was therefore an integral part of the strategy developed by Hirst Conservation. Various cleaning methods were tested. Poultice methods were considered unsuitable, since they relied on substantial mechanical action and water usage, both of which could cause etching of the alabaster surface. Laser cleaning was found to deliver the most effective and controlled method for removing the crust and proved effective even on the most friable surfaces (Figure 5). Cleaning was carried out over a period of four weeks using a low-power NdYAG laser (1064nm; 5ns; 4.5W) mainly at a fluence of 0.5–0.7 J/cm2 with a beam approximately 5–7mm in diameter. The fluence and repetition rate (mostly 15–20Hz) were adjusted according to the condition of the surface, with the most fragile areas being cleaned at slightly lower fluence and a repetition rate reduced to 5Hz. With subsequent repairs and consolidation, conservation was completed in 2018.