Historic Churches 2021

18 BCD SPECIAL REPORT ON HISTORIC CHURCHES 28 TH ANNUAL EDITION Conservators in the process of cleaning the Romanesque frieze in the purpose-built shed; one of the lasers can be seen on the floor in the right of the picture. CLEANING ISSUES Cleaning trials began on the Romanesque frieze in May 2018. One unique feature of Lincoln limestone is the beautiful, golden ochre patina that it naturally develops over many years in sheltered areas. Fragile and easily damaged to reveal the tell-tale creamy stone beneath, this acts as something of a ‘canary’ when cleaning – its loss indicating that the operative has gone too far. Micro air-abrasion (MAA) using aluminium oxide had been the technology of choice when cleaning nine of the ten northern Romanesque frieze panels in 1987–90. This widely used technique is capable of giving a satisfactory clean over fragile surfaces and polychromy. Subsequent cleaning in 1997 was then undertaken using a combination of laser and, where polychromy was identified, MAA. When used exceptionally carefully and sensitively, MAA can give a visually satisfactory result. However, subsequent scanning electron microscopy (SEM) images on samples of Lincoln stone clearly showed damage caused at microscopic level by the abrasive particles, which we were keen to avoid. In addition, when trialling cleaning techniques on the frieze and Gallery of Kings in 2018, the same visual results could not be replicated, partly because we were unable to source a satisfactorily sensitive micro air-abrader, and partly because of a necessary change of aggregate from aluminium oxide to aluminium silicate due to health concerns over the former. LASER CLEANING In the last 30 years or so, laser cleaners have come to play a greater role in the restoration of our building heritage. Lincoln Cathedral has a long association with laser cleaning, initially working with leading figures John Larson and Professor David Emmony in the 1990s. John Larson, formerly Head of Sculpture Conservation at the V&A and National Museums and Galleries Mersyside, began investigating the idea of using a laser for sculpture cleaning in the mid-1980s. David Emmony headed up laser research at Loughborough University and collaborated with research at Lincoln Cathedral. This collaboration led to the successful early use of on-site laser cleaning in the UK with the cleaning of a fragment of the Romanesque frieze panel in 1997. As a consequence of this successful project, in 2012 Lincoln Cathedral purchased its own laser for future cleaning projects. The basic principle of laser cleaning is that the dark layer of dirt absorbs the radiation (or energy), while the lighter surface beneath reflects most of that energy away. The Q-switched Nd:YAG laser, most commonly used for stone, works by causing the top layer of dirt to be selectively thermally vapourised, or ablated, from the object’s surface – using what is essentially a very short pulse of heat – an action that occurs so swiftly that the dirt is removed before there is significant heating of the stone substrate. Cross section images show 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. It is a precise and very controllable technique. However, the damaging effect of laser radiation on a number of common pigments is well known. Vermillion and lead-based pigments are especially sensitive, with vermillion becoming permanently blackened. Iron-based pigments tend to be moderately sensitive; while copper- based pigments are generally more stable, although care is still needed. The binding medium in any of these may also play a role. As a result, where polychromy is suspected on a sculpture, lasers must be used with extreme care, and the more information known about the pigments beforehand the better. WORKING PRACTICES Following the erection of scaffolding in 2018, a large shed was constructed with 240V electrics, a clear polycarbonate ceiling and good lighting, providing optimal working conditions for the conservators in all weather conditions. Built with the potential of laser cleaning in mind, the workshops were tightly constructed, and contained an internal locking system to prevent inadvertent access. It should be noted that there are a number of health and safety issues to consider when cleaning with an infrared Q-switched Nd:YAG laser, with potential for damage to the eyes and skin, through inhalation, or as a fire hazard, and training is essential for all operatives to fully understand the risks. Our conservators therefore worked wearing full Tyvek suits and gloves, with 3M Versaflo masks and laser goggles. Local extractors as well as a large overhead extraction system ensured good airflow and ventilation. Before cleaning, polychromy mapping was undertaken on Adobe Illustrator by using a digital overlay on