Historic Churches 2021

BCD SPECIAL REPORT ON HISTORIC CHURCHES 28 TH ANNUAL EDITION 19 a photograph to indicate all the areas of coatings and paint we found (illustrated on theprevious page). This mapping, as well as providing a record of the sculpture as found, was a working tool for the conservators, highlighting areas where extreme caution or alternative methods of cleaning were to be used. Following cleaning tests and trials the decision to use the laser was a straightforward one, but we were left with the issue of how to approach cleaning the areas of polychromy vulnerable to potential laser damage. Our solution came in two forms. Firstly, the use of a polymer gel applied to laser-sensitive areas, which gave full control when removing light dirt. This comprised a polymer-based poultice used to soften and remove thin sulphation layers (carbopol and triethanolamine base with 1–2% tri-ammonium citrate for the upper dirt layers) applied with variable dwell times, and removed using de-ionised water. Secondly, where dirt was more tenacious, our ‘back-up’ system was an IBIX 9 Eco-Blaster dry-air abrasion unit, with a 1mm pencil nozzle using fine garnet powder, which gave a sufficiently sensitive result in challenging areas when used between 0.5–1 bar pressure. Cleaning began with the removal of any loose debris and build-up of guano deposits: thick black sulphation crusts were removed using fine riffling files to save time. The area of carving to be cleaned was checked for any known signs of polychromy, and if found to be clear, testing was undertaken to fine tune the laser parameters. Following this a first pass of the laser was undertaken. If the dirt was found to be stubborn, a light misting of water was used to help increase absorption of laser energy by the dirt, potentially enhancing the cleaning effect and speed. Although highly discriminating, the term ‘self-limiting’ for laser cleaning can be misleading – if an object absorbs energy, damage is possible. The lowest energy fluence (or strength) was used, usually between 100–115 (the full range being 90–210), and a pulse rate of 15 per second (out of 1–30). In terms of polychromy, the fluence level needed to remove the dirt layers is important – the lower the fluence, the less likely there is to be inadvertent discolouration or damage to the pigments. If an area of polychromy or an unidentified coating was observed or suspected, laser cleaning was avoided. If required, this area was then revisited with gels for more controlled cleaning, or, where necessary, the IBIX. On a practical note, the model used was Lynton Laser’s Compact Phoenix, which is roughly the size of an office A4 printer and can be carried onto site by two people if required. Due to its temperature sensitivity – the water cooling system means that allowing it to stand in freezing weather would be highly inadvisable – it sometimes required wrapping up or bringing down from site in very cold weather. Luckily, it has proved suitably robust in being regularly man-handled. The speed of laser cleaning is highly variable dependant on the thickness and type of dirt and the location of the sculpture. In order to avoid piercing the rich ochre patina or producing a ‘leopard’ print effect with high spots of black dirt remaining, the process required constant vigilance and sensitivity. Cleaning of the nine Romanesque frieze panels at low-moderate strength and speed took nine months in total, with generally two conservators working side-by-side (a second machine was hired due to time pressures). The Gallery of Kings is in its final stages at the time of writing and will have taken 10 months with one conservator. As an aside, on-site testing showed that this rate was considerably faster than comparable MAA/IBIX cleaning. THE RESULT The effective removal of decades of dirt gave us fascinating glimpses of the sculptures’ original polychromed appearance. It revealed small human details such as pencilled graffiti from those who went before us, and previously unseen details such as 18th-century resin repairs (still holding up!). The aim of our finished clean on the West Front sculptures was to remove 90–95 per cent of soiling rather than 100 per cent, giving us the opportunity to ‘pull back’ when we felt that areas were vulnerable, while still achieving an overall harmonious appearance. Cleaning on any conservation project can be a multi- faceted and complex process; rarely does one-size fit all. On-site cleaning with a Q-switched Nd:YAG portable laser is generally a superior cleaning method for stone, but can be expensive to hire or purchase and maintain. It presents some major practical obstacles, requires safe working practices and ultimately, as with all methods, the quality of the end result depends on the skill and sensitivity of a trained and experienced practitioner. However, at Lincoln Cathedral the laser successfully provided the high level of delicacy and control needed for this project, especially when integrated with other techniques such as gel poultices, selective micro air-abrading and polychromy mapping, giving minimal damage and optimum results. JANE COWAN ACR is Lincoln Cathedral’s Head of Conservation (https:// lincolncathedral.com/history-conservation/ ). She is a stone carver with a master’s in stone conservation from Bournemouth University and an Icon-accredited conservator. Detail of a cleaned carving from the Gallery of Kings canopy (approx 30mm (1.18 inches) in length) with remains of green polychromy and white ground around the eye