June 2016 | Kimberly Reczek, head conservator at DBR Conservation

One of the finest examples of Coade stone work, the Nelson Pediment in Greenwich is in need of cleaning. Initial trials have shown how the job can best be done.

The Nelson Pediment, an enormous memorial set in a tympanum in the King William block of the Old Royal Naval College in Greenwich, is a remarkable tribute to Admiral Lord Nelson. Designed by Benjamin West and sculpted with Joseph Panzetta, the 10 x 40 foot alto-relievo piece is a stunning allegorical depiction of the great British hero who died at the Battle of Trafalgar in 1805, just a few years before the work’s commission.

At the centre is an epic Britannia, the moment she receives the body of Nelson, while a great bearded Neptune and his attendant Tritons appear to command the scene. Winged Victory, also present, supports the weight of the dead hero, and the British lion roars his dominion of the seas above a tablet inscribed, ‘CXXII BATTLES.’ The grief of the nation is represented by the weather-beaten face of the British seafarer, broken with sorrow, and a disconsolate trio of sisters, personifications of the English, Scottish and Irish kingdoms, who lament together amid frothing marine horses, ocean-washed rocks, cannons and sinking ships.

The Nelson Pediment has a debt to pay to the arts of ancient Greece. Benjamin West, just a few years before the design of the Pediment, was one of the first to view and study the Elgin Marbles that had arrived in London in 1807. That West was profoundly affected is apparent not only in its frieze arrangement and placement, but also in individual figures that are borrowed directly from the Parthenon sculptures. The Marbles are echoed in the horse’s heads, the torso of Neptune, and the lay of fabric across the three female mourners.

The Nelson Pediment is also significant for its ambitious and highly skilled use of Coade stone, a material reaching perfection in the late 18th century. Also called Lithodipyra (meaning ‘stone twice fired’), Coade stone is composed of ball clay, grog, flint, fine quartz and crushed soda lime glass that is kiln fired at extremely high temperatures over many days.

The process requires extreme control, and the enormous size of the Nelson Pediment’s composition would have necessitated the firing of astonishingly huge masses of clay. The result is considered by many to be the finest example of Coade stone work. Fortunately for us, its highly vitrified nature endows it with a hardness and low porosity that is crucial for resisting weathering and pollution. As a result, despite being over 200 years old and positioned outside, in the King William’s courtyard, the Nelson Pediment is in surprisingly good condition.

In 2015, the Greenwich Foundation commissioned DBR Conservation to investigate the Pediment’s current condition and to secure its stability. The first step was to erect a scaffold for access and to complete a detailed photographic survey. This revealed an urgent need to replace all previous cement repairs or failing fills with lime mortars to match the right palette of greys, pinks and beiges. Replacing these old fills was imperative, because while Coade stone is highly durable, the iron cramps and dowels used to secure it are not; water ingress and subsequent corrosion of the ferrous fixings is the main cause of deterioration. Additionally, in a few locations such as the mane of the lion, the terracotta had spalled, losing its fireskin, and here weathering had accelerated.

Because of the need for both small mortar caps as well as larger fills, dispersed hydrated lime (DHL), mixed with various sands, stone and brick dusts, was chosen. DHL carbonates more rapidly than conventional lime, and it reacts even in the thinnest of layers. Additionally, mortars based on DHL can have a higher proportion of aggregate to binder. For the very large voids, broken shards of new Coade stone, sourced from Coade Ltd, were used as large aggregate, to minimise shrinkage and to bridge gaps.

Once the mortar joints had been secured, work turned to cleaning the surface, which had become disfigured by pollution. This proved to be the most exciting aspect of the project. There were two types of soiling. First, a compact black layer of sulphation/pollution had built up in crevices and along horizontal surfaces. Second, large grey patches disfigured the broad swathes of the sculpture, especially across the vertical planes of limbs and drapery, which appeared to be pollution build-up in the pores of the Coade stone.

Various cleaning methods were trialled: simple mechanical cleaning with brushes and natural and synthetic rubber sponges (wishab and smoke); water poultices with various dwell times and mediums (laponite synthetic clay, cotton, arbocel paper pulp and sepiolite clay) with the addition of a variety of detergents and chemicals (including Vulpex, ammonium carbonate solutions, white spirit, IMS, ammonia and hydrogen peroxide). Any alkaline chemical was used with extreme care, as they are known to cause salts on terracotta, which could damage the fireskin. Only small discrete areas were tested, and thoroughly rinsed with water and steam.

These initial trials were less than satisfactory. The first type of black soiling was only slightly reduced with water poulticing, and the second type was impervious to any of the above cleaning methods. Fortunately, the funding allowed for a series of laser trials using the Nd:YAG Compact Phoenix laser system (Lynton Lasers), which emits infrared light at a wavelength of 1064 nm.

The results were exemplary. Using an energy density or fluence of 0.3–0.5J/cm2, even the most built-up pollution from the sculpture’s deepest detail was safely and selectively removed, even off the more delicate surfaces such as the rose of England. The large patches of ingrained grey soiling across the drapery and the limbs were also removed with gentle laser treatment in conjunction with fine mists of water. Microscopic analysis of the Coade stone’s surface before and after laser treatment confirmed that the cleaning was conducted within safe parameters.

Additional funding is now being sought for a comprehensive laser cleaning programme based on these promising results.

This article has been reproduced from Context, March 2016, with the kind permission of the Institute of Historic Building Conservation. Find out more at www.ihbc.org.uk

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