BCD 2018

95 C AT H E D R A L COMMU N C I AT I O N S C E L E B R AT I N G T W E N T Y F I V E Y E A R S O F T H E B U I L D I N G CO N S E R VAT I O N D I R E C TO R Y 1 9 9 3 – 2 0 1 8 MASONRY 3.2 MARBLE SCULPTURE The conservation, consolidation and repair of external marble statuary and ornament ANGUS LAWRENCE E XTERNAL MARBLE statuary is a central feature of many historic and contemporary landscapes. As well as statues, often depicting allegorical figures, a wide variety of decorative ornaments, water features and furniture can be found in gardens, parks and urban spaces. Taking their lead from examples on the continent, British designers and architects started to incorporate key pieces of decorative work into carefully constructed landscape designs at the end of the 17th century. Initially, objects were imported from abroad and installed as aesthetic and intellectual focal points in increasingly complex schemes. Commissions carried out by celebrated European carvers and the rise of increasingly skilled and fêted homegrown talent, saw this trend continue throughout the 19th century as the fashion for neo-classical pieces flourished. Derived from the Greek marmaros meaning a white, shining stone, ‘marble’ is used to describe a metamorphic rock formed from limestone (CaCO₃) which has been broken down under considerable pressure and heat before cooling and recrystallising, producing a granular mosaic of calcite crystals of roughly equal size. During this process, the original sedimentary elements of a limestone are lost and the resultant pure marble is therefore monomineralic, free from fossils and white in colour. However, small amounts of impurities are often incorporated with the calcite during this metamorphism, resulting in the vast range of coloured marbles. Marble is found and quarried across the globe but Italy has long been the primary source for sculpture and ornament in Europe, with white statuary marbles such as Carrara being used extensively for external pieces. Unblemished by colour or pattern, marbles of this type were considered most suitable for the portrayal of human and natural forms in the ancient world, and the choice was reaffirmed in the Renaissance period. Although Britain has a long history of carving and exporting alabaster, the first examples of marble in this county were antiquities and newly carved imports from the continent, and it was not until the 18th century that marble statuary was being commissioned and carved here. SOILING AND DETERIORATION Biological growth, such as algae and lichens, thrives in the damp climate of the British Isles, resulting in persistent surface soiling to all external stonework but is particularly visible on marble. As the surfaces become gradually more eroded, roughened and porous by repeated weathering cycles, bio-colonisation becomes more prominent and tenacious. As with all calcite stones, the major causes of the decay of marble in an external environment are direct surface erosion through wind and rain, soluble salt crystallisation, etching by acids in the environment, and frost damage. The initial deterioration of finely finished marble surfaces is caused by thermal and hygric fluctuations. A non-reversible deterioration both at and below the surface occurs through the formation of microcracks which are then subjected to increased moisture uptake. Studies of daily temperature and moisture levels in northern Europe have concluded that the wide fluctuations which occur throughout the year have significant effects on marble deterioration levels. The cycle of degradation caused by acids in the environment has been extensively studied and documented. The petrological composition of marble makes external sculptures highly susceptible to atmospheric weathering, and marbles with a high calcite content such as Carrara are particularly sensitive to acidic environmental conditions. When dissolved in moisture, carbon dioxide, nitrogen oxides and sulphur dioxide (the latter from burning coal and volcanic activity) form weakly acidic solutions which can cause the breakdown of the stone. Other products of pollution include particulates of carbon and tar which cause dirt and staining. The deposition of atmospheric particulates, particularly from Marble garden statuary at Powis Castle showing typical organic soiling with pitting on exposed surfaces (Photo: Jonathan Taylor) Loss of surface detail caused by erosion (Photo: Taylor Pearce Restoration Services Ltd)