BCD 2018

97 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 Resin fills often look excellent in the short term but fulfil few of the criteria necessary for long-term sustainability in external situations. Traditional lime mortars, mixed with fine aggregates and, sometimes, earth pigments, have proved to be the most compatible fill materials. Conservation-grade resins have been extensively used in the repair of marble pieces in internal and museum settings where lighting and environmental conditions can be manipulated and controlled. They have also been used in small quantities for filling fine cracks in external sculpture where standard lime-based treatments have proved ineffective. In recent years, however, the development of dispersed nanolimes capable of penetration into fine cracks has enabled new possibilities for more compatible treatments. Proprietary resin-based products, which may have some useful qualities such as lightfastness and general durability, should not be used for the repair of marble statuary. They are generally developed and marketed for filling flaws and damage to new marble elements such as floor tiles and wall cladding and will inevitably fall short of the criteria outlined above. There are also distinct drawbacks in inserting carved indents into weathered sculptural objects. Any new addition, which in standard conservation practice should be carved to the level of the original uneroded surface, will inevitably sit awkwardly with adjacent weathered surfaces. There is also likely to be an excessive loss of sound original material required to fit replacement sections. Surface coatings Many materials have been used as protective surface coatings but few successfully and they are often cause damage. Waxes and oils have been found to readily discolour, attract particulate pollutants and reduce permeability, while materials like soluble glass (sodium silicate) which are used to make the surfaces less porous, damage the marble by trapping moisture. Once applied, these materials are difficult to remove. Readily available proprietary products such as lacquers, water proofers and ‘hard waxes’ should not be used for external statuary and sculptures. Hydrophobic treatments have been favoured for their reversibility but any applied layer that is impervious to water can prevent surface evaporation, trapping any soluble salts within the marble leading to subsurface degradation. A standard conservation practice, which is regarded as effective and suitably benign, involves the application of a thin layer of microcrystalline wax which can provide some protection to marble surfaces in semi- enclosed, unregulated environments such as grottos and colonnades. One surface coating which is compatible with marble is a lime sheltercoat. This can be applied to eroded roughened surfaces to fill pores and cracks as a sacrificial layer, decaying preferentially to the stone itself. It is highly probable that new compatible coatings for marble will follow soon from developments in nanolime technology. Temporary protection Although it has been noted that climatic conditions throughout the year can contribute to the deterioration of marble, it is the weather during the winter months that causes the most concern for those entrusted with caring for external marble pieces. One solution which is frequently employed is to provide an object with a temporary form of protection. At present, winter covers take many forms from close wrapping (usually in waterproof but vapour- permeable sheeting), blankets, duvets and sleeping bag assemblages, tent-like structures and bespoke wooden boxes. There appears to be no firm consensus what is the best approach with some conservators favouring maintaining some degree of airflow and others championing an airtight seal. It can be argued that close wrapping of an object still allows the scale and form of the shrouded piece to be suggested while preventing the marble from being subjected to the worst of the climatic conditions. It is thought that a vapour-permeable membrane can provide a degree of protection against prolonged and repeated saturation, but they entail considerable expenditure both in terms of labour and materials and the jury is still out on their effectiveness. Research into this aspect of conservation is ongoing and it is hoped that a comprehensive set of observations and guidelines may soon be forthcoming. In the meantime the use of winter covers, in whatever form, should always be accompanied by close inspection, recording and photographic documentation at all stages. The condition of the object should always be carefully checked before covering and again when the cover is removed. There are also instances where sculptures are temporarily moved and displayed inside or placed into storage. As with winter covers, this can often be an appropriate approach particularly when locations are closed or public access is restricted during winter months. There is, however, always a risk in moving any sculptural or ornamental feature and any programme of moving should always be carefully assessed and planned and only carried out by experienced and trained staff. Replacement Ideally, statues and other decorative features should always be displayed in their original locations. However, there will inevitably be a point when an object is deemed to be in danger of catastrophic deterioration and an argument will be put forward for permanent removal into a more controllable environment. This poses the question of what to put in its place. Like- for-like replicas carved in marble by hand in the traditional manner often rely on the knowledge and skill of a carver to interpret what may be quite badly eroded and softened detail. Making casts from moulds is a common approach, with casting material based on cementitious mixes. Cement lacks the crystalline qualities of marble but acrylic- modified cement casting composites (such as Jesmonite) can be closely matched to natural stones. Taking moulds from an already friable surface can be a precarious and, in the wrong hands, damaging process. State-of-the-art scanning and cutting techniques (including laser and water jet) now allow very accurate replicas to be produced in a range of natural and artificial materials. In conclusion, the conservation of external marble statuary presents a complex set of decay mechanisms and treatment options. For objects that are in reasonably sound condition or which have undergone conservation treatment, straightforward procedures such as regular inspection and maintenance are essential and the use of winter covers should also be given serious consideration. ANGUS LAWRENCE BA Dip Cons ACR has over 25 years’ experience working on a wide range of conservation projects including major works at Westminster Abbey, the Natural History Museum, Norwich Cathedral and the City of London. He is a director of Taylor Pearce Restoration Services Ltd (see page 107) and is an assessor for the PACR (Icon) accreditation scheme. Marble garden sculpture tightly wrapped for the winter in waterproof but vapour-permeable sheeting (All photos this page: Taylor Pearce Restoration Services Ltd) Replicating a marble statue in Jesmonite, an acrylic- modified cement casting composite