BCD 2018

96 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 Cementitious repairs are relatively impervious, trapping moisture and causing further decay at the edges (All photos this page: Taylor Pearce Restoration Services Ltd) Thick gypsum crusts must be carefully removed using hand tools before poulticing with ammonium carbonate Applying ammonium oxalate in a paper pulp poultice to consolidate and strengthen friable, sugary areas of marble the height of the industrial revolution until the Clean Air Act in the 1950s, led to the formation of thick, hard, unstable gypsum crusts on areas of surface which are sheltered from direct rainwater washing. Under these crusts, sugar-like friable subsurface marble formed through repeated dissolution and recrystallisation of salts, leading to the loss of intergranular cohesion. An additional cause of accelerated decay has been the use of inappropriate repair materials, most obviously hard cementitious mortars, which form a relatively impervious barrier and introduce damaging salts into the marble. Considerable damage can also be caused to marble by the corrosion of metal, with expansion and staining of ferrous fixings being a common problem. The staining associated with iron corrosion is both visually disruptive and can also be the first indication of a more serious issue. Cracks and splits caused by expanding iron often lead to material damage and loss of structural integrity, as well as increased moisture ingress. Although copper and bronze fixings do not increase in size through corrosion, extensive staining to marble is a common problem. CONSERVATION The conservation of deteriorated external marble involves a complex set of judgements which will include effective and appropriate methods for cleaning, consolidation, repair and protection. A full inspection and understanding of the condition of an object is always required before establishing a programme of works. Cleaning Cleaning may be required to restore the visual integrity and uniformity of a marble surface, particularly in the case of figurative or highly decorative pieces. The removal of damaging sulphate deposits and metallic staining may also be a priority. Removal of superficial surface soiling can be carried out by simple dry brushing and the use of water and steam on a controlled scale. Steam cleaning can be carried out using small portable dental cleaners, which allow very controlled cleaning to be carried out at a pressure of around 3–5 bar (40–75 psi). Larger scale steam cleaners such as the Doff or Thermatech systems are controllable enough for use in cleaning external marble pieces where, for example, removal of tenacious bio growth or rinsing after poulticing is required. The use of high temperature steam is also thought to sterilise the surface, thus slowing down future bio- colonisation. Careful examination of the marble is required prior to this treatment though, and only sound surfaces should be cleaned by this method. Thick gypsum crusts can be reduced by the careful use of hand chisels to remove the bulk of the calcium sulphate, followed by the application of ammonium carbonate poultices. (These should be used sparingly with as few applications as possible to prevent over-saturation of the marble). Low-pressure micro air abrasive systems and, more rarely, laser systems have been employed with good effect to remove dark sulphation residues and, in the case of water features, hard lime-scale deposits. It is worth emphasising that the many proprietary marble cleaning products available have mostly been developed for the maintenance of new architectural surfaces and are not suitable for use on marble statuary. Usually highly alkaline or acidic in nature, they can be relatively effective at cleaning marble but do not allow the degree of control necessary for sensitive cleaning of sculptural surfaces. They can also leave damaging residues on the surface and introduce harmful salts. The removal of most types of metallic staining, however, requires the use of chemicals. Many conservators have in the past produced ‘field’ mixes, but a few suitable proprietary products are available which have been carefully formulated for historic surfaces. Tests should always be carried out and the manufacturer’s instructions strictly adhered to. Consolidation Dealing with disaggregated marble poses a considerable challenge for conservators. As with all stones, effective consolidation of deteriorated marble is difficult to achieve in practice, particularly externally where control of environmental fluctuations is usually impossible. Some degree of success has been achieved with the use of ammonium oxalate to convert degraded calcium carbonate into calcium oxalate. This treatment has both consolidating and protective properties. Deteriorated calcium carbonate is converted to calcium oxalate which is harder and more resistant to acid attack, with a lower solubility than calcium carbonate and with an improvement in surface cohesion occurring after treatment. This means that extremely friable, quite sugary areas of marble can be consolidated and strengthened allowing sacrificial mortar repairs and sheltercoats to be applied. Repair Carved features that have become loose or detached through deterioration, vandalism or accidental damage may require drilling and pinning. Depending on the structural characteristics of a repair, broken elements can be re-attached where necessary using austenitic grade stainless steel, carbon fibre or perspex rod set in polyester or epoxy resin. Conservation repair fills should: • adhere well but be readily reversible • be similar in composition and performance to the substrate material while being essentially inert • satisfy the aesthetic demands of colour and texture inherent in the supporting substrate • be environmentally friendly and economically viable • be stable in UV light and able to endure environmental fluctuations if used in external locations.