technology in the 21st century
|Isle Abbots at the moment of truth, as the last of the scaffold poles are removed|
The conservation of limestone churches using the 'lime technique' or 'lime method' has become firmly established and widely used over the last 25 years. However, some aspects of the technique have been refined over time, and others have been abandoned. This case study, concerning the conservation of exterior limestone masonry and fine sculptures of a Somerset church tower, provides the opportunity to review the technology, its development, and how its interpretation and application have impacted on the fabric of our churches.
The tower of St Mary the Virgin, Isle Abbots near Taunton rises above the watery landscape of the Somerset moors, gloriously adorned with elaborate niches, sculptures, beasts, and demi-angels, as well as pierced stonework in the windows. The tower is crowned with a parapet positively bristling with pinnacles. Constructed from Ham Hill, Lias and Doulting stones it is not grand in scale, but perfect in form, design, light, space and setting. It is one of the most complete and beautiful of the decorated towers of Somerset.
In 2004 the attention of the church's architect, Bob Chambers, had been drawn to the condition and stability of the parapet and pinnacles as well as the 39 carved elements decorating all the elevations. It was with considerable excitement that the conservation firm of Strachey and Strachey subsequently received a request to advise on the project, and two years later their team of conservators, carvers and masons were delighted to be awarded the contract to carry out a programme of conservation and structural repair on the tower. They were joined by the sculpture conservator Hebe Sampson and the building archaeologist Jerry Sampson, whose work on the cathedrals of Wells, Salisbury and St David is well known.
Apart from addressing the serious structural problems at parapet level, the project could be seen as a classic programme of lime conservation, treating every stone on the west front from top to toe, as well as the decorative elements on the north, south and east elevations. In order to understand where we are today with the conservation of limestone I feel it is worth taking a look back to the 1970s and the development of the lime method of conservation for limestone sculptures.
THE DEVELOPMENT OF MODERN LIME TECHNOLOGY
During the 1960s and into the 1970s heritage bodies and organisations were becoming increasingly concerned at the accelerated decay of the overall fabric and detail of many of our churches, cathedrals and historic buildings. The three main culprits were environmental pollution, the extensive use of cement mortars since the 1860s and the presence of ferrous fixings throughout these complex structures.
By the 19th century the increase in pollution was rapid, due to an explosion of industry and population in cities, towns and countryside, and the consequent acceleration in the burning of fossil fuels. Limestone, as well as being a porous building material, is rich in calcium carbonate. This makes it highly susceptible to atmospheric sulphurous acids from the burning of fossil fuels. The calcium carbonate binder in the stone undergoes a chemical change to become calcium sulphate, which is one of the soluble salts most closely associated with the decay of the limestone through repeated cycles of salt crystallisation. By the middle of the 19th century some of the carved surfaces of our historic buildings had accumulated a thick carbon deposit behind which the soluble salts were wreaking havoc. The result was the exfoliation and blistering of surfaces behind which the stone had been reduced to powder.
|Above left: St George, south elevation, before conservation: the detail of the carving has been obscured by heavy lichen growth but the exposed iron dowel can be clearly seen running through the back half of the horse. The Ham Hill shafts have been edge bedded and show evidence of lamination and loss of surface. Above right: St George after the final application of the sheltercoat: great care was taken to push the sheltercoat into the hairline cracks and to rub it back enough to reveal the character of the stone|
Hot on the heels of environmental pollution came the invention of Portland cement in the first half of the 19th century. This new and exciting material was highly regarded for its hydraulic properties, fast setting times and great strength. It is easy to see why the custodians of our churches and cathedrals must have thought that Portland cement was the ideal repair material which would cope with the increase in stone decay due to environmental pollution, particularly as it had the added benefit of colour matching the blackened surfaces. However, Portland cement is now known to be a potential source of sulphates, thereby contributing to the decay process. Of even greater significance is the fact that Portland cement has a dense and impervious nature which encourages concentrations of moisture and soluble salts at the interface of the cement repair and the limestone. In most cases where a cement repair is removed, the limestone beneath will be found to have been reduced to powder by the crystallisation of salts within the interface.
The third main culprit is ferrous fixings, such as wrought iron cramps, which are embedded within stone structures. Whether large or small, ferrous fixings expand over time through corrosion, resulting in the splitting, cracking and dislodging of sections of stone. The end result of this process is that the structural integrity of the sculpture, pinnacle or larger architectural element can be threatened.
Despite the overwhelming problems facing building conservation in the 1960s and 1970s, there were a number of architects, scientists, historians and artists who were up to the challenge and together pushed back the boundaries in the treatment of external stonework. Among these were Dr Clifford Price at the Building Research Establishment, John Ashurst, Principal Architect of the Historic Buildings and Monuments Commission, Peter Burman at the Council for the Care of Churches and the conservation departments of the Victoria & Albert Museum and the British Museum. Into this heady mix we can add Professor Robert Baker who by the 1970s had already achieved an illustrious career as an artist, teacher and as Director of Design at Royal Worcester. Professor Baker's approach was to return to the application of a traditional system of materials based around lime. There was nothing new or revolutionary about this approach, as most aspects of the lime technique had been used historically in the care and upkeep of buildings. His achievement was to draw traditional technology into a specific methodology, with a heavy emphasis on the attitude and technique of the conservator, as well as an aesthetic approach to the sculpture. This development was timely, coinciding with changing opinion in favour of conservation rather than restoration as the way to move forward; that is to say the belief that, in order to retain as much of the original surfaces and detail as possible, repair should always be considered before replacement.
The lime technique which emerged in the 1960s and held sway for the next 20 years or so involved:
- cleaning with a hot lime poultice kept moist for two to three weeks followed by timed water sprays of a four-second wetting time with 'dry intervals' of four minutes
- the removal of cement and where possible 'live' ferrous fixings
- consolidation by approximately 40 applications of limewater (Limewater was a traditional method of consolidation in which the principle was the deposition of calcium hydroxide into the decayed stone where it will carbonate in contact wit hair. Its purpose was to replace the calcium that had been lost from migration, gypsum formation and weathering.)
- grouting and packing internal voids with a fine adhesive lime mix
- dowelling and micro pinning unstable elements
- the application of lime mortar repairs
- the application of a lime 'sheltercoat', a thin protective layer applied in the last stage of conserving a building or sculpture.
|The Resurrection after conservation with all the glorious detail revealed|
|The head of Christ in the Resurrection, illustrating how the sheltercoat has been carefully applied up to and not over the areas retaining paint, which include earth browns over the hair and skin and a copper blue/green on the crown|
In 2006 as we consider the conservation of the tower sculpture and masonry at Isle Abbots what are the key changes to this technique and how has it been adapted and extended?
The two elements that have been virtually removed from the original system are cleaning with a hot lime poultice and consolidation with limewater. In the case of cleaning, the developments of safer and more effective methods of removing harmful or unsightly materials from the surface has been one of the most exciting areas of stone conservation. By 1984 the lime poultice was being replaced by micro air abrasion and ammonium carbonate poultices. Nebular spray systems were being refined to allow the conservator to be far more accurate as to where and to how long the surface of the stone would be moistened. Over the next few years this arsenal was widened with the introduction of the Jos/Torc vortex water/abrasive system, steam cleaning with the Doff and Deroter systems and with a particular eye to the cleaning of external polychrome using the ND:YAG laser. The correct choice of cleaning technique is absolutely critical and should be arrived at through a careful study of the geology of the stone, its condition and the decay mechanisms present as well as trial studies.
The use of limewater has been gradually omitted from the conservation of limestone due to increasing concerns over saturating salt laden stone and so activating the movement of soluble salts. It should also be avoided with certain limestones that have a high clay content such as Ham Hill stone. Although limewater is a traditional method of conservation, its effectiveness is hard to quantify despite continuing research. We now prefer to consolidate friable surfaces which need repair with two or three applications of an adhesive lime mix which has been thinned down to the consistency of milk.
THE NEED FOR INTERVENTION AT ISLE ABBOT
It was important before starting an extensive programme of repair to such a beautiful structure as Isle Abbots tower, to review what we were trying to achieve. There was no doubt that at parapet level addressing the poor condition of the decorative stonework and the instability of the whole structure was a priority within the contract. Although the rate of decay of the Doulting sculptures at the heart of the tower appeared to be slow, with little evidence of immediate dramatic loss, there were further considerations. The fine detail of the carving had been blurred by a heavy growth of lichen, and the character of the stone completely lost. The discovery, made from an access scaffold, that traces of polychrome survived on the lower west front sculptures was seen as an incentive to carry out a detailed cleaning programme in order to record and assess the condition of these historic paint layers. Furthermore the true condition of the sculptures could only be accurately assessed once the lichen was removed.
The Ham Hill canopies and niches which protect the sculptures were also in urgent need of attention as they were already suffering from lamination, hairline cracking and loss of surface, and their condition was deteriorating. This limestone is particularly sensitive to incorrect bedding due to weathering through the exposed beds and in particular through the clay/sand beds, as these layers are more vulnerable to erosion than the surrounding stone. At Isle Abbots the canopies were face bedded (that is to say that the sedimentary layers in which the stone formed were placed vertically, so as they decay, whole layers are liable to separate). They had already suffered some dramatic loss of surface, and it was clear that an early intervention was required to maintain effective protection for the sculptures. The same problems as those found on the canopies were also found on the windows, beasts and angels, and the appearance of many of these had been marred by heavy lichen growth.
At the start of any contract it is always advisable to have some vision of what the overall appearance should be after the scaffold has been struck. Although the presence of lichen on historic buildings is often considered picturesque, it was felt that at Isle Abbots it had destroyed the warm tones of the Ham Hill stone and the cooler hue of the Doulting, as well as greatly reducing the legibility of all the carved detail. It was important to retain this patina on untreated surfaces of the walling but also essential to visually link all the treated elements together so that the eye can be effortlessly drawn up the tower, and something of the intention of the original builders could be revealed.
The inclusion of an archaeological survey is essential on a contract such as Isle Abbots. It can not only uncover valuable information about the building's history, but also assist in the assessment of rates of decay through archive images as well as setting the conservation work in context. To this end the archaeologist Gerry Sampson was assigned to the conservation team at the outset, and he worked closely with the masons and stone conservators throughout. As a result it was possible to recognise and categorise earlier periods of repair and conservation. In particular the survey confirmed that the tower had been completely rebuilt from the top of the west window with impressive sensitivity and care in the 1870s.
The sculpture catalogue formed a large part of the archaeology and each carving was recorded in great detail. In this case it proved critical to the conservation programme, as all ten sculptures retained traces of original polychrome. Alongside Gerry Sampson's detailed observations of the carvings, a full condition survey of each sculpture was made which looked closely at decay mechanisms such as incorrect bedding of the stone, surface soiling and lichen growth, previous repairs and fixings, weathering and water flow patterns.
CLEANING AND REPAIR
The cleaning of both the Doulting sculptures and the Ham Hill elements was carried out by first mechanically removing as much of the lichen as possible using dental tools and scalpels. The next stage was to use hand water sprays and soft brushes to lift the residue off the stone surface avoiding those areas which retained historic paint layers. The isolated areas of gypsum crusts and black carbon coating were treated where appropriate with ammonium carbonate poultices to keep the use of water to a minimum.
The removal of the lichen revealed only a limited amount of lamination and surface decay on the Doulting figures, but extensive lamination and decay on the Ham Hill elements. As we suspected the elaborate canopies were riddled with weak beds which had developed into extensive lamination with associated hollow areas. We were extremely fortunate at Isle Abbots to find that the carved detail had not been covered with the dreaded Portland cement. A weaker cement mix had been used, and it was possible to remove it with dental tools and small masonry chisels.
The consolidation of the canopies was a high priority and involved extensive grouting and packing with adhesive lime mortars around a system of stainless steel pins. An adhesive lime mortar was made from sieved lime and finely graded stone dusts and sand, to which ten per cent brick dust had been added to it on site. It was worked up into the correct consistency for either packing or grouting. The dowelling of delaminating Ham Hill stone was a challenging task. Successive layers between the voids could be quite different densities so that a normal layer might be followed by an iron-hard one (relatively speaking), immediately followed by layers that let the drill through as if they were soft cake. The pins were all angled down and set in an adhesive lime mortar. In all cases the ends of the dowels were sunk 3-5mm beneath the surface of the stone and capped with lime mortar. This same method was used on all the Ham Stone detail including the parapet and windows as well as the Doulting sculpture. A record has been made of all fixings which will be submitted with the final report.
Apart from St George, the sculpture and niches were relatively free of live fixings. The carving of St George is a particularly vivid and dynamic portrayal, with the hind quarters of the horse breaking the frame of the niche. A subsidiary block had been attached with two leaded iron dowels which over time had expanded and caused the loss of a section of stone 12cm wide and up to a depth of 4cm. The removal of this block and the extraction of the lead and iron were time-consuming but successful, and allowed the section to be re-fixed with stainless steel dowels and repaired with lime mortar.
As with the consolidation and pinning of friable stonework, the repair programme to Isle Abbots also reflected the expansion of the lime technique from its origins for the conservation of sculpture to its application to every architectural element of masonry and walling. Although much has been written about lime mortars over recent years it is worth reminding ourselves of the key points of its purpose, preparation and application.
The primary reason for lime mortar repairs is protection; it is to provide a barrier between the weather, atmospheric pollution and the endangered stonework. Whether it is a mortar joint to prevent water penetration to a wall or a repair to a vulnerable piece of sculpture, the mortar should have a variety of different properties. The mortar should be softer than the material it is protecting, and be porous enough to prevent an interface forming with the stone where soluble salts can be deposited and recrystallise. It should also encourage residual salts to travel into and through the mortar, so preventing further damage to the stone. A mortar should have good adhesive properties which are dependent on the quality of the lime, the correct aggregate ratio and the correct application of the mortar. For example, sufficient wetting of the prepared surfaces, the initial application of a slurry mix and suitable compression and control over the drying time. The correct plasticity of a mortar is crucial to its successful application. It should be stiff enough to enable compression without moving the volume from one area to another and should never be crumbly. The mortar must be supple enough to allow it to be pressed into small crevices and corners, but never sloppy. To sum up, the priority considerations are the quality of the lime, the selection of aggregate, aggregate size, the lime/aggregate ratio, mortar mixing, application, porosity, frost resistance, surface texture and colour matching. All these considerations can make it seem almost impossible to consistently achieve the optimum lime mortar for the job whether it is for sculpture or building conservation. What this technique does scream out for is attention to detail in every stage of the process and nowhere more so than the application of the sheltercoat.
The sheltercoat is the final stage of the lime technique. The ingredients are similar to that of the adhesive lime mix with the addition of ten per cent casein as this material imparts waterproofing qualities. The consistency of the sheltercoat should be similar to thin cream and applied to the surface in up to three thin coats. Each coat is carefully rubbed back and the drying out of the thin layers is closely controlled. The purpose of the sheltercoat is to provide a physical barrier against weathering, as well as knitting together both physically and visually the original stone surfaces with the lime mortar repairs. The sheltercoat should fill all the fine hairline cracks and cavities that are too small to be repaired, and it is particularly useful on undercut surfaces by acting as a barrier between the original stone surface and future soiling. The drying time is greatly effected by humidity, temperature and moisture content of the stone and it can take up to five or six weeks for the sheltercoat to reach equilibrium with its substrate.
The sheltercoat offers the opportunity to achieve a level of aesthetic unity in the treatment of a complex surface with such a range and richness of colour and detail as the Isle Abbots tower. This demanded from the practitioners both a technical and intuitive response in the proper and selective use of a skilfully colour matched sheltercoat. In this case we put all our efforts into attempting to enhance the natural colour of the stone, bring out the dazzling detail of the carvings and to some small degree re-establish the intention of the designer in defining the divisions of the architecture as the eye moves up the tower.
The final result at Isle Abbots is somewhere near the vision I had at the beginning and if I ever felt I had achieved exactly what I wanted then it would be time to change jobs. What I am struck with in writing this piece is that it is now impossible to look at the conservation of limestone sculpture without taking into account every aspect of the architecture in which it sits.
- Vitruvius, The Ten Books on Architecture
- John Ashurst, 'The Cleaning and Treatment of Limestone by the Lime Method' in Conservation of Building and Decorative Stone (edited by John Ashurst and Francis Dimes), Butterworth Heinemann, Oxford, 1990
- Tim Ayers (ed), Salisbury Cathedral, The West Front, Phillimore and Co Ltd, Chichester, 2000
- English Heritage Research Transactions Volume 2: Stone, Gower, Aldershot, 1988
- Alison Henry (ed), Stone Conservation Principles and Practice, Donhead, Shaftesbury, 2006