Many thanks to Michael Plageman, who has provided the below. Michael is a Chartered Architect, Registered Conservation Architect, at Davies Sutton Architects and is a Design Review Panel member, attending Design Review Panels across the South West; in Cornwall, Devon (both Plymouth and Exeter), Dorset, Wiltshire and Swindon.
"To anyone who has looked closely at historic stone buildings and monuments, the deterioration of stone is an ongoing and evolving characteristic. As part of an un-carved, mass rubble stone wall, such deterioration can be aesthetically attractive and has been referred to on occasion as ‘pleasing decay’, however, the effect on more refined stonework, such as buildings designed in a classical style for instance, can be more detrimental. Indeed, at its extreme, it does not take much for the deterioration of a carved piece of stone to lead to a sculptor’s original intention being lost altogether. Therefore, with a large proportion of Britain’s cultural heritage constructed in stone, there is an ongoing battle to prevent the significance of these monuments being compromised and lost to future generations.
Cardiff based practice, Davies Sutton Architects, embraced the philosophical and technical challenges presented of managing such decay mechanisms, in their undertaking of stone repairs to the South façade of Apsley House – a Grade 1 listed building of considerable national importance and significance in central London. The practice specialises in the conservation of historic buildings, together with new buildings on sensitive sites and their portfolio of works includes interventions to an array of outstanding buildings and sites such as Chiswick House, Cleeve Abbey, Tintagel Castle and St Woolos Cathedral. Working on such buildings demands a particular level of care, dedication and research to be carried out before implementing conservation repairs. Consequently their experience of such working methods and requirements positioned them appropriately to accept English Heritage’s appointment to undertake the works.
General view of the South elevation of Apsley House from the South West, circa 1853
Located on the junction of Hyde Park corner and Mayfair, Apsley House once ‘stood at a focal point of the topography of London and, equally, its social and political fabric.’ It was the first house to be seen after the tollgates to the City of London, and as such, has subsequently been commonly referred to as No.1 London. Originally designed by Robert Adam between 1771 and 1778, as a brick building for Lord Apsley, the house was passed to the Duke of Wellington’s family in 1807, and the 1st Duke soon commissioned Benjamin Wyatt to reorder and reorganise the house in the latest fashion to reflect his rising status.
The property now comprises of some of the country’s finest interiors, including retained decorations by Robert Adam and the Waterloo Gallery by Wyatt – an extravagant Regency interior which marks the Duke of Wellington's famous victory over Napoleon. In addition to this, Wyatt also remodelled and reclad the property in Bath stone, thereby projecting the Duke’s growing sense of grandeur, elegance and order into the public realm where it has been the backdrop for London life ever since.
The re-routing and widening of roads in the 20th century, together with surrounding development, means that the house is now predominantly isolated and disconnected from its historic context. There has also been a marked change in the natural environment that contextualises the house, with the thunderous and heavily trafficked roads of Piccadilly and Park Lane to the South and East, replacing the less polluted and sedate environment that the building used to enjoy.
Commissioned by English Heritage, Davies Sutton undertook an initial feasibility study and assessment of the South elevation of the house as part of an ongoing , phased repair programme that had previously addressed the North and West elevations. The study identified that the South elevation had suffered environmental damage, strongly associated with its urban location, that was detracting from the significance of the building. A cursory inspection was also made of the remaining elevations of the building to both contextualise these findings and to ensure an overview of the condition of the building as a whole was understood.
Visual inspection of the South facade established that there was a general blackness to the stonework. This was caused by sulphuric acid and calcium carbonate in the surrounding environment ,dissolving in rainwater, mists, fog and general condensation, and being absorbed into the stonework. The acidic water was reacting with the lime in the Bath stone to form calcium sulphate, and as the water evaporated from the surface of the stone, the resulting calcium sulphate crystallisation was fixing dirt onto the surface of the stone, forming the black pollution crust.
Comparison of the West (left) and South (right) elevations of Apsley House, shows the appeal of rich, warm colour of the copperas decoration to the Bath stone retained in sheltered areas of the South elevation. Note also the context of the house.
The sulphate crystallisation was subsequently not being washed away – particularly in sheltered spaces such as the underside of the cornice, the dentil course, architrave and the stringcourse – leading to concentrated deposits of pollution crusts to sheltered areas of the façade. In addition to this, a number of poorly executed repairs had been carried out which were both a bad visual match with the surrounding stone, and that were also causing further damage to the stonework. There were two types of repair that had been undertaken – cement based repairs and lime-based repairs. With regards to the cement based repairs, the dense impervious nature of the material, whilst visually being an unsuitable match for the Bath stone, was leading to moisture and salt concentrations around them, and concentrating areas of decay. This was leading to small sections of masonry falling from a high level to the public forecourt below - an obvious health and safety issue.
With regards to the lime based repairs to the building, whilst it was unlikely that these repairs were damaging the surrounding stonework, the mortar used for the repair was unfortunately a poor match. The colour was too dark and the exposed aggregate gave a contrasting texture with the original stone. Such repairs were therefore affecting the presentation of the building and this was on a reasonably significant scale.
The feasibility study allowed the practice to understand the type of decay mechanisms at work, and to guide further investigations. It also enabled initial ideas to begin to be developed for interventions to repair and manage subsequent decay. These proposals were set against a desire to preserve, conserve and enhance the presentation of the building which was embodied in the wider Conservation Management Plan for the property. The general and overarching principle was consequently to retain as much of the original fabric as possible whilst undoing previous repairs where they were causing damage or were inappropriate to the presentation of the building. Underpinning this approach was the premise that the decay of historic fabric, particularly stonework, can affect the architectural impact of a building, as well as encourage and accelerate further deterioration. Therefore, the early repair and maintenance of the historic fabric in situ, in advance of significant deterioration, would play a substantial and important role in guiding conservation decisions going forwards.
Detailed Assessment and Repair Proposals
Following the initial feasibility study, the practice undertook a detailed survey of the South façade of the House together with commissioning a structural overview of the elevation. This was aimed at mapping out the condition of the facade on a stone by stone basis so that quantifiable repair proposals could be developed. The survey also provided an opportunity to undertake and investigate trial repairs and interventions on discrete areas of the building to inform suitable repair techniques going forwards.
It was proposed that previous cement-based repairs would be cut out and replaced with Bath stone to exactly match the type and profile of the existing stone to the building. Through discussion with stone specialists, together with visual matching, this was identified as being a Bath stone being quarried at Hartham Park. Its selection for repairs meant its properties would be commensurate with the existing stone of the facade and therefore give the best chance of minimising any adverse reactions with the existing stone, together with being a much better visual match.
With regards to the lime based repairs to the building, whilst these were effectively sacrificial elements to the facade, they significantly affected the presentation of the building as they deteriorated. It was therefore proposed that these repairs would also be cut out with new stone pieced to provide a repair with more longevity that better matched the existing stonework.
Cement render & delaminating
stonework to the parapet, presented a danger as it could fall to the forecourt below
Soiling on top of the string course at high level. Note the pollution crusts to the dentil course, the damaged string course and the plastic repairs carried out in a lime based mortar that does not match the colour of the existing stone
Repairs to the Corinthian capitals of the pilasters carried out using cement
In order to assess the impact of cutting out the cement repairs, vibration testing was carried out in conjunction with English Heritages’ in-house building scientists. This allowed the focused assessment of the effect that cutting out former repairs would have on the interior of the property where tall mirrors were located on the opposite side of the South wall. In addition to this, it also allowed the measurement of dust levels to the interior so appropriate methodologies could be specified during the works to manage this.
Consolidation of Friable Stonework and Repointing
The detailed assessment was carried out with full access to the facade provided from a cherry picker. The closer analysis revealed further issues with the elevation including the following:
1. The pores of some stones to the East of the facade were much more open than elsewhere on the elevation. This was allowing increased levels of moisture and salts to enter the stonework, with the subsequent evaporation and salt crystallisation making the stonework friable. In order to address this, a lime based shelter coat was proposed to the surface of the stone. Shelter coats protect the stone by filling the pores with a porous medium that allows free movement of moisture, decreasing the rate of soiling and are a relatively inconspicuous solution that meant further stone would not require removal.
The proposed repairs were document on a stone by stone basis in relation to the facade
2. There were several movement cracks in the stonework. These predominantly occurred to the stone joints and were not felt by the structural engineer to be a cause for concern. Where cracks had opened up, they could therefore be repointed to exactly match the existing mortar, and analysis of mortar samples was therefore undertaken to ensure an exact match could be made.
With regards to the blackness of the stone, a further benefit of the detailed survey period was the allowance it afforded to engage conservators to carry out cleaning trials on the building prior to the construction phase, aimed at removing the soiling. The trials demonstrated that lightly pressure washing the building with controlled, heated water, was effective in removing the pollution crusts by simply dissolving the calcium sulphate, and washing it away. It highlighted that care would need to be taken during the works to evenly clean the façade and to avoid patchiness, together with the avoiding over watering the stonework and saturating the stone. The process led to the discovery of a previously unidentified copperas wash that had been applied to the facade. This ferrous sulphate solution gives the stone a warm, golden colour and would originally have been used to homogenise the appearance of the stonework. In its patchy, weathered state, it gave the building a welcome patina of age and removing it was discounted on the grounds that it would unnecessarily involve the loss of historic decoration of which little was known. In reviewing other elevations of the building it could be seen that over-cleaning of the stonework had previously been undertaken to the West façade. This had resulted in a relatively dull and lifeless elevation in comparison to the warmth of the copperas wash on the South façade. It was consequently imperative that the cleaning during the main works was undertaken by a trained Conservator to avoid this happening.
In addition to this, the cleaning trials demonstrated that areas of detail, such as the capitals to the columns, required alternative approaches to cleaning in order to remove more stubborn soiling. These areas required extensively brushing down using synthetic nylon brushes to avoid marking the stone, with the most stubborn soiling requiring the use of poultices. These are a form of chemical cleaning that break down the bond that can occur between some dirt and the stone, and also the bonds that hold the dirt itself together. The trials therefore established the correct poultice required by testing in a limited area. This meant that a light touch could be taken to the cleaning of the facade, avoiding the use of more invasive approaches that could potentially harm the stone during the main works.
The detailed survey period enabled Davies Sutton Architects to identify, quantify and agree the repair proposals and methodologies for the building going forward. This ensured more accurate tenders could be returned helping to highlight and manage potential risks during the construction phase. Together with the physical works there were a number of logistical challenges to overcome including a busy central London site with limited access, the ongoing occupation of the house during repair works, and a limited period to undertake the works due to the requirement for them to be completed in time for the Waterloo Celebrations. This ultimately led the works to be undertaken during winter months which placed an added pressure onto their implementation from the weather.
A semi enclosed scaffold was therefore constructed onsite that was designed to allow ongoing access at ground floor level whilst the works were undertaken above. The scaffold further included a number of security provisions in order to protect the valuable collections inside the building.
The construction works effectively fell into three sequential elements:
the basic cleaning of the building
repairs to the existing fabric
the decoration of the building.
Following the cleaning and repair of the building consideration had to be given to its subsequent decoration. Whilst it was originally thought that there was minimal decoration applied to Apsley House, the discovery and identification of the copperas wash changed the approach required.
Video showing the gentle cleaning of the facade with controlled heating of the water
The stone capitals were cleaned using brushes initially and subsequently with poultices, and cutting out and piecing in new stonework.
The remnants of this little known decorative technique were predominantly located to sheltered surfaces i.e. below the portico, to the underside of string-courses and dentil courses etc. By mapping the location of the wash, it was demonstrated that it had been applied extensively, with only the string-course above the rusticated base, the columns to the portico and the cornice above the dentil course being spared the treatment.
The decision was therefore made to retain it where it existed, and consequently, where new stone repairs were carried out in areas where copperas wash was to be retained, it was acknowledged that there would be a clear colour difference with surrounding stone. In order to address this, and following trials on stone samples, individual stones were treated with varying concentrations of a newly developed copperas wash to blend in. This involved relearning historic techniques and the building is now being visually monitored to assess the success of the approach.
Cutting out and piecing in new stonework.
The scheme was ultimately delivered to align with its challenging requirements. The success of the project relied on the skills of the project team - including the early research, assessment and investigation of the proposed repairs - together with good communication between consultants, conservators, contractor and Client representatives in its delivery. On heritage work, the relationship between members of the team is particularly important, as even with the most rigorously researched and prepared specification, it is rarely possible to anticipate all aspects of the work required. The finished conservation works have retained the overall refined exterior of the building, lifting its appearance in time for the Waterloo celebrations. Perhaps most importantly however, it has conserved the building for the enjoyment of future generations."
The Completed Building
Established in 1993, Davies Sutton Architects have nearly 25 years’ experience of repairing, extending and providing new buildings within historic and environmentally sensitive sites.
Our expertise in the conservation of historic buildings informs our approach when designing new buildings in such settings, via an enhanced understanding of:
the local vernacular and indigenous natural materials
the context and setting of listed buildings and conservation areas
a detailed knowledge of the planning system
Our clients include the Welsh Assembly Government, English Heritage, the National Trust, various Local Authorities, several schools and voluntary charitable trusts. Further information can be found at www.davies-sutton.co.uk