The Preservation of Terracotta Buildings
Peter Gibbs
In the late 19th century a system was developed in North America and the United Kingdom for constructing large city-centre buildings with steel frames hidden by walls of terracotta. Use of the new construction system became widespread in the early 20th century because it was inexpensive, relatively lightweight, fire-resistant - and it was cheaper than traditional masonry due to the manufacturing process. Countless terracotta blocks and tiles representing familiar stone architectural details could be mass-produced quickly and economically from a single mould, removing the need for on-site carving in traditional stone.
However, less than one hundred years later, many of these terracotta buildings had begun to develop serious structural problems. In 1974 a pedestrian in Chicago was killed by a falling piece of terracotta. A subsequent investigation revealed that over 45 per cent of terracotta building facades surveyed were in an unsafe condition. Chicago and New York introduced facade inspection regulations to prevent this type of incident, but in 1999 a portion of the terracotta facade at 75 East Wacker Drive, Chicago, collapsed, resulting in the closing of an entire street so that the remaining façade could be made safe. The slender tower of 75 East Wacker Drive had already lost its dome, despite being otherwise restored to former glory in 1983 (illustrated above) while other buildings have had certain architectural details removed for safety reasons. One such example is The Reliance Building, Chicago (illustrated right), a steel and iron-framed building dating from 1895. The base of the building is clad in red Scottish granite, with the upper floors clad entirely in a cream glazed terracotta. Although declared a Chicago landmark in 1975, the building was not restored until the 1990s - with its missing cornice reinstated in aluminium.
The Reliance Building has been acclaimed as the direct precursor of the International Style skyscraper that emerged in Europe some 30 years later. The same steel-frame construction that made it possible for buildings to 'reach for the sky' is behind many of our own landmark buildings. America's problems with spalling terracotta facades are therefore likely to be repeated here in the UK's medium rise buildings within the next few decades.
MATERIAL CONSIDERATIONS
The basic ingredients of terracotta are clay, grog (previously fired clay products) and water. These are mixed together and kneaded in preparation for moulding. Plaster or wood models are carved slightly larger than the architect's required size to allow for shrinkage during firing. A mould is then made of the model and the clay is pressed into the mould by hand. After drying, it is removed and worked further by hand if necessary. The clay body may then be sprayed with a slip or glaze. Slip, a mixture of fine clay and water, gives terracotta a smooth finish, while a glaze is a coating of silica and minerals that vitrifies during firing to give a glassy finish and to add colour. Glazed tiles and blocks are generally referred to as ' faience'.
Early terracotta blocks were hand pressed into the moulds to give a front face approximately 40mm thick. The tile was strengthened by forming clay ribs around the edges of the block and at intervals across its width. These ribs were also usually pierced to provide a mechanism for attaching the terracotta to the building with steel ties or anchors. Terracotta of the early 20th century is typically associated with buildings constructed of iron and steel; most notably steel-framed, masonry infilled buildings. Steel beams and shelf angles often support the ashlar blocks which are tied to the masonry back-up walls or steel frame using steel wire ties or bent flat plates. The ties are passed through preformed holes in the terracotta ribs and are normally located within 25mm of the surface. Not every block had metal anchors. However, cornices, projecting elements and window heads are those typically hung from the steel frame using J-bolts and rods passed through the terracotta ribs. The hollow terracotta blocks were commonly filled with concrete or rubble. The quality and type of infill materials is an important factor and the use of poorly graded builders' rubble or clinker breeze aggregates can cause significant durability problems.
DETERIORATION
Despite its many advantages, terracotta is prone to a number of deterioration mechanisms. Defects at the time of manufacture include crazing of the glaze, which allows moisture ingress. Insufficient packing into the moulds may result in variable thickness and weak planes parallel to the external surface that tend to spall. Terracotta is intrinsically brittle and so is prone to thermal cracking, particularly where the facade design lacks expansion joints. Freeze/thaw actions can compromise the glaze and crack the tiles. Fungi, mould and algae can cause biological deterioration, for example, by penetrating and creeping beneath the glaze. Inappropriate cleaning can also compromise the original finish and lead to water damage. Add to this list corrosion of fixings and ties, metal contamination in the infill, and corrosion of the main beams and columns, and protecting terracotta facades becomes a major issue.
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| Restoration of the Joshua Hoyle Building, Manchester, UK, was commercially unviable using traditional repair techniques (right before work started). Cathodic protection was used to reverse the corrosion of its steel frame and the terracotta façades are now restored to their former glory |
Whatever the trigger, the principal cause of terracotta failure is water penetration. Although many terracotta glazes are virtually impervious, the clay body beneath will absorb water readily. Once the glazing is breached, small quantities of water can enter and deterioration will accelerate. A survey of the faience facade at Earl's Court Station, London, revealed that all the cornice blocks were imperfectly filled and all had vertical cracking. The cracks corresponded precisely with the position of the original clay ribs. These cracks had allowed water ingress and the deterioration of the facade had then been accelerated by freeze/thaw action. In addition, general water penetration through the porous mortar joints had led to the corrosion of supporting steel beams and fixings as well as the expansion of clinker breeze infills.
Of course, water may also penetrate from behind the facade, whereupon any terracotta with an uncompromised glaze or dense fireskin layer acts to hold the water in. Moisture collecting in this way penetrates the infill, which in turn holds it against the iron/steel frame and fixings, making the onset of corrosion inevitable. The expansion of corrosion products behind the facade causes the cracking, displacement and spalling of external terracotta that is such a cause for concern. The number of potential sources of terracotta deterioration, the extent of the damage and the desired conservation or costdriven end result all influence the choice of repair technique.
REPAIR ISSUES
Restoration of terracotta buildings requires meticulous planning. Failure to do so will nearly always result in significant cost penalties to the building owner. Poor planning may result in project delays by not taking into account material supply times of up to 26 weeks. Alternatively, poorly matching replica blocks may be produced or existing blocks may be damaged by poor cleaning and repair, resulting in lower asset values or a building that is difficult to sell or let. Supervision of surveys, inspections and remedial treatment is therefore a specialist skill. One of the most critical examinations is that of the steel frame using local breakouts and measurement of the corrosion damage. Levels of deterioration may range from superficial expansive corrosion to entire loss of section. The majority of the worst cases are often attributable to the use of acidic clinker concrete infill, which results in rapid deterioration of the steelwork. Even where only superficial damage is determined, the replacement of individual terracotta blocks must be undertaken with care to avoid causing damage to adjacent tiles.
Where more serious deterioration of the underlying steelwork is found, it is essential that the steel frame corrosion be stabilised before replacement of the terracotta takes place. Traditionally, the treatment of steel frame corrosion has required removal of the facade to clean back the affected steel, treatment of the steel surface and the reinstatement of the facade. This can be costly and time-consuming. Steel treatment costs vary widely, ranging from £500 per linear metre for simple details to £8,000 per linear metre for complicated architectural details such as cornices.
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| A cross-section of replacement terracotta tiles manufactured to the same specification, but showing significant colour variations. |
Taylor Woodrow's Engineering & Consultancy team pioneered the adoption of cathodic protection techniques as a less disruptive, more cost-effective alternative. Cathodic protection (CP) is based on the principle that corrosion is a preventable electrochemical process. The corrosion of the steel is the result of an ionic current that flows away from the area of deterioration. Cathodic protection effectively prevents this process by applying an electrical current through the masonry in the opposite direction. The company first applied CP to restore an entire faience facade in 1997. The subject was the Joshua Hoyle building, Manchester, a Grade II listed structure spanning the Rochdale Canal. The building was acquired with the intention of converting it to a three-star Malmasion hotel. It had been unoccupied for some time and its brick and terracotta facade was suffering from extensive cracking and spalling from the upper storeys.
Traditional repair of the full facade would have made the project commercially unviable. Following a detailed examination of the building, cathodic protection was proposed as an alternative treatment. The CP system at Joshua Hoyle was trialled and proved feasible in June 1996. Costings for the finalised design showed substantial savings on initial outlay over traditional repair techniques. The full CP system was applied to the front and side facades in early 1997 and the 112-bedroom hotel opened in 1998. In addition to making the refurbishment project commercially viable, it is estimated that the use of CP in this case yielded initial savings in the region of 50 per cent compared to traditional facade repair options, with the added benefit of long-term reductions in maintenance.
Of course, stabilisation of steel corrosion is only a tool in the process of restoring terracotta buildings. In situ repairs such as crack injection, replacement of damaged units and cleaning are nearly always required. It is worth noting, however, that where CP has been used to halt corrosion, the number of blocks requiring replacement can be dramatically reduced - in some case by up to 75 per cent - making the technique attractive in terms of initial costs, life-cycle costs and conservation.
The appointment of terracotta specialists is essential to ensure the correct specification of replacement materials, to establish the level of replacement required, and to ensure procurement and delivery schedules are maintained. The terracotta specialist may also undertake liaison with appropriate heritage bodies, and specification of appropriate fixing methods and cleaning regimes. The use of partnering arrangements with terracotta suppliers is also worthy of consideration. The involvement of the specialist manufacturer at concept and procurement stages will nearly always result in client savings. In particular the partnering arrangement should remove contract delays due to planning issues and reliability in delivery times.
COLOUR MATCHING
One of the principal claims for terracotta is its attractive range of colours. Many shades can be produced by varying the ingredient materials and the manufacturing process. Control of these factors also provides a means of colour matching new terracotta with the original, thus preserving appearance on a repaired facade - although many trials may be required to achieve an acceptably close match. Even with relatively few trial specimens, visual colour matching can be extremely difficult as colour perception depends on the observer and is strongly influenced by light source, surrounding colour and scale. Furthermore, subjective descriptions lead to difficulties in the communication, agreement and recollection of colour differences.
The variables are reduced significantly by using a colour measuring instrument (a colorimeter or spectrophotometer) as the light source and the 'observer' (detector) can be standardised to internationally agreed specifications. The optical properties of a specimen, defined by hue, lightness/tone and intensity/brilliance, can then be measured quantitatively, giving an objective and reproducible description of its colour. With the availability of sophisticated hand-held instruments that incorporate data logging facilities, further benefits arise: numerous measurements can be performed and processed very rapidly; the original terracotta can be characterised in situ, independent of lighting conditions; trial specimens can be evaluated remotely. Additionally, production trials can be assisted by directly relating the specific colour parameters to manufacturing variables.
This electronic approach proved to be of great value during the refurbishment and redevelopment of the Royal Albert Hall. The creation of the new South Porch presented a number of challenges: colour, texture and existing details had to be replicated to blend seamlessly with the original. Assisted by quantitative colour assessment, over 100 cream coloured terracotta samples have been screened. A bewildering range of candidate shades has been significantly reduced in number to allow the economic production of a few full size blocks for a realistic and manageable submission for approval by English Heritage. Maintaining and restoring the fine terracotta facade examples that exist today requires significant investments of time and skill. With the experiences of America as a benchmark, specialist engineering and construction companies in the UK and Europe are developing and refining repair techniques that will help owners and heritage bodies to restore and maintain these buildings as economically and faithfully as possible.
