Guidance on Matching Terracotta
|A finial on the London Road Fire Station, a Grade II* listed building in Manchester which is on the Buildings at Risk Register (Photo: Jonathan Taylor)|
British Standard 8221-2: 2000 makes clear that in conservation work replacement units must match the originals as closely as possible in terms of surface colour, texture and finish. This article explores the problems and potential resolutions associated with meeting this standard where architectural terracotta is concerned.
Replacing historic fabric is always a contentious issue and should be the last resort. When necessary, with naturally occurring vernacular materials such as wood or stone, accurate replacements can often be readily sourced and, although the difference in material will be noticeable, structural integrity will not have been compromised by the material. However, with the increasing standardisation of manufacture, difficulties in implementing this advice for man-made materials are far more complex.
REPAIR OR REPLACE?
In England, statutory guidance advocates that ‘original materials normally only need to be replaced when they have failed in their structural purpose’ (PPS5 Planning for the Historic Environment: Historic Environment Planning Practice Guide, Section 6, para 149). Repair is always the preferred option but terracotta can be very difficult to repair adequately. Mortar repairs are often found on terracotta structures due to their relative ease of application. However, the surface texture will rarely match that of the smooth terracotta and although colour matches can be achieved, over a period of time this colour will be bleached through exposure to ultraviolet light and erosion. Another common ‘repair’ technique is the use of paint, which is often applied to obscure discoloured terracotta. This presents a homogenous appearance that is often conspicuous when compared to existing blocks. Newly developed filler materials and consolidants which include polymeric binders are becoming increasingly common, although their longevity is unproven.
There is a widespread misconception that once the fire-skin of a terracotta block has been eroded, either through natural weathering or insensitive cleaning, the integrity of the block beneath is compromised and it should be replaced. Although this is true for certain types of terracotta, such as the under-fired pink Doulton pieces, most Victorian and Edwardian terracotta was fired at such high temperatures and consistently enough that the removal of the skin has little effect on durability.
Terracotta is susceptible to a multitude of failure mechanisms that stem from its method of manufacture and how it is fixed to a building. The problem with repairing terracotta units is that deficiencies in the wares are often internal, caused by rusting cramps or dowels, expanding fills, freeze-thaw damage or salt crystallisation (cryptoflorescence) for example. Such problems can compromise the entire unit and often repairs cannot be carried out without damaging or destroying the originals. For both aesthetic and structural reasons, terracotta units are often replaced in these circumstances. The question that must then be asked is: to what extent can they be accurately matched?
After the first world war architectural terracotta fell out of fashion and by the mid 20th century its manufacture had virtually ceased. The manufacturers that survived did so by branching out and making other architectural ceramics such as sanitary ware. Around the end of the 20th century the demand for terracotta increased as a result of the deterioration of existing architectural terracotta alongside the increasing recognition of the importance of Victorian and Edwardian architecture and demand for its conservation. During the mid to late 20th century a large number of terracotta structures were subjected to inappropriate remedial works. The first and most widespread of these was the use of inappropriate cleaning techniques including hydrofluoric acid cleaning and sandblasting. The former often caused surface streaking, while the latter led to the erosion of the fire-skin, leaving a pitted surface.
The new units employed during this period were far from adequate, reflecting the manufacturers’ lack of experience. Due to the difficulty in matching existing terracotta units, replacement pieces often had a uniform layer of tinted clay (an ‘engobe’) applied to the surface resulting in a homogenous colour. In some cases a glaze was inappropriately applied resulting in an excessively glossy surface. A number of structures also had replacements made with other materials such as glass-reinforced plastic, although thankfully this practice now seems to be universally frowned upon.
Today, with ever-increasing demand for the material in conservation work and new-build alike, the quality of the new units is of a much higher standard. Despite this the conservation professional must be aware of both the potential and the limitations of these new materials.
|Above left: Hand-pressed terracotta units often vary in depth because of the way they are formed. Above right: Early replacement works often appear homogeneous with no colour variation between blocks. (Photos: Steven Handforth)|
MATCHING NEW TO OLD
The first problem is matching the plethora of different types of terracotta that were historically produced. The colours can range from dark reds, to browns, to buffs, to whites. Secondly, the variation of colour within each block can differ widely from the mottled appearance of the terracotta used for Edmund Sharpe’s early 19th century ‘pot’ churches to the more homogenous appearance of the later Victorian wares. To add further confusion, the texture of the blocks can vary from smooth skins to undulating surfaces caused by vagaries in the manufacture and firing process. Finally, the original finish of the pieces could vary from smooth ashlar blocks to tooled or combed surfaces imitating stoneware.
In addition to provenance and manufacturing concerns, the conservation specialist may struggle to identify the original colour on first inspection due to heavy soiling or inappropriate cleaning. Surface deposits caused by pollutants often obscure the surface of terracotta, particularly in urban areas. On the other hand, the colour of blocks which have been cleaned may be difficult to identify due to streaking from insensitive acid cleaning, or a pitted surface from sandblasting, and some structures were so extensively cleaned that their colour has been completely bleached.
According to modern conservation principles, replacement materials should be as authentic as possible, manufactured from the same clays and mixture of grogs (fired clay ground to a specified particle size and added to unfired clay to reduce shrinkage). They should also be modelled and moulded in the same way, and fired using the same methods as the original units.
|‘Authentic’ under-fired pink terracottas are no longer produced due to their inherent weaknesses. (Photo: Steven Handforth)|
Historically, clay sources would have been extensive thanks to the large number of manufacturers, which peaked in the mid to late 19th century. Today, due to the shortage of manufacturers, there are far fewer clay banks used for terracotta replacements than in the Victorian period. In addition, manufacturers use a limited number of clays as their tolerances and shrinkage rates are understood.
This is important because new blocks need to work to specific tolerances, compressive strengths and frost resistances in compliance with the European Standard Specification for Clay Masonry Units (EN 771-1). Although BS 8221-2:2000 highlights the significance and importance of using different clays, the imposition of standards such as EN 771-1 has actually resulted in suppliers using fewer clays. The requirement for durability has also had an impact on the production of authentic under-fired units such as early Victorian wares or pink Doulton products.
Visually this is not necessarily a problem, however, as extensive laboratory analysis is often used to identify oxides or colourants that can be added to the clay recipes to provide as accurate a match as possible. In any event, the chances of obtaining the original clays and manipulating them in the same way are extremely low. Units produced using modern methods would almost certainly appear very different to the units they are trying to replicate. There is also evidence that chemical additives were used in the late 19th century, providing a precedent for their use today.
Historically, methods of terracotta manufacture varied. Early 16th century wares were produced using timber moulds, while Victorian wares were often made using plaster casts. Modern manufacturers can replicate these techniques, with the clay being hand pressed into the moulds to a depth of approximately 25mm-50mm. Another modern technique is the slip casting method, where a liquidised clay mix (or ‘slip’) is poured into a porous mould and left to set, providing a uniform depth to the body. Of the two methods, slip casting is the most recent although its origins are over 100 years old. There are slight differences between the two processes with slip casting often giving a more smooth and sharp appearance. Other benefits of this method are that it creates a more uniform depth to the terracotta block and reduces the risk of air pockets, which are more common with hand-pressed wares and often lead to later laminations. It is therefore important for the conservation professional to sample the different types of ware to get an accurate match to existing units depending on their requirements. Undulations are also still possible in both techniques, often caused by slumping during the manufacturing process.
Matching the finish of terracotta is normally the most straightforward part of the process. In the early 19th century terracotta was used to imitate stone and would often incorporate tool marks on its surface. This can still be achieved through similar methods and is undertaken prior to firing.
Historically, terracottas would be fired in down-draught, muffle or Hoffman kilns, none of which produced particularly even firing temperatures. As the colour of natural clays changes substantially with firing temperature, the appearance of the wares would vary depending on their position in the kiln. However, modern demands for uniform, regular and reliable products have resulted in better kilns. The most common method of firing is now the shuttle kiln, which is an intermittent, low thermal mass, gas-fired structure. This creates a uniform heat removing any variations in the appearance of the terracotta blocks. To create a match for less evenly fired originals, in addition to using colourants, colour variation between batches can be achieved by using slightly different firing temperatures.
|Left: Sandblasting of terracotta units often leads to a pitted surface. Due to the temperature and duration of firing, the internal
body of the block is still resilient to weathering. Top right: This modern slip-cast example shows natural imperfections
in the block. Lower right: Mortar repairs often become unsightly as their pigments are bleached by exposure to ultraviolet light. (Photos: Steven Handforth)
There are benefits to using these modern kilns in terms of the structural stability of the terracotta produced. The higher uniform firing temperatures dictated by EN 771-1 remove all the sulphates from the clay, reducing the risk of efflorescence appearing on the surface of the ware. Higher firing temperatures also result in a denser body, which in turn makes the block more resilient to weathering and freeze-thaw damage. While in the Victorian period it might have been necessary to make two to three pieces of a unique unit in case of defects, modern manufacturers benefit from low wastage and high yields.
THE BEST AVAILABLE SOLUTION
Although there are only a few manufacturers left, and despite their reliance on significantly different production processes, the modern architectural terracotta manufacturing industry has never had better conservation credentials. Its output in terms of matching is often of a high quality, with detailed research undertaken to ensure accuracy. Ironically, if original techniques were used, the chances of producing accurate replicas would be far slimmer because of the many variables that influence the appearance of Victorian and Edwardian terracotta. Modern regulations and production techniques may have ruled out the use of truly accurate replica materials, but they have enabled us to produce accurate visual matches using durable materials that are more resilient to weathering. Although the process used in terracotta manufacture today is not in keeping with conservation best practice in terms of its sourcing and integrity, the products being made today are far superior to those of recent decades. Problems with identifying the provenance of the unfired material combined with financial and regulatory constraints mean that the process currently employed is the best available.
- J Ashurst and N Ashurst, Practical Building Conservation, Volume 2: Brick, Terracotta and Earth, Gower Technical Press, Aldershot, 1988
- British Standard 8221-2:2000, Code of practice for cleaning and surface repair of buildings Part 2: Surface repair of natural stones, brick and terracotta, 2000
- J Fidler, ‘Fragile Remains: An International Review of Conservation Problems in the Decay and Treatment of Architectural Terracotta and Faience’, in Architectural Ceramics: Their History, Manufacture and Conservation, James and James Ltd, London, 1996
- GR Hollis, ‘Manufacturing Faience and Terracotta’, in Context, Issue 52, 1996
- PPS5 Planning for the Historic Environment: Historic Environment Planning Practice Guide, EH/DCLG/DCMS, London, 2010
- M Stratton, The Terracotta Revival, Victor Gollancz, London, 1993