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Recasting Terracotta

Cast, glazed and fired, clay is the original mass-produced building material. Today, innovative production technologies are opening a new chapter in the biography of this versatile substance

From the humble Mesopotamian brick to the exuberant faience tiles of Victorian facades, the crystalline sludge we call clay has played a long and distinguished role in architectural history.

Described as ‘The Clay Hypothesis’, this theory relates to a general notion that ‘life arose from clay’, which Cairns-Smith elaborates in Clay Minerals and the Origin of Life. Cairns-Smith suggests that the aperiodic crystalline structures of clay may encode information by difference and dislocation and are analogous to DNA in that the structure of clay could similarly encode information, replicable through crystal growth under the right conditions.

St James's Gateway Piccadilly

St James’s Gateway Piccadilly

There is a question as to whether as architects and designers we should be responsible for the ways and means by which a design is made materially manifest. A contemporary tendency in architecture is to turn construction methods and materials into products and legitimise these products through marketing and the conceit of professionalism.

The problem with this specialisation is that the imaginative potential of the designer is, de facto, limited by the range of marketed ‘products’ available, and/or known, to the designer. Clay-based building products are a useful case in point where the repertoire of architect-specified derivatives of said material may extend to tiles: of roofing, flooring, kitchens, bathrooms; or bricks: stock, glazed, engineering etc.

In some cases the application of fired clay products may even extend to the extruded rainscreen panels recently seen in various hues at Renzo Piano’s Central Saint Giles building in London. However, these clay ‘products’ are merely the marketable result of an efficiency chain that issues standard sized or finished products based on imperatives that may not match those of the designer.

Clay slip being poured into a plaster mould, the plaster column helps to build a head of pressure

Clay slip being poured into a plaster mould, the plaster column helps to build a head of pressure

While I would not question the usefulness and tactile utility of the metric brick (215mm x 102.5mm x 65mm), the substance of clay is such an extraordinarily plastic material (in its undried and unfired state) that it would be timely to be reminded of the versatility of this hydrated silicate. Faience describes a facade made from glazed ceramic in a centuries-old process imported from Italy that was extremely popular in the UK from the 1860s up until the middle of the last century.

Skill_1_01Cornice sections, click the image to view the complete system

The designer was no longer limited to the small clay units of brickwork or the heavyweight craft of the stonemason, but employing the skills of both sculptor and ceramicist and the emergence of the steel frame, a new construction method was formulated. Of the great faience and terracotta buildings, Alfred Waterhouse’s Natural History Museum (1881) is an extraordinary model, in both detail, finish and perhaps surprisingly in mass production. While our eyes may be drawn to the idiosyncratic modelled animal detail of this Victorian gift, we are also looking at repeated cast clay elements replacing the logic of individually hewn stones.

As a consequence of this proto mass-customisation process, faience became popular as a hardwearing pollution repellent cladding material for literally hundreds of London Underground stations and as the decorative detail facing of Oscar Deutsch’s Odeon cinema chain, designed by Harry Weedon and constructed during the 1920s and ’30s.    

A piece of the St James’s Gateway facade ready to extract from the mould

A piece of the St James’s Gateway facade ready to extract from the mould

Architect Eric Parry has produced a number of contemporary buildings employing faience as external cladding material, including the articulated box of the Holburne Museum extension in Bath and the zigzagging couture facade of London’s 50 New Bond Street. His most recent project to employ these fired clay building blocks is his St James’s Gateway scheme for Piccadilly (scheduled for completion in 2013).

Parry, working with sculptor Richard Deacon, has positioned the cornice at the same height as Regency near neighbours in a measured deferential ‘nod’. As a sculptor acquainted with the deft manipulation of matter, Deacon studied and has remade the cornice as a ‘chopped up’ performance of 14 variable prismatic forms, each mutated from a single genotypic cross-section, but confined in height, or what Parry described as ‘the field of play’ to a not inconsiderable 1200mm. In addition, each of these 39 sculptures, extending over 25m, is highly coloured with facsimiles of Deacon’s painting using screen-printed waterslide transfers (decals) in a process originally invented for the pottery industry; each facet of the cornice blocks is differently coloured, further emphasising the geometric transformations.

Parts of the St James’s Gateway facade are dry laid outside to check for colour consistency

Parts of the St James’s Gateway facade are dry laid outside to check for colour consistency

Shaws of Darwen, established in 1897, has produced all of Eric Parry’s faience work. Based in the Lancashire countryside, Shaws split their manufacturing between fireclay Belfast sinks and architectural terracotta or faience. Both parts of their business use the clay slip casting process. On a tour of the factory, operations director Andy Hampson explained that clay pellets are mixed with water to form a liquid clay (or slip), which is then poured into plaster moulds.

The thixotropy or relative viscosity of the slip in relation to movement is important as the clay slip is piped around the factory to the various manufacturing halls. The mould making and modelling process is vital to both the sink and architectural faience work with all moulds made from plaster. Some of the most skilled work is in the modelling and mould-making studio where ‘positives’ are handmade or hand-finished before being set in plaster to create the moulds, reusable several times over before some definition is lost. Digital fabrication is also now used in the more speedy production of polystyrene ‘positives’, a process that can also be used for restoration work, where replacement components are measured, three-dimensionally modelled and quickly milled from foam.

Once the two-piece plaster moulds have been produced they are filled with liquid clay slip. In the case of complex architectural forms, plaster columns are used on top of the pour holes to build up a head of pressure ensuring all the internal detail is picked up on the final piece. Typically the clay is left to dry in the mould for approximately two days before being removed from the plaster form where further natural and mechanical drying takes place before the application of glaze (faience) and firing.

Pieces of Richard Deacon’s cornice sculpture

Pieces of Richard Deacon’s cornice sculpture

The glaze process is a science in itself, with technical manager Anthony Cristoforo explaining how colours, finishes and technical performance are achieved with ‘recipes’ typically containing metal oxides for various colours and textures. Initially the clay components are sprayed with a first layer of engobe, which acts as an undercoat to improve colour saturation, and then a specific colour glaze.

Anthony Cristoforo is currently in the process of compiling a new database of glazes, which because of evolving health and safety guidance would not now contain dangerous heavy metals (even including the less than benign uranium), to achieve certain hues and visual effects. The process of application of the glaze is also important for the final finish. For the Bond Street project, copper, manganese and iron oxides were hand applied and over-dripped thus the hand of the artist (craftsperson) needs to be the same. This is less of an issue with smooth monotone finishes, although even then a consistent glaze ‘recipe’ and application procedure has to be maintained to ensure an even colour match.

The process of fabrication at Shaws ends in a ‘dry lay’ where whole sections or in some cases the whole facade of a building is laid out on the ground to check for colour and dimensional consistency. At this stage you can see how geometric complexity, detail and finish are manifest in a family of relatively lightweight, hardwearing and non-standard pieces.

Artwork samples

Artwork samples

The facade elements for Eric Parry’s St James’s Gateway project are hollow 30mm thick forms hung off a steel frame, an aesthetic decision as this type of ceramic component has excellent compressive strength and could be used as part of a loadbearing facade.

Both the white-glazed, gum coloured edifice of the Wrigley building and the terracotta mullions of the Monadnock Building in Chicago use faience and terracotta and both were recently restored in part by Shaws. In this instance, the Clay Hypothesis and evolutionary design information (DNA code) is not contained in the molecular crystalline structure of the clay per se, but is revealed in a typology of forms, finishes and surface treatments found in old and new building fragments in a factory yard in the Lancashire hills.

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