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Monday28 July 2014

Eric Parry Architects’ Holburne Museum in Bath

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The blue-green glaze of Eric Parry’s Holburne Museum extension brings new depths to its Bath surroundings.

I am out on the roof terrace of Eric Parry’s London office among a menagerie of ceramic forms that might be artefacts from some ancient civilisation. Their glazed surfaces are almost metallic, like dark cast bronze with spots of weathered copper that glint silver when they catch the sun. These are the cladding samples for an extension to the Holburne Museum in Bath, which is undergoing a £13 million redevelopment due for completion this autumn.

Fundamental to our image of Bath is the weathered limestone that binds it into a homogenous city. So how do these dark ceramics fit in? This question has dogged Parry since they were first proposed in 2005 and has taken three planning applications to overcome. The first was rejected in 2007, partly because of the proposed colour of the ceramics. The glaze was reluctantly changed to a light brown, similar to Bath stone and the revised application was approved.

But Parry was not prepared to let his vision be compromised by what he felt was a misunderstanding of the desired effect and by using material samples he slowly brought the various conservation groups round to the idea. In 2009, with the ceramic panels already in production, a third application was submitted to change the finish to a darker blue-green colour.

“As soon as you start lightening it you lose the depth,” says Eric Parry. “We got the Georgian Group on our side with the earlier samples by describing it as a bit like the colour of my daughter’s eye, which they melted at.”

Permission was finally granted just a day or two before a decision was needed to avoid causing a delay to the project.

Concepts of ambiguity

The Georgian development of Bath is characterised by the integration of built form with the natural landscape. When Great Pulteney Street was laid out on the east side of the Avon it was designed as a straight boulevard wide enough to turn a carriage and terminated by the Sydney Hotel. Beyond, Sydney Gardens were laid out as a pleasure garden with a grotto and maze. This surviving 18th century landscape is the context in which the new extension will be seen.

“I wouldn’t propose ceramic in another place in Bath, certainly not in the street,” says Parry.

Inspiration came from the way light plays on the north-east facing wall of the existing building, particularly at dusk.

“It’s like that Magritte painting of the park with a lamp and a day-lit sky; there’s a fantastic ambiguity and theatri-cality,” he says.

The mottle coat had to be applied in the same direction on each piece, creating a foliate pattern

Originally the Sydney Hotel had a loggia at the rear with a first floor bandstand addressing the gardens. One of the prime moves in Parry’s alterations is to reopen this connection. A new staircase and lift are being built just off axis outside the rear wall, linking the building at all levels to the new extension.

The extension has a basement housing a picture store/archive and the kitchen, allowing the ground floor café to be completely glazed on all sides. The first floor and mezzanine house interconnected galleries for smaller objects and the top floor is a single, top-lit gallery that links directly with the galleries in the existing building.

To maintain column-free galleries, the steel structure above ground level is hidden in the outside walls. The concrete floors are tied back and pocketed into the walls of the existing building for bracing. The facade is hung off the structure and each floor cantilevers out from the floor below so there is a physical depth to the elevations. The structure is quite simple but it is working hard.

Curatorial demand for a gallery on the second floor gives the building a bulky, blank-walled top. Parry has used this anomaly to invert the tripartite composition of the existing building so the extension becomes lighter and more transparent towards the bottom.

Elevations have been designed around the concepts of ambiguity and veiling to create an animated facade defined by changing col-ours and patterns of light. The top floor is clad in flat ceramic panels with ceramic fins concealing the vertical joints. Some of the fins continue down to first floor level in front of a glass rainscreen with a void behind and then an inner wall faced in the same flat ceramic panels as above.

The glass extends down to ground level, where the café is enclosed by a double-glazed curtain wall inside the rainscreen. At ground floor level the rainscreen glazing is low-iron for optimum transparency while the upper levels have a very light reflective coating.

Changing rhythm

The ceramic fins are intended to give the building a crafted feel, like something woven. Covering the vertical joints is intended to expunge the perception of a grid, particularly on the glazed areas. The flat ceramic panels are made in 900, 750 or 500mm modules and the fins follow this irregular pattern. The module of the glass is wider than that of the ceramic so the rhythm changes down the building and the height of the fins is deliberately different to that of the flat panels to emphasise their verticality.

The whole facade will be naturally ventilated. An 80mm gap at ground level beneath the outer glazing will allow air in at the bottom and the gaps between the panes will be left open. On the soffit on the underside of the second floor perforated metal grilles will allow air to rise up into the cavity and out through open horizontal joints between the second floor cladding panels.

The ceramic elements are being made by Shaws of Darwen. Its early input allowed the detailed design to take into account the natural variations between the pieces.

“It’s a hand-made product going on to a new facade so tolerances are critical,” says Shaws’ production manager John Wilson. All the fixing holes have to be made before firing so each piece is individually supported on stainless steel brackets which are fixed via slotted holes to allow adjustment. The fin elements can be packed with EPDM shims and are bolted in place.

Each faience piece takes around 18 days to make. First a thick slip is poured into a mould and left to dry for two days. Once removed from the mould, it is cleaned up by hand and wrapped in hessian to prevent it drying out too quickly. At almost a metre long, some of the flat panels are near the maximum size that can be made without distorting. The pieces shrink by about 5% as they dry. After nine days the hessian is removed and the panels spend two days in a drying mach-ine. Then they are ready for glazing.

Two coats of glaze are applied. First a base coat rich in manganese and copper, which produces a green-brown colour, and the following day a mottled pattern is applied containing manganese and titanium oxide, which is white but reacts with the basecoat to form the distinctive blue-green pattern. Finally the pieces undergo a single firing and are laid out on the factory’s floor to check for imperfections. Experimentation determined the mottle coat had to be applied in the same vertical direction on each piece, creating a foliate pattern like hanging ferns in a grotto.

Had the extension been clad in Bath stone it would not have the depth or contrast that the glaze gives the ceramic. The fragmentation of the surface is intended as a landscape gesture, establishing the new building as a pavilion in the gardens as much as an extension to the museum. Perhaps this dram-atic contrast between the Georgian street and the shadowy, magical world of the pleasure garden is what was always intended by its creators.

Project team

Architect Eric Parry Architects, Client Holburne Museum of Art, Facade consultant Arup Facades, Facade sub-contractor MBM Metalwork Construction, Ceramic fabricator Shaws of Darwen, Main contractor Sir Robert McAlpine, Project manager Cragg Management Services, Structural engineer Momentum Consulting Engineers, Services engineer Atelier Ten, Electrical services contractor Concab Electrical, Mechanical services contractor CMB Engineering

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