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Guessing what this building鈥檚 skin is made of is quite a teaser. The mirror-like panels of the 拢27m, 233-bedroom extension to the Gatwick Hilton hotel measure up to 1.2 脳 0.6 m in area. They resemble cement-particle panels such as those made by Eternit, yet their surface is notably

more polished. Perfectly flat, with fine tolerances at the open joints, they could be mistaken for stove-enamelled steel panels.

In fact, the large panels are made of

extra-high-density vitrified ceramic granite, as used in small, hard-wearing bathroom floor tiles. The shiny surface was produced by polishing rather than applying a glazed finish. Made by Mirage in Italy, this is the first time they have been used to clad a building in the UK.

Ironically, the Manser Practice pioneered the material in order to match the 40-year-old cladding system on the original hotel building next door. Designed by Yorke Rosenberg Mardall and built in the late 1960s, the original hotel took the form of four severe Miesian slab blocks with flush-textured cladding in travertine marble. The big stumbling block was that, even though the size and specification of the rooms were upgraded for the extension, travertine marble was ruled out as too expensive for its exterior.

鈥淲e were looking for an economical material that would emulate all the qualities of natural stone and complement the travertine, but that would also distinguish the new executive wings from the original building,鈥� explains Manser鈥檚 project director Marvin Farr. 鈥淲e wanted to create two flush seamless boxes that would reflect the landscaped gardens and changing skies, and this required a material with a high degree of reflectivity.鈥�

Aside from appearance, the key functional requirement of the external envelope, given that the hotel overlooks the airport runway, was to block out as much aircraft noise as possible. An impressively high sound insulation of 48 dB Rw was achieved by building up a steel-framed external wall system of dense layers of plasterboard, acoustic insulation, galvanized steel liner trays, breather membrane, cement particle board and, finally, the ceramic-granite panels.

The effect inside the hotel bedrooms is that planes can be seen taken off and landing as if with the sound switched off. The top layer of

ceramic-granite panels operates as a rainscreen cladding system with open joints and a cavity behind.

The panels are made from a patented

self-coloured mix of ceramic clays, granite dust and other natural minerals that is compressed to a density three times higher than natural granite and a thickness of 12 mm before being fired in 100 m long kilns. The strength and slenderness of the panels dramatically reduces the weight compared with traditional stone panels 鈥� hence the reasonable cost.

Production costs were further reduced by Farr, who rationalised the total facade area of 5000 m2 into just eight panel sizes. The entire external wall was built for a unit cost of 拢400/m2, which quantity surveyor Davis Langdon & Everest claims is at the lower end of the range of comparable systems.

The Manser Practice was introduced to the

new material by supplier Shackerley Holdings, which had formed a joint venture with Mirage to serve the UK market. A visit to the factory in Pavullo, northern Italy, and buildings in Switzerland and Italy that had been clad in the material for up to 15 years convinced both client and architect.

Despite its enthusiasm for the product, the Manser Practice rejected the mechanical fixing system recommended by Mirage astoo clumsy, with its bulky steel clips at each corner and

wide gaps between panels. Instead, cladding subcontractor Exterior Profiles, which had E

E worked with Manser on several projects, came up with an adhesive fixing system using Sika impact adhesive, which could be installed with precise 10 mm gaps between panels.

To achieve the highly impressive flush finish of the facades, the critical part of the installation process was setting out the grid of aluminium channels to carry the panels. The channels were set out by precision lasers and packed out with shims. Spacer tape precisely 3 mm thick was then stuck to the channels, adhesive applied to the same thickness between the tapes, and the panels were offered up, with temporary 10 mm spacers around the edges.

Since the material had never been used before in the UK, two failsafe techniques were adopted by the project team. Construction testing specialist Sandberg was called in to monitor quality control during the installation process. And as a belt-and-braces support, Exterior Profiles recommended a back-up fixing system of two inconspicuous stainless-steel angles to hold up the bottom edge of each panel.

To achieve the design concept of seamless boxes, in which 鈥渢he distinction between glass and wall panels disintegrates,鈥� Farr blended the bedroom windows into the facade by designing out window frames. The outer window pane

was silicone-bonded to the adjoining cladding panels, and its edges were fritted with off-white dots to play down the distinction between transparent glazing and opaque cladding.

Sound insulation in the windows was increased to 42 dB Rw by creating a sealed cavity 280 mm deep between inner and outer window panes

and lining the reveals with 40 mm thick acoustic-absorbent tiles.

The polished finish and high density of the panels have the practical advantage of not retaining moisture or dirt. 鈥淲e were looking for something as maintenance-free as possible in an environment that produces pollution,鈥� says Farr.

Now the scheme is complete, the project team is still flying high over the sleek facades to the new hotel wings. Their delight is backed by assurances from Mirage and Exterior Profiles that the ceramic-granite panels and their full fixing and support systems come with a designlife of

50 years. It is little wonder that, as Mirage

claims, a grand total of 20 million m² of it鈥檚

ceramic-granite panelling is currently on order to clad buildings throughout Europe.