Glowing panes

For the architect charged with designing the blue-glazed wall that forms the backdrop to the exhibition floors of the London's Science Museum's Wellcome Wing extension, it turned out to be the commission from hell.

The idea for the wall came from Richard MacCormac himself. Collett explains: "He wanted the wall to have an electric glow evocative of the electronic age in which we live. Not a constant glow, but one that changed as the daylight changed." The blue wall fills the west elevation of the £50m Wellcome Wing. The new extension houses a 450-seat Imax cinema and three floors of interactive exhibits explaining the latest developments in genetics, digital technology, bio-medicine and artificial intelligence. As Collett found, the wall would have to be every bit as high-tech.

Backlit by the luminous blue glass wall, the three floors or "trays" take on the appearance of rectilinear spacecraft hovering silently above the gallery's floor, eerily detached from the building's structure. It was just the effect the architect intended.

The wall was fundamental to the competition-winning scheme. But finding the right combination of glass and materials proved to be far more complex than anyone had imagined. "Rather frighteningly, the contract to build the extension was let to Kier before we'd established how the blue wall would work," says MacCormac. There were no precedents. For Collett, the pressure was on.

Light in the darkness

The success of MacCormac's concept rested on achieving a fine balance between daylight and artificial light. Too much light and the images on the screens of the interactive exhibits would disappear in a blue wash; too little light and the electric blue of the wall would dim to black, creating a featureless box.

"At the concept stage, our biggest worry was whether we could achieve the blue effect in both summer and winter with their vastly different levels of daylight," explains Collett. The search would take them into uncharted territory, as moving louvres or blinds were vetoed to eliminate the possibility of mechanical failure and to minimise maintenance costs.

The architect found help in the shape of Arup environmental engineer Chris Twinn and Dutch lighting specialist Hollands Licht. "Rogier van der Heide, principal of the studio, found out about the project on the Science Museum's web site and just turned up at our offices unsolicited," says MacCormac. His expertise in theatre lighting made him an ideal addition to the team.

The design team set about the wall's design without the aid of computers, opting instead to use models, scale mock-ups and experimentation. "Computer solutions depend on the information you input," says Collett. "All we had at the outset was a vision." Initial studies were carried out at the Bartlett School of Architecture and at Erco Lighting in Germany, where tests were carried out using laboratory equipment developed to simulate the sky.

Extensive testing found that a three-layered form of construction came closest to achieving the electric blue effect the architect sought. The composition consisted of a perforated aluminium backing panel, a double-glazed unit comprising a clear 6 mm outer layer of glass and an 8 mm inner layer of blue glass. A row of polished aluminium louvres slotted between the two glass panes forms the third element.

Two lines of defence

The wall works by using the perforated screen to reduce daylight levels, and the louvres to take out all direct sunlight. "At the outset, we got these two functions slightly confused and it didn't really work," says MacCormac.

On the outside, the perforated aluminium backing plate mounted about 1 m away from the glazing is the primary line of defence against bright sunlight and solar heat gain. By transmitting only 16% of the solar heat incident on them, the panels reduce the energy needed to cool the interior and cut light levels on the glass to 40% of the daylight levels. "This is only possible because there is no requirement for a clear view to the outside," says Collett.

The second line of defence against the penetration of direct sunlight is a series of miniature aluminium louvres. These small reflectors are sandwiched between the glass panels of the double-glazed units with their polished surface angled to reflect daylight into the building's interior. "This defence is not impenetrable," jokes MacCormac. "On 21 December, at the winter solstice, the angle of the sun will allow it to peep through the louvres into the interior for 90 seconds – so we expect the place to be packed with Druids." The system was developed as a model; the next step was to refine the design and verify the test results using a full-size mock-up of a section of the wall. The design team decamped to the studio of Hollands Licht. Here, a sulphur fusion lamp was used to simulate sunlight to allow the design to be fine-tuned as well as giving Tim Molloy, the Science Museum's design director, a unique preview. The experiments proved the blue hue would not be a problem for the exhibits as long as light levels stayed within the range of 50 to 200 lux – about the level of an intimately-lit restaurant.

Out of the blue

With the system designed, the challenge now was to find a manufacturer that could produce the glass to the precise shade required. Once again, the team had to look to the Continent. "We found that suppliers in Britain are not interested in research or experimentation," says MacCormac.

The team originally investigated the use of laminated glass with a blue resin interlayer, but this the risk of blue fading in direct sunlight was thought to be too great.

It was the agent for glazing supplier Alistair Price who stumbled across the answer at an exhibition in Germany. Schott-Desag was showing samples of the blue glass the company makes for optical instruments and cups for dispensing eyewash solutions. It was a fortunate discovery – the colour was almost identical to the one chosen by the team. And because it was commercially available, costs could be limited. Costs for the wall were in the region of £1100/m sq, comparable with the cost of a curtain wall for a commercial office.

The wall is constructed from a series of 2.1 × 1.2 m glazing units with supporting transoms and mullions. Measuring more than 30 × 30 m, the wall's entire weight is carried by the two 30 m high concrete stair towers that flank the glazing.

A steel structure comprising three 400 mm diameter circular hollow steel sections and three vertical steel 250 mm diameter columns form the main structure. Within this primary structure, a complex arrangement of bow trusses resists the wind loads. On the outside, V-shaped diagonal bracing rods direct the wall's weight to the two supporting brackets at the top of each stair tower from which the entire glazed wall is hung.

To the team's relief, the wall performed just as hoped, and the Wellcome Wing was officially opened by the Queen on 27 June. And Collett could sleep again.