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The flame game: Heatherwick's Olympic cauldron

The visual spectacle of Heatherwick Studio’s Olympic cauldron was hand-crafted in the workshop and refined with digital modelling, writes Felix Mara

The quirky, narrative qualities of the London 2012 Olympics opening ceremony were nicely balanced by moments of pure, abstract visual delight, in the design of the lighting and above all in Heatherwick Studio’s magnificent cauldron.

Because the Olympic cauldron’s form appeared simple, it could be read on many different levels, but the essential message was that 204 nations, each represented by a unique copper petal with a flame where it meets its stem, had assembled in one spectacle. The spectacle’s emblem - and the ceremony’s climax - was the convergence of these petals and flames, arranged in concentric rings, as their stems slowly pivoted upwards to form a ball of fire.

After the flames of the perimeter petals were lit by young torch-bearing athletes, they appeared to be relayed in circular and centripetal sequences from one ring to the next as they formed a cluster. This was offered up to the night sky by the cauldron’s copper hands in a sacrificial gesture that resonated with the rituals of the original games and with the ‘workshop of the world’ sequence earlier in the ceremony. The raising of the cauldron and its reverse replay at the closing ceremonies was as expertly choreographed as the event’s dance sequences; balletic, like a scene from Swan Lake with copper feathers.

‘We wanted to hear a continuous ‘Oh!’ of surprise as people in each part of the Olympic Stadium got it at different times,’ says Heatherwick Studio project architect Katerina Dionysopoulou who, together with architect Andrew Taylor, was involved in the project from its inception in 2010. ‘The plan-form of the cauldron, which in its closed configuration reflected the elliptical geometry of the stadium, and the sequence of its rise and descent  had to be fully integrated with the opening and closing ceremonies.’ Heatherwick Studio worked closely with the opening ceremony’s director, Danny Boyle, and Stage One, a scenery contractor with a very strong technical and engineering bias, to co-ordinate the sequence, ensuring that it was sufficiently short to retain spectators’ interest.

‘The process was unusually linear for a Heatherwick Studio project,’ says Dionysopoulou. ‘But we knew from the start that we wanted to find a way to represent each of the constituent nations and their convergence.’ The Olympic rings mandala informed the decision to have a five-stepped circular ziggurat at the cauldron’s base and in the early stages of the design there were five concentric rings of petals, but this number increased to 10 as Heatherwick developed the distinctive ‘pin-cushion’ geometry of the cauldron’s open configuration, which was the optimum form for spectators viewing from all angles.

Although Heatherwick initially relied on an instinct for what was buildable, the design developed empirically as Stage One and project manager Robbie Williams Productions tested the designers’ proposals. The form of the petals was difficult to replicate using physical models and Andrew Taylor built a Rhino 3D model which was then exported as solid geometry to be used in Stage One’s Autodesk Inventor software. This could then be used to model the cauldron’s mechanics, gas services and other components, as well as for CNC output. The form was not significantly modified in Inventor, but was repeatedly imported back into Heatherwick’s Rhino model so that refinements could be made, for example to the pivots, which were originally more bulky.

In Heatherwick’s ambitious design, the appearance and performance of the flame and the pivot mechanism were critical to the success of the Olympic cauldron, versions of which had faltered at the Vancouver and Sydney Games. In its closed configuration, the petals at the top of the cauldron formed a concave bowl, but Heatherwick wanted the flames to follow a convex profile which was highest at the centre, so more gas had to be supplied to the burners in the inner arrays of petals. The supply of gas was regulated by what Dionysopoulou describes as a ‘mother computer’ below the cauldron and was closely calibrated by Boyle and the designers in the two weeks of intensive undercover preparations in the stadium before the opening ceremony.

Similarly, Robbie Williams had identified the pivots of the lifting mechanism as a potential risk and baulked when Heatherwick proposed to have 204 of them. The solution was to introduce mild steel ring beams, which were raised and lowered to operate the pivots using mechanical technology rather than hydraulics, thus spreading the risk. The other components of this mechanism were machined stainless steel and aluminium.

‘We wanted the stems to disappear,’ says Dionysopoulou. ‘They needed to be as thin as possible, but their diameter was restricted because they were conduits for gas and the electricity used to ignite the flames in the burners at their ends,’ adds Taylor. These ignitions created the illusion of flames spreading from one petal to another and enabled Boyle to fine-tune the sequence. ‘Each stem comprises four different swaged circular section stainless steel tubes which were squeezed and spun to form tapers before being welded end-to-end,’ says Stage One project manager Neil Franklin. The smallest tapers to a diameter of 25mm. Discolouration by the welding process wasn’t a concern because the stems have a finish known as ‘bad black’, which makes them look thinner and, depending on their background and lighting, more invisible.

Heatherwick considered various materials and finishes for the petals, seeking a precious quality but wanting to avoid the gold, silver and bronze used for Olympic medals, eventually settling on copper sheet, typically 0.9mm thick, partly because its orange glow combined well with the blue-black finish of the stems.

Although the petals have similar shapes, each was intended to be slightly different and had to interlock with its neighbours to create a satisfactory form in the cauldron’s opening and closing configurations. They also had to be easy to fix to the stems and to remove at the end of the ceremonies, when they were taken away by the competing nations as souvenirs. This was achieved by bayonet light bulb-type connections and notches in the petals that enabled them to be fitted around the burners, which gas safety regulations required to be pre-welded to the stems, thus keeping the petals’ funnels as narrow as possible.

It took five hours to fabricate each petal. The fabricator used CNC wooden plugs, which were the negative of each petal, as formers and hard-worked the sheet with hammers as much as possible before quenching it up to 30 times to achieve the final form. The copper was then washed with acid to remove tarnishing from the anealing process, then polished and buffed using various grades of coarse paper. Each petal has the name of one of the competing nations inscribed on its rim. This was achieved by unfurling the top of each petal so that a template could be made for the rim which was then fitted, with the country’s name acid-etched on its surface.

Taylor considered using Grasshopper parametric software and scripting to automate the modelling of the petals, but the form-finding process was particularly complex, especially in the case of the outer petals, so he relied heavily on what he describes as a manual process which involved manipulating digital meshes. Each petal took three hours to model. ‘The cauldron involved a nice mix of traditional craftsmanship and digital technology,’ says Taylor.

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