With a height of around 35 metres, the atrium of the Mercedes-Benz Museum impresses its visitors. Futuristic lifts on three walls arranged in the shape of a cloverleaf glide up into the exhibition. The lift shafts are connected with so-called twists, imposing double-curved concrete components. They grow out of the walls and, leaning against the nearest shaft, spiral outwards in a slight curve. With an area of 400 square metres and a weight of 2,500 tonnes each, the twists shape the dynamic appearance of the Museum and are one of the most spectacular architectural innovations ever. Their beautiful, smooth surface in exposed concrete shows the main building material of the Museum. Only those who look very closely will discover five large squares in the twists. Are these doors? At this height?
Yes, the squares are doors. They are inconspicuously embedded in the concrete wall, similar to the camouflaged wallpaper doors in many a castle. They are quite large – each of the five doors is 4 metres high and 3.50 metres wide. All the trucks, buses, vans and passenger cars enter the Museum’s Collections Rooms through these openings. For this purpose, a crane system has been installed in the atrium ceiling. If required, this lifts the transport platform with the vehicle to the necessary height and docks at the desired door.
A rough calculation shows that each door would weigh around six tonnes if it were made of concrete. Theoretically, it would be possible. But in practice, each door would require gigantic hinges as well as an extremely powerful opening drive. That’s why a clever trick is used in the Mercedes-Benz Museum: instead of concrete, the doors are made of a much lighter hollow steel construction covered with gypsum fibreboard. So despite the enormous size, each one weighs less than a tonne.
The concrete look is preserved on all five doors despite the steel and gypsum fibre boards. This was made possible by extensive trompe-l’œil work during the construction of the Museum in 2005 and 2006. This technique is actually more familiar from feudal or sacred buildings: it creates deceptively real-looking representations of things. Including three-dimensionality on a two-dimensional surface, if desired.
“Deceive the eye” – that’s how one can translate trompe-l’œil. Jürgen Kuntel carried out this elaborate artisan technique in the Mercedes-Benz Museum at the time. He is a specialist in the restoration of architectural surfaces, plaster and stucco in monument preservation, and also in the renovation of exposed concrete surfaces on prestigious new buildings.
Kuntel’s work on each “concrete door” in the Museum involves several steps. The preparation alone is time-consuming: after priming the gypsum fibreboard, it receives a thin layer of concrete filler. “With this materiality, it blends in with the surroundings, and besides, this way you already have a basic grey tone,” Kuntel explains. The wall surfaces in the atrium show the joints of the concrete casting everywhere – with the help of a chalk line, the conservator continues their courses on the door surface and then correspondingly moulds them exactly along the lines. Because if it were a real, poured concrete wall, it would have these joints. He also marks the correct positions of the small round formwork anchors, which are sealed with concrete plugs and can be found at regular intervals in all the Museum walls. The restorer also incorporates their joints plastically into the door surface with fine spatula craftsmanship.
After this preliminary work, it’s time for colouring. “Extending concrete patterns is indeed a challenging task,” says Kuntel. “On the door, the colours and patterns from the right, left, top and bottom flow together.” First, he mixes the appropriate grey colours – the basic shade and three to four glazing shades. The paint is then applied in the best trompe-l’œil: with brushes and sponges, in several layers, with superimposed different shades of grey, in the appropriate gradients. And each stroke transforms the door more into the supposed concrete door.
The last step is sanding with fine and finest grits. “The surface must be identical to the surroundings in terms of abrasion so that the applied colour is as textured as the real concrete,” Kuntel explains, “otherwise shadow effects occur that undo all previous perfection.” The expert repeatedly strokes the surface with his hand and is only satisfied when the smoothness on the door exactly matches that on the concrete walls.
The work on the doors takes weeks. Kuntel works together with a co-worker at a lofty height – on the platform of the mobile crane. A machinist steers them to the front of the respective door and then lifts them there bit by bit so that the experts can gradually work the entire door height of four metres. They do not complete one door at a time and then the next, but carry out each major work step on all the doors in turn. So the crane basket is moved again and again. “All the processes on the Museum construction site were highly interesting for me, and everything also worked out perfectly. We received first-class support from construction management,” Kuntel sums up some fifteen years later. It was a very exciting time for him, he still says today. “I have never worked in that scale since.”
Each “concrete door” has an electric motor at the bottom of the opening side with two rollers to support it on the floor and move it at the same time. The power delivery is comparatively low: the motor has a torque of 70 newton metres. By comparison, the engine of the famous Mercedes-Benz SSK model from 1928 has a maximum torque of 450 newton metres. But the door motor is easily sufficient for opening and closing. Seen from the atrium, there is a short anteroom behind the door that leads towards the Collections Room. At its end are two hinged doors. These are also made of sheet steel and are visually matched to the wall on the side of the showroom with a white paint finish.
But even a door measuring 3.5 by 4 metres becomes a bottleneck when heavy-duty trucks or buses have to pass through. Usually they reach their destination without any problems. But not the Mercedes-Benz Travego coach in Collections Room 1: Gallery of Voyagers. The vehicle is a bit higher than four metres, so the Museum curators have to find a creative solution. As a result, the air is let out of the tyres. That’s enough to roll the Travego into place – out of the atrium, through the “concrete door” and into the exhibition room.