Feature

The weight of history

Snøhetta has carved a monolithic visitor centre into a French hillside as the Lascaux cave paintings go digital

Massive, monolithic concrete was the only material that could do justice to the remarkable cave paintings at Lascaux, writes Tony Whitehead

In the autumn of 1940, a teenager walking his dog near the village of Montignac in south-west France discovered the Lascaux Caves – the site of some of the world’s most extraordinary prehistoric cave art.

Around 2,000 detailed images, some estimated to be 19,000 years old, decorate the limestone walls of the complex. After the war, it became a huge visitor attraction, with more than 1,000 tourists a day flocking to the cave. But by the late 1950s it became apparent there was a problem: opening the cave to visitors had changed the atmosphere inside, and the paintings had begun to deteriorate. The caves were closed in 1963 and, ever since, only rare visits by conservationists and historians have been permitted. A replica, “Lascaux II”, opened 200m away in 1983, to become the most-visited prehistoric cave in the world.

Now, however, at the new Lascaux IV visitor centre, tourists will once again be permitted to gaze upon the astonishing art of our distant ancestors – or something very nearly as good. Laser-scanning technology has enabled an extraordinary facsimile of the main cave to be reproduced to within millimetres of the original, while artists and sculptors have toiled to exactly recreate the artwork of our distant ancestors.

But how to frame and present these pictures from the past? Designing a visitor centre which will, in effect, become humanity’s only opportunity to experience the caves, perhaps for centuries to come, is clearly quite a challenge. Naturally enough, the first decision facing the design team from Norwegian architect Snøhetta concerned the choice of material.

“We had a big debate about this,” says project architect Rune Veslegard. “One option was to use limestone – the stone in the cave and also the local building material. But somehow a steel structure with thin panels of limestone cladding stuck on it would have felt wrong – too lightweight. By choosing concrete instead, we have this feeling of weight, of massiveness, as if it is all carved out of the hillside, like the cave itself.”

In fact, the in-situ concrete which forms the facade and most of the centre’s structure is of a colour very similar to the local limestone. The pleasing result is that the long lateral lines of the centre seem almost to have been revealed by brushing away topsoil from the hillside into which it is set. Getting the shading right was a priority for Veslegard: “It’s not simple. We tried and tested 13 different mixes before we were satisfied that we had got the colour and texture exactly as we wanted.”.

Given how strongly he felt about this, the consistency and quality control offered by precast concrete could have been an option. Veslegard, however, was adamant that only in-situ concrete fitted the aesthetic: “Even with precast you would still have had the feeling of something attached to a frame. Using cast-on-site concrete gives us these beautiful, continuous forms – one shape flowing into another. Altogether the project uses 10,000 tonnes of concrete, and it provides this sense of mass and heaviness which is all-important here.”

Looking at the long, low and angled facade, it is easy to see what he means. Comprising a series of angled reinforced concrete vertical slabs some 250mm thick, the fact that it appears to float above the glass frontage only serves to emphasise its obvious weight. Movement joints have been kept to a minimum and are all but concealed within the vertical creases formed by the facade’s regular change of angle – a trick that helps to maintain the feeling of unbroken solidity.

The facade is supported by a combination of concrete beams and metal trusses which run back into the interior of the centre and which cantilever off internal load-bearing concrete walls. “In addition, there are two large inclined concrete columns near the central entrance which also support these beams,” explains Veslegard. “In general, though, we avoided columns as there are no pillars in caves and we wanted to keep the sense that the interior is carved out of the hillside.”

Inside the 8,365m2 centre are the usual facilities: a reception area, shop, exhibition space and also a small theatre. The flooring in these areas is all formed from polished in-situ concrete to which a quartz aggregate has been added to help keep it pale, like the walls. Interior walls are all formed from the same mix of self-compacting concrete used for the facade, and this remains exposed and visible in most areas. The banded pattern visible on the facade continues inside, though some areas, such as the exhibition space, have a simpler finish which reveals the standard plywood boards used for the formwork (see “Get strata”).

Before entering the replica cave, visitors must pass through a canyon-like circulation area known as the orientation space. This comprises a kind of wide corridor flanked by tall, tilted concrete walls up to 13m high, and lit from above by a glass ceiling.

“The walls are up to 300mm thick and are set at varying angles from vertical,” explains Veslegard. “They range from 9° negative to 2° positive. This meant quite a lot of special pieces of formwork to ensure that the angles all met correctly on plan.”

Each vertical section was done in one pour, he adds, although each could take up to four loads of concrete to complete. “We decided, even though the concrete is self-compacting, to vibrate it in the break between trucks. This ensured that it did not change appearance between truckloads.”

In addition, care had to be taken with curing “It was very important that all the concrete was in the formwork for the same amount of time or it would affect the colour of the finish. For example, we would not cast on a Friday and take it down on Monday.”

Prehistoric precast

While designer Snøhetta initially strove to create the Lascaux IV visitor centre entirely from in-situ concrete, there are certain areas of the project where precast concrete was a pragmatic and practical alternative.

One such example is the patio garden, a tall-walled but open-air continuation of the orientation space, and here precast concrete has been used as the backing to a water feature.

“Water runs down the wall continuously here, so naturally we had to be concerned about water penetration,” says architect Rune Veslegard. “The density and quality control of the precast element gives us more control over that. Behind it is a membrane and then behind that the wall reverts to in-situ concrete.”

Externally too, precast slabs have been used. “In France, where you have a membrane, there are rules about access which are difficult to comply with if you are using in-situ concrete,” Veslegard explains. “So on the roof of the centre, around the walkway above the glass roof of the orientation space, we have used precast for the low walls and steps.”

As Veslegard says, the colour match with the in-situ concrete is so good that the difference does not catch the eye – and the fact that these areas are away from the facade also reduces any visual dissonance.

The finished walls are strong enough to self-support their tilts from the vertical, though they are also tied back to supporting walls. However, because of the weight of the concrete involved, unusual amounts of support were needed to maintain the formwork in position while it was curing.

“The formwork was all steel, from bottom to top on both sides, and each 3m-wide section had around 20 steel rods supporting it,” says Veslegard. “It meant that a dense criss-cross of rods made access very difficult in the area which is now the orientation space.” With pours of up to 13m, it was essential that the metal formwork was tightly sealed with silicon: “Otherwise, the pressure of the concrete, especially towards the bottom, would have caused leaks.”

Entrances to this area have been formed by timber boards, and the technique is evident in the as-struck board-marked finish of the doorways. Air out-take holes near the roof and ducting for lighting were also cast in. “To keep the massive, carved-out feeling, we didn’t want things attached or drilled if we could avoid it,“ says Veslegard. “We cast in everything we reasonably could.“

To access the cave itself, visitors must travel by lift to the roof, though because of the way the centre is built into the hill, this is almost level with the ground outside. “We wanted to recreate the feeling of discovering the cave, so entering from inside the centre would not have felt right,“ he says. 

Get strata: Creating the layered concrete walls

The horizontal banding, or striations, which run across the facade, much of the interior walls, and also the canted walls of the orientation space, are the most distinctive visual feature of the concrete at Lascaux. Reminiscent of sedimentary layering, these bands provide a unifying theme and add to the geological references created by Snøhetta’s deployment of in-situ concrete.

The banding comprises alternate, slightly wavering lines of rough and smooth concrete and, as project architect Rune Veslegard explains, the effect required considerable attention to detail. “The smooth part is made by the combination of metal forms and self-compacting concrete,” he says. “The rough is created, after the formwork has been removed, by applying a metal mask or stencil to the concrete and sandblasting away the outer layer to reveal the aggregate beneath.”

Achieving consistent finishes of both rough and smooth meant experimenting with the mix. “First the limestone aggregate was too yellow. Then we found that, even though the concrete was self-compacting, the flow was not good enough.”

This, says Veslegard, created unwanted patches of visible aggregate in the finish. “We solved it by increasing the amount of round sand compared with angular sand.“

Creating the masks was also a subtle exercise. “Where the walls are vertical, it is quite straightforward to reuse the mask and get the striations to match up in continuous horizontal lines,“ says Veslegard. “But in the orientation space, because the walls are inclined by different amounts, this was much harder. It meant that for each angle of inclination we needed to make a special mask to ensure everything matched perfectly.”

In the orientation space, 30% of the surface has been roughened, but on the external facade the pattern is reversed, with 70% of the surface sandblasted. Veslegard is particularly pleased with the effect of sunlight on the facade. “From one direction the sunlight makes the rough surface light, from the other it appears dark. It means the pattern changes depending on where you view it from, and also the time of day.”

The famous images of hunters and bison have been painted onto a resin/aggregate substrate and, as in the original, these images are above waist height. The lower half of the cave has been created with shotcrete sprayed onto a steel mesh shaped to replicate the rock of the original cave. Sculptors have moulded clay on top of the shotcrete to provide a more natural finish.

Just 200m away from the new centre is the entrance to the original Lascaux cave, though these days of course it is seldom used. So, how to assess the accuracy of the experience provided by a visit to the replica? Not many people are in a position to comment, but Veslegard, as one of the few to have ever visited the original, is pleased with the results. “It is not just the shape of the cave and the detail of paintings which is so accurate,” he says. “It is the whole experience: the acoustics, the temperature and the humidity.”

The odd thing is that we shall just have to take his word for this. “Yes, it is strange,” he admits. “We have effectively made a new reality for the generations to come.”

PROJECT TEAM

Architect Snøhetta
Main contractor Lagarrigue
Concrete supplier Sopreco
 

Photos: Boegly + Grazia; Eric Solé