Herzog & de Meuron now know this building task very well: Take a giant brick building built for commercial purposes on the bank and convert it for cultural use.

The Tate Modern in London, which was planted and grafted onto a former power station on the Thames, marked the international breakthrough for the Basel architects' office in 1999.

And with the Elbphilharmonie, completed in 2016, which billows on a former granary in the port of Hamburg, they crowned an entire city and also their own work.

Matthias Alexander

Deputy head of department in the features section.

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In between, the two Pritzker Prize winners had completed a far less spectacular project at Duisburg's inner harbor, but one that was very well received by experts: the conversion of the huge Küppersmühle, built around 1908, into a museum for modern art, initiated by the local collector Hans Grothe. While the interior of the three-wing building was radically rebuilt for the new use, the interventions in the facade were more cautious: Several window openings were bricked up with bricks and a few narrow vertical window strips were cut. There was also a red-washed extension that took up the sloping gable roof line of the existing building and the sculptural staircase made of red-colored concrete. The whole thing was a demonstration of decency.

The Grothe Collection passed into the hands of Sylvia and Ulrich Ströher in 2005. The public-shy Darmstadt collector couple decided to remain loyal to the location at Duisburg's inner harbor, which had long since blossomed into a flagship project of structural change in the Ruhr area that had long been recognized across the region. Because more space was needed for the presentation of the combined collections with their 1,500 works of predominantly German art from the period after 1950 to the present, an extension for the Küppersmühle Museum was to be built. It has now been completed and will be open to the public from Saturday - including the new hanging of the collection.

The architects, for whom Robert Hösl is responsible for the project, picked up where they left off in 1999: the four-story extension, which almost doubles the exhibition area by 2500 square meters, exudes an ensemble spirit in terms of material, volume and shape. For its part, it is divided into three buildings of different heights and made almost closed in clinker brick, interrupted only by vertical, floor-to-ceiling window slots. The clinker bricks were broken in the middle in an elaborate process, which ensures a particularly lively appearance on the facade, similar to the one in the Schaudepot that Herzog & de Meuron built for Vitra in Weil am Rhein. The lettering "Küppersmühle" was inserted into the eastern facade facing the A 59 motorway,it consists of around 1000 hand-cut bricks.