Herzog & de Meuron, as seen in their approach to brick at Tate Modern, build every project starting from a question about the material, not from a predefined form the material must simply adapt to. At the de Young Museum in San Francisco (2005), rebuilt after the previous building had been irreparably damaged by the 1989 Loma Prieta earthquake, this question finds one of the most elaborate answers in their entire body of work: an envelope entirely in copper, conceived from the outset not as a static cladding but as a material destined to transform visibly over the decades following construction.
Before the de Young: the Basel signal boxes
Herzog & de Meuron's relationship with copper began more than ten years earlier, in Basel, with two service buildings that many travellers see from the train without knowing who designed them. The signal box "auf dem Wolf" (1994) and the central signal box near the station (1999) house the electronic equipment that controls rail traffic. They are concrete volumes entirely wrapped in copper strips about twenty centimetres wide, arranged horizontally like the windings of a coil. At the windows the strips twist slightly, letting light filter through and revealing from outside, almost imperceptibly, where the openings are.
The choice of copper here also has a technical reason. A continuous, earthed metal envelope works as a Faraday cage, protecting the equipment inside from the electromagnetic interference produced by the railway lines and by lightning. It is a clear example of the practice's way of working: the same choice solves a functional problem and produces a precise image, an object that seems wrapped in metal fabric and changes colour over the years. The de Young takes up this experience at a much larger scale, shifting the emphasis from protection to the relationship with the landscape.
A pattern derived from the park's light
The museum stands in Golden Gate Park, surrounded by tall trees whose canopy casts a constantly shifting play of light and shadow on the ground. Herzog & de Meuron translated this natural effect into a precise architectural pattern: photographs of backlit tree canopy were digitised, pixelated and enlarged, and the resulting pattern was translated into a combination of perforated holes (in six different diameters) and concave and convex embossed reliefs in the sheet metal (in eleven different depths), spread across nearly 7,000 copper panels, each individually designed so that no two are identical. The result, developed technically together with the specialist metal fabricator Zahner, is a facade that filters daylight into the museum's interior with a dynamic, shifting effect similar to sunlight filtering through leaves — an ornamental detail born from a specific environmental datum of the site, not from an abstract decorative repertoire.
Behind the effect lies a production process that, in the early 2000s, was still experimental. Every panel was different, and drawing them one by one would have been impossible: software translated the pixelated image directly into instructions for the punching and embossing machines, which worked the sheets without a traditional shop drawing for each piece. It is one of the first examples, on a large building, of the continuous digital chain between design and fabrication that is now normal for many complex façades. The museum also has a tower about 44 metres high, slightly twisted on itself, with an observation deck on the top floor open to the public: from there you can see how the copper roof merges with the tree canopy of the park and the city beyond.
Copper designed to disappear into the landscape
The choice of copper as a material was no accident: Herzog & de Meuron selected it explicitly in anticipation of its chromatic evolution over time. The bright orange copper of installation, over the building's first years of life, progressively darkened and eventually began to develop its characteristic green patina — a process that, for a building of this scale, is measured in decades, not years. The practice's stated intent was for the museum to chromatically merge, over time, with the vegetation of the Golden Gate Park that surrounds it: a building conceived to become part of the landscape precisely through the ageing of its own material, not in spite of it. It's the same principle, applied to metal instead of brick, that guides the practice's entire material research: the material isn't chosen for its appearance on opening day, but for its entire trajectory over time.
It should be said that in San Francisco's climate, marine but relatively clean, the transformation proceeds slowly: almost twenty years after opening, the museum has turned a dark, matt brown, with the first signs of green only on the areas most exposed to rain and fog. It is a good reminder for anyone designing with copper: the timing of the patina depends on climate, pollution, orientation and even the slope of each surface, and cannot be predicted precisely. Anyone who wants a certain result from day one has to choose a factory-treated finish.
Finishes and alloys: copper is not one colour
Anyone choosing copper for a façade today can start from different states of the material. Natural copper, bright and pinkish, is the de Young's, and will change colour over time. Pre-oxidised copper is treated at the factory to give an immediately brown tone, similar to the one natural copper reaches after a few years. Pre-patinated copper reproduces the green of old roofs from the outset. Then there are the alloys. Brass, copper with zinc, has golden tones and ages by darkening; bronze, copper with tin, is harder and historically linked to sculpture and monumental doors; tombac, a brass rich in copper, has a warm colour between copper and gold. Some modern alloys with aluminium and zinc are designed to keep a golden look for a long time without developing a green patina.
The choice between these states is not only aesthetic. Natural copper conveys the idea of a building that changes; pre-patinated copper declares the final appearance from the start and gives up the transformation. It is interesting to note that even buildings that evoke bronze or copper do not always actually use them: the National Museum of African American History and Culture in Washington, designed by David Adjaye and opened in 2016, has a corona of thousands of cast aluminium panels with a bronze-coloured coating, lighter and cheaper than real bronze. It is a legitimate choice, but it changes behaviour over time: a coated finish does not develop a patina and may eventually need renewing. By contrast, the Copper Box Arena built for the London 2012 Olympics has a façade of real copper, largely recycled, which is following its own colour evolution.
A lexicon that goes beyond Herzog & de Meuron
The de Young Museum paved the way, over the following twenty years, for a broader vocabulary of metal envelope as an ornamental, changeable surface in contemporary architecture — perforated or punched metal panels with patterns derived from environmental, cultural or geometric data specific to the site, rather than from a generic decorative repertoire applicable to any building. The underlying logic, common to many later projects by other practices working with metal cladding (aluminium discs, expanded metal mesh, perforated panels), remains the one Herzog & de Meuron demonstrated first: metal cladding is no longer necessarily a neutral, uniform surface, but can be a carrier for specific information — a pattern, an image, a datum — that ties that building, irreplaceably, to its own site and its own history.
The same practice has applied this logic to other metals too. At CaixaForum Madrid (2008) Herzog & de Meuron turned an old brick power station into a cultural centre, removing its stone base so that the building seems to float above a covered plaza, and added new upper floors clad in perforated cast-iron panels left to rust. The reddish-brown colour of the cast iron echoes that of the old bricks, while the pattern of holes, similar to the de Young's, lets light filter into the upper rooms and makes the added volume legible but not alien. Here too the metal is not a neutral cladding: it is chosen for the way it ages and for the dialogue it sets up with what existed before. Visitors to Madrid will find it on the Paseo del Prado, next to Patrick Blanc's large vertical garden covering the wall of the neighbouring building: two opposite ways, metal and vegetation, of making a façade change over time. Visitors to Madrid will find it on the Paseo del Prado, next to Patrick Blanc's large vertical garden covering the wall of the neighbouring building: two opposite ways, metal and vegetation, of making a façade change over time.
How a copper façade is built
A contemporary copper façade is almost always a rainscreen: the thin panels are fixed to a substructure with an air gap between the cladding and the insulated wall behind. The cavity lets moisture dry out, reduces summer overheating and allows the panels to expand freely. The critical details are the same as for historic roofs. Copper must not be in direct contact with less noble metals such as aluminium or galvanised steel, because in the presence of moisture these would corrode quickly: the substructure and fixings are therefore copper, stainless steel or separated by insulating elements. Water running over copper also carries small amounts of copper salts and leaves green stains on glass, light stone and render below: drips and water collection must be designed from the outset.
Copper is not the only metal that changes over time, and the comparison helps in choosing. Weathering steel forms a stable, orange-brown rust within a few months, but in the early years it releases oxides that stain nearby surfaces red, and it is unsuitable for constantly damp areas or places near the sea. Titanium zinc develops a matt grey patina, more discreet, and is cheaper than copper. Copper has the longest and most colourful transformation cycle, a lifespan that can exceed a century and a very high recovery value at the end of its life. These are three different ways of accepting that a building ages, and the comparison between the metals is developed in the article on alternatives to copper.