Crystal case: the Rhineland Regional Museum in Bonn is a model of its kind in both urban and cultural terms.

The Rhineland Regional Museum started as long ago as the 1820s, and has accumulated a distinguished collection ranging from the 40000 year old skeleton of Neanderthal man to contemporary artworks. The original purpose-made museum building was set up in the 1890s on a site stretching north-south between two streets just south of the main railway station. An extension was added in 1909.

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During the Second World War, the main building was bombed, leaving the 1909 extension at the north end intact, and a boxy new museum building in sub-Mies vocabulary was made to replace it in 1967. By the late 1990s, this had become technically unsatisfactory, submitting its valuable contents to unacceptable variations in temperature and humidity, quite apart from the sheer unattractiveness of the uninspired and ageing fabric. At first, the museum authorities intended to rework and update the thirty year old structure, but this promised to be an expensive task, hardly less than renewing the whole. Having just lost the status of capital. Bonn was being handed generous cultural money, so a new building to the highest technical standards was possible. A competition was held and won by Architektengruppe Stuttgart, who decided to make a new block to the south, its main entrance fronting a shallow square, while preserving and internally converting the 1909 extension to the north. Rather than working directly with the exhibits, the architects were asked to produce a range of exhibition rooms flexible in character, allowing for changes of interpretation. The exhibition design was placed in other hands as a separate operation.

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Most remarkable in the new museum is the layered treatment of the south facade, which flips between transparent and reflective as your viewpoint changes. A single-glazed outer screen-wall serving as rainscreen and climatic buffer stands some 4m forward of the timber inner facade surmounted by a completely glazed roof. This glass case is not just an empty symbol for a museum, but also a transition space. It provides a protected outdoor area for the cafe enjoying the afternoon sun, and it also houses a couple of exhibits which belong outside but require protection from frost and acid rain: a Roman arcade and a Gothic cross. The naked wooden inner facade behind is presented in contrast like a series of display cases or open drawers shallowly angled to project from the facade plane. The twist in its components makes the facade more three-dimensional, brings down the scale, and exaggerates the degree of openness. In fact it is largely solid, though there are narrow windows between the boxes framing views to southeast. The timber treatment continues inside, its texture enhanced by the sidelight, so the visitor easily makes the connection.

The organization of the new museum is commendably clear and makes a virtue of the marriage of the buildings, for nowhere does it seem a strain. The ground floor central entrance introduces the main axis along which the complex is deployed. It leads on through a glass wall to a visually open but fully controlled layer housing ticket hall and cafe, and near-central stairs in a large well lead down to cloakrooms.

Entrance to the museum involves passage through another glass wall which brings one to a well with stairs to one side and numerous other flights and ramps passing overhead. This atrium is the heart of the building, mediating between the shallower floor heights of the new part and the more generous old ones in the 1909 part. It is a clear reference point for reorientation and is spatially the most interesting volume, but so little daylight is admitted by the clerestories of the rooflight that it feels completely internal, dominated by electric illumination. From the well you can move into exhibition rooms in either direction. The new building offers a wide central hall, a narrower room on each side, then a yet narrower one, a linear gallery with daylight only at the ends. But if you continue instead on axis into the old building, you discover a suite of taller rooms: a single central hall with six columns and gallery, and traditional side-lit rooms at each level on each flank. Set under an updated version of the original glass lantern with an inner translucent ceiling in barrel-vaulted form, the central hall is bathed in daylight, bringing the whole museum to an appropriate climax. Somewhat church-like, this space has appropriately been used to accommodate religious and monumental objects of stone, such as Roman funerary inscriptions and Romanesque capitals from lost churches.

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In presenting the collection, the curators decided against a traditional chronological progression, reorganizing the material around nine themes including Periods, Power, From wilderness to city, From gods to God, Secrets of discovery, and Rhineland and the World. It is like the themed arrangement of Tate Modern in London, and has similarly brought both praise and criticism. It seemed to me to work well, and has at least the advantage of demonstrating that classification is neither fixed nor neutral, and it also gives the curators a more visibly active role. That it may all be reorganized by fresh curators with a new world-view seems no bad thing, and is a good argument for the kind of general-purpose loose-fit attitude taken by the architects. The exhibition designers have added a certain amount of deliberate scenesetting, but the building takes it quite well. Fortunately, the whole treatment is more sober than most recent museums, and the signage relatively restrained. It evokes some atmosphere of reverence and one can enjoy the objects without the intervention of the shouting gimmicks and interactive gameshows that spoil many recent museums in the UK. Reconstruction models are generally helpful, and a computer simulation of the changing local landscape over millennia is really engaging. The decision to commission life-size wooden sculptures of local heroes from Agrippina--after whom the Romans named their first settlement Cologne, (Colonia Agrippina)--to Max Ernst shaping one of his sculptures, has also paid off.

Since the initial reason for changing the building was technical, the new one is environmentally well controlled, with a high thermal mass due to its concrete construction, temperature control through heat exchange using pipes embedded in floors and walls, and humidity control through air-conditioning kept at a moderate level. It was one of the architects' stated aims to avoid showing off the technical apparatus, and the construction too is rendered rather basic and pure, with deceptively simple detailing that could even be called overprecise. My greatest disappointment was the general gloom inside the building and the suppression of relations with the outside world.

The curators felt that history does not stop but goes on around us, so they actively wanted visitors to be aware of the city beyond. They encouraged the architects to provide regular views out, but the phobia against daylight has for the most part won the day, for even key viewing windows are toned down by screens, and side-lighting in old rooms is filtered by solid if translucent blinds. Strong light can of course damage many kinds of materials, and museum objects are meant to last for ever, so the caution of curators is understandable. At the same time, exhibition designers most easily achieve control by applying artificial lamps of their own, and have made this their automatic habit. But many of us prefer to see objects by daylight if at all possible, and its variability--the very thing that puts curators and exhibition designers off--is also its virtue. It changes at different times of day and year, and helps locate us in time. It is possible to calculate an object's speed of destruction in variable light and put it in darkness when it is not being seen. It is also possible to filter and control daylight and sunlight so that they are not excessive. But this requires close collaboration between architects, designers and curators rather than the assumption that exhibition areas are essentially black boxes.

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