Most of the glass we see in contemporary cities isn't transparent in the sense that Gothic stained glass or a shop window once was: it's reflective. Low-E coatings and solar-control glass, applied to reduce a building's thermal load, have the side effect of turning the facade into a large mirror that reflects the sky, the clouds, the building across the street — showing everything except what's actually happening inside. It's in this context that the work of the Japanese studio SANAA, founded in 1995 by Kazuyo Sejima and Ryue Nishizawa, takes on an almost polemical meaning: an entire career built on the premise that glass should be looked through, not looked at.
The 21st Century Museum of Contemporary Art, Kanazawa
Kanazawa's 21st Century Museum of Contemporary Art (2004) is perhaps the clearest statement of this principle. The building is a perfectly circular disc, with an entirely glazed perimeter and no recognisable main facade: from wherever on the circle you approach, you see straight through the building to the park on the other side, and the entrances are distributed around several sides of the circumference, with none taking precedence over the others. Toyo Ito, in whose office Sejima had trained, described this approach in a 1996 essay as "diagram architecture": a building that translates the diagram of its activities directly into plan and that rejects the traditional hierarchy between front and back, between main entrance and secondary facades, precisely because the transparency of the perimeter glass makes every point on the circle equivalent to every other. A visitor walking around the building always sees, through the glass, the exhibition rooms and the people inside them — not a reflection of themselves or the sky.
Inside the circle, 112.5 metres across, the galleries are not carved out by subdividing one large volume: they are independent boxes, of different sizes and heights, arranged like the blocks of a small city. Between the boxes run corridors that work like streets, and several glazed courtyards bring daylight right into the centre of the plan. Some rooms are lit from above through skylights, others are completely closed. The result is that the perimeter glass does not merely light the building: it connects it. Someone standing in a corridor sees, through two or three layers of glass, the park, a courtyard, another gallery, a visitor on the far side of the building.
The museum also makes a management choice that turns transparency into more than an effect: part of the ring, with a library, workshop rooms, a café and circulation spaces, is free and open to anyone, while only the central galleries require a ticket. The boundary between the public zone and the paid zone is not a wall but a line drawn within the same luminous space. The building opened in October 2004 and in the same year earned SANAA the Golden Lion at the Venice Architecture Biennale. Six years later, in 2010, Sejima and Nishizawa received the Pritzker Prize.
Omotesando: when transparency becomes a veil
It would be wrong to reduce SANAA to an office that simply uses the clearest glass available. In the Dior store on Omotesando, Tokyo (2003), the façade is a double skin: on the outside a surface of flat glass, on the inside panels of translucent acrylic curved like curtains. By day the building reads as a stack of milky white layers; at night, lit from within, it hints at the floors without showing them in detail. It is a filtered transparency, built by pairing a material that lets the gaze through with one that lets only light through.
The same reasoning returns, with different materials, in other projects by the practice. The New Museum in New York (2007) is a stack of offset boxes clad in expanded aluminium mesh, which turns opaque volumes into vibrating, slightly transparent surfaces. The Louvre-Lens (2012) instead uses façades of anodised aluminium that reflect the landscape in a blurred way, without the sharpness of a mirror: the reflection exists, but it is deliberately imprecise, and the building merges with the northern French sky instead of duplicating it. The common thread is not glass as such, but control over how much an envelope should reveal, filter or give back.
The Glass Pavilion at the Toledo Museum of Art
At the Toledo Museum of Art in Ohio, SANAA completes the Glass Pavilion in 2006: a series of rooms with curved walls, made from float glass bent while hot — not flat panels joined at an angle, but continuous, curving surfaces — with a supporting structure reduced to the bare minimum, almost invisible. The building itself houses the museum's glass-working studios: the choice to clad in glass a building dedicated precisely to glassworking is no accident, it's a deliberate short-circuit between content and container. The curved walls, with no sharp corners and no visible mullions, create a sequence of spaces glimpsed one inside the other through multiple layers of glass — an effect of depth and overlap that the flat transparency of an ordinary glazed wall couldn't produce.
The pavilion, opened in 2006, also houses one of the most important glass collections in the United States, and here the most interesting technical choice is not immediately visible. Many of the walls between rooms are double: two curved panes separated by an air gap. Those cavities work as thermal buffer zones. This matters in a building that places climate-controlled galleries next to a workshop with glass-melting furnaces that give off heat continuously: the cavity separates spaces with opposite thermal needs without introducing a single opaque wall. Transparency here is not achieved by giving up performance, but by moving performance into the thickness of the glass.
Transparency versus reflection: why the difference matters
The difference between a building that uses glass to reflect and one that uses it to be seen through isn't just aesthetic: it's a choice about the relationship between the building and the city. The mirrored skyscraper, however elegant, communicates a message of self-sufficiency and distance — it reflects the outside world instead of letting it in, and hides the building's inner life behind a veil that's opaque to sight despite technically being "glass." SANAA's buildings, by contrast, treat transparency as a civic value: they make an institution's inner life visible from outside — whether it's a museum, a university library, a civic centre — turning the facade into a threshold permeable to the gaze rather than a reflective barrier. This requires precise technical choices: extra-low-iron glass to minimise the greenish tint that would compromise the colour neutrality of the view through multiple layers, and solar control achieved not through reflective coatings but through shading, curtains, or simply the building's orientation and geometry — accepting a more demanding energy trade-off rather than sacrifice true transparency.
The glass you need to be truly transparent
Anyone trying to achieve a similar effect on a real project soon discovers that "transparent glass" is not a specification. The first issue is iron content. Ordinary float glass contains enough iron oxide to give it a slight green tint, barely visible when looking straight through a pane but obvious at the edge and, above all, when the view passes through several layers or thick panes. Extra-clear glass, also called low-iron glass, reduces that tint considerably: the light transmission of a 10 mm pane rises from about 86% to about 90%, and colours seen through the glass stay neutral. In a building like Kanazawa, where the eye passes through façade, courtyard and internal wall, the difference between standard and low-iron glass is the difference between seeing the park and seeing it tinted green.
The second issue is curvature. There are two routes. Hot bending brings the pane to around 600 °C and lets it slump onto a mould: it allows tight radii and precise shapes, but every shape requires its own tooling and optical quality must be checked carefully, because tiny distortions become visible over large surfaces. Cold bending, by contrast, elastically bends a flat pane on site by fixing it to a curved frame: it is cheaper, but it only allows gentle curves and leaves a permanent stress in the glass that has to be accounted for in the calculations. The flowing walls of Toledo belong to the first family; many gently undulating commercial façades belong to the second.
The third issue is the behaviour of toughened glass, which is required, or nearly so, in many applications for safety reasons. Toughening leaves stresses in the glass that, under polarised light, such as skylight at certain times of day or light reflected from glossy surfaces, become visible as iridescent patches and bands. On a façade meant to be a neutral veil, those patches are a clear defect. Today it is possible to specify an anisotropy limit measured at the factory, something that was not done until a few years ago. Toughened glass also carries the rare but real risk of spontaneous breakage caused by nickel sulphide inclusions: heat soak testing, covered by the EN 14179 standard, greatly reduces this risk and is worth requiring for large panes in exposed positions.
The price of transparency: heat, glare, birds
A highly transparent envelope lets in a lot of light, but also a lot of heat. The parameter to watch is the solar factor, written g: the share of solar energy that passes through the glass and reaches the interior. Clear, uncoated glass has a high solar factor, and in a building glazed on every side this means substantial cooling loads. For years the only answer was reflective glass, which is exactly the mirror SANAA rejects. Today there are almost neutral selective coatings that let through most visible light and block a significant share of infrared, with limited external reflection. They are not as transparent as bare low-iron glass, but they greatly narrow the gap between the two requirements.
The rest of the work falls to geometry: roof overhangs, plan depth, the orientation of the most exposed façades, internal curtains that can be drawn without changing how the building looks from outside. Then there is glare: a room glazed on several sides receives light from different directions, and working at a screen in such a space can be tiring. Finally there is a problem that design culture has only recently begun to take seriously: birds. Transparent and reflective glass are among the main causes of bird deaths in urban areas; a 2014 study estimated between 365 and 988 million birds killed every year in the United States alone by collisions with buildings. Countermeasures exist: frit patterns or markings spaced a few centimetres apart, which birds read as an obstacle and which are unobtrusive for people. A well-designed transparent building today includes them from the start.
What it leaves the designer
SANAA's lesson is not "use more glass". It is a question to ask before choosing a product: what should people outside see, and what should people inside see? If the answer is the life of the building, then you need low-iron glass, few reflective coatings, joints kept to a minimum and solar control entrusted to geometry and shading. If the answer is light but not detail, laminated glass with an opal interlayer or a double skin as at Omotesando may work better. If instead the building must shield the people working inside from view, total transparency is the wrong choice, however elegant it may be. In every case transparency is a condition to be designed, layer by layer, not a property that glass supplies on its own.