The reduction of the frame from sliding to mere edge (by the possibility of using glass as a structural element) is actually an extremely simple idea. This technical, formal choice was synthesised, very effectively, by the Swiss architect Andrea Bassi and the entrepreneur Eric Joray, who launched the Vitrocsa trade name in Saint-Aubin en Sauges (Switzerland) in 1992. However, knowing that the development of a product is a time-consuming technical process – as we discussed in the previous chapter – may help explain the gap between the launch of minimalist frames and their commercial success. Indeed, the Vitrocsa “3001” series had to wait more than a decade for companies from different countries to show any interest in marketing the system following its presentation at the “Intérieur” Design Fair in Kortrijk (Belgium). The product was of disarming simplicity and elegance and included a series of truly revolutionary elements: a modular roller bearing track that allowed better performance than any previous solution and very easy maintenance and replacement; the extremely slender and sturdy bolt latches; and a structural bonding system that ensures that glass and aluminium are perfectly solidary and geometrically accurate. In addition, the vertical profiles were slimmer than any other system thus far (18 mm thick), which has made it a landmark in sliding frame systems.
In 2003, Jofebar introduced the product in Portugal. Its first application was in the houses of two Jofebar shareholders, the emblematic Houses in Ponte de Lima, designed by Eduardo Souto de Moura. Shortly afterwards, in the Braga Stadium, minimalist windows and doors are again used, associating the product with Souto de Moura’s architecture. The widespread international dissemination of these projects along with other large-scale buildings, such as residential towers and real estate developments with hundreds of houses, form the outline of a market that had never existed for this type of frame. In 2005, Jofebar launched the product in Spain and, two years later, in the United Kingdom and the United States of America, where it supplied and installed Goldbrecht’s first projects (currently Vitrocsa’s US distributor). The market grew very rapidly and, in the ensuing years, reached an unparalleled success, with Jofebar becoming Vitrocsa’s main client, transforming and installing much of its production.
Very initial Vitrocsa doors and windows. House in Switzerland (2000). Arch. Andrea Bassi. Photo: panoramah!®
Vitrocsa modular roller bearing track. Photo: Filipa Coimbra / Jofebar.
Although the product was technically innovative and operated with sliding panes up to 6m2, allowing seamless floor, ceiling and walls integration (which ensures total transparency between the interior and exterior), it was still a compromise solution when it came to watertightness and thermal and acoustic insulation. This was a technical limitation that was hardly compatible with the extremely demanding Swiss market but has found, in the more temperate climate of Portugal and Spain, an environment suitable for the rapid and exponential success of the product.
High-performance glass was now easily available and as of 2007 it was clear that the market for minimalist windows required a much larger industrial capacity so to meet demand. It was imperative that minimalist windows would keep pace with this improvement in glass performance and could overcome their deficiencies, of water leakage, air permeability and energy efficiency.
Other competing companies start to gain ground and build a truly industrial project, the most important one being the Swiss Sky-Frame, and the system is subject to improvement and progressively overcomes its initial limitations, becoming technically more efficient, economically more competitive and more varied in the range offered. This unfolding of events has led Jofebar, without the support of its Swiss counterpart, to forcefully focus on the development of custom-made solutions, almost tailor-made, that allowed them to bridge clear performance gaps. This would eventually lead both companies to progressively move apart from each other.
Taking advantage of its metalwork experience and know-how in doors and windows, Jofebar has continually risked pushing the possibilities of the materials to the limit and simultaneously developing its products in tandem with the creative designs of the architects and each project’s specificities. Very early on, it began the task of adapting the system, being particularly interested in replacing glass with other materials and transforming the system in a process for moving any structural element. As early as 2005, therefore, the profiles of minimalist frames were accommodating different types of shutters in non-scalable solutions, since they were detailed specifically for certain projects. This is the case of the wooden shutters of Rocaford House (Valencia, Spain), to the design of Ramón Esteve, and the geometric meshes with irregular motifs, initially designed by the Israeli designer Itamar Burstein, which served as a basis for applications in various works. In the following year, Jofebar took part in a project which, although not completed, became emblematic for the company: the residential and tourist complex of Porto Senso in Altea (Spain), designed by Jean Nouvel with Ribas & Ribas. The first test with a hidden bottom track has been developed for this project, and built into a system of sliding shutters in a metal structure with stone gabions. This project paved the way for the development of solutions where the sill was invisible – a kind of execution detail long tried by different architects, such as the sliding walls of Alvar Aalto’s Imatra Church (Finland, 1958) and the Capela das Aparições in Fátima (Portugal, 1982) by José Carlos Loureiro. The motorisation of window frames was also developed, in sliding prototypes for Casa Fez (Porto, Portugal, 2006), by Álvaro Leite Siza, and the first large sash window in the House on Bassett Road (London, United Kingdom), by Paul+O Architects, completed in 2008. Note that the first use of minimalistic window frames with a sash opening – without motorisation – had been launched shortly before, with the Vitrocsa sash series, used in the Edifício Transparente (Porto, Portugal, 2006), renovated by Carlos Prata.
Geometric meshes with irregular motifs used as sliding structural elements. Photo: Jofebar.
Totally hidden bottom rail system developed for the metal shutters with stone gabions of Jean Nouvel’s Porto Senso project in Altea, Spain. Photo: Jofebar.
It is only in the post-2008 economic recovery that minimalist windows had become a mainstream product and other players like Keller Minimal Windows have appeared, and that established multinationals such as Schüco, Sapa and Reynaers got genuinely interested in the technology. For Jofebar, the focus on larger glasses was a lever to improve the system. The sliding window frames of Casa do Bom Jesus (Braga, Portugal, 2009), by Topos Atelier, were the first to achieve large-scale glass panes in a vertical layout (2.20 x 5m). Prior to this, a first solution with a double parallel bearing bottom track was developed for the Archaeological Museum of Côa (Portugal, 2007), a design by Camilo Rebelo and Tiago Pimentel. It allowed for glass dimensions up to 15m2, much greater than those then allowed by the rival minimal frame systems, but with glass panels arranged horizontally. In addition, a first “pocket” solution with a balustrade was developed specifically for this work – whereby the window retracts into the wall and gives way to a glass balustrade. It was this project, moreover, and the prototypes developed for it that gave rise to the further development of the panoramah!® brand and products. The new trade name allowed the dimensional and typological range to be improved and an enhanced thermal performance if compared with Vitrocsa – now a competitor – to be achieved. At the time Jofebar decided to take this step, another competitor, Sky-Frame, was already showing significant progress in terms of thermal and air/water performance, so it was necessary to close this gap without compromising the functional and aesthetic competitive advantages of the panoramah!® system.
The PH38 sliding series was launched at Villa E (Geneva, Switzerland, 2008), by Christian Geissbühler. The first pivot door solution was installed in Casa F, in Lugano (2008), designed by Carvalho Araújo, for which a pivoting frame originally developed for the shop window of the Spanish brand Dynamobel was perfected. The PH38 version of the sash window appeared shortly afterwards in the AA/Origami House (Barcelona, Spain, 2009) by Carlos Ferrater and Xavier Martí. Later on, a sash system was developed with the window pane retracting into the floor at Villa La Californie (2012), in the South of France, in a Norman Foster design. In 2010, a triple glazed sliding frame (2.60 x 2.60m) was installed for the first time in Villa B in Geneva (Switzerland), in a P-H Gindre & Associés design. However, the limitation of the PH38 series to a glass thickness of 38mm means, with triple glass, a maximum surface area of 7m2. To overcome this limit, a more robust 54mm series was developed for Villa NHV in Vandoeuvres (Geneva, Switzerland, 2012), a dla designlab-architecture project [pp. 318-323]. The PH54 quickly became the most award-winning minimalist series ever, obtained the stringent Minergie certification, attributed only to windows of the highest energy efficiency.
Totally hidden bottom rail system developed for the metal shutters with stone gabions of Jean Nouvel’s Porto Senso project in Altea, Spain. Photo: Jofebar.
From 2008 onwards, and from an industrial point of view, it also became more frequent for glass processors to be equipped with “Jumbo” tempering furnaces, which allow the tempering of larger glass (6 x 3.21m), somehow meeting the aspirations of many architects. It should be noted that the glass panes used are now increasingly larger than the 6m2 that were part of the initial proposal from Vitrocsa, and it becomes paramount to partner with a preferred glass manufacturer, able to work with standards of quality much more demanding than those that the glass processing norms define. This explains the relationship that Jofebar and the Vidromax/Maxividro group began to establish in 2006, deepening as this glass processor adapts its installed capacity to the needs of its best customer, and culminating in the acquisition of part of Jofebar in 2011. The remaining part was acquired in 2013 by the same shareholder that owns Vidromax/Maxividro and the process of integration and verticalisation of the activity that today has led to panoramah!® being the only manufacturer of minimalist windows in the world that processes and manufactures the main components of its system.
Projects in India – done in close collaboration with long-time partner Durall Systems – have since long explored large frame sizes. Specifically, the house in Juhu Beach (Mumbai, India) with windowpanes of 19m2 and 6m-high – which corresponds to the Jumbo dimension – was, in 2011, the building with the largest sliding windows ever built. In 2012, also in India, 7.2m-high double glazing was installed in another house in central Mumbai. In 2015, the biggest triple-glazed glass pane (3 x 5.5m) was installed in a panoramah!® sliding frame at Villa D in Vaud (Switzerland) by Grégory Garcia. Also in 2015, in Alderbrooke, UK, the 8m height was surpassed, with 26m2 double glass panes (8.2 x 3.2m), a solution that formed the prototype for the PH60 series, launched in January 2017. This series has been designed specifically to meet the requirements of the Migergie-P and Passivhaus standards. Moreover, it achieves larger glass dimensions and higher thermal performances, a requirement that led to the reconfiguration of the profile, altering the positioning of the thermal break section.
It should be noted that the assembly of the Alderbrooke window obliged appropriate machinery to be built for its installation. In situations where they lack greater strength due to glasses’ large size, aluminium profiles are modified and change proportions. There is a whole engineering process that leads to the design of structural components of great slenderness to ensure the desired structural performance. It is easy to understand, therefore, that this qualitative leap requires ever-increasing precision in manufacturing and assembly to ensure perfect operation since large glass panels need to be operated with great lightness and ensure ease of maintenance. This requirement, therefore, rests both on the prior meticulous calculation that has to be made specifically for each window – adjusting the profiles to the forces to which each frame will be subjected – as well as on the accuracy of execution of the structural bonding between glass and aluminium during manufacture and on overcoming the difficulties on each site. In fact, these are the attributes that distinguish companies specialising in engineering, manufacturing and assembly from the multinationals that are developing competitive systems. While the former maintain these processes, the latter have been forced to dumb down the product and consequently eliminate much of the added value that it offers.
Other less conventional forms have also been object for development, such as solutions with curved and oblique geometries. Curved double glazing was first tried in minimalist frames as a fixed element in the Salão de Festas in Cortes (Leiria, Portugal, 2008), by Marini Bragança. Later, the PH Curve product, with curved sliding windows, was launched for Casa Circulo in Begur (Girona, Spain, 2015). This product was also developed with a hidden rail solution, installed for the first time in an estate in Valle Bravo (Mexico, 2016). The triple curved glass was supplied to the Villa AT in Søgne (Norway, 2016) by Sauders Architecture. In the same year, conical double-glazed windows were also installed in a house in Quinta do Lago (Algarve, Portugal). As for the sloped solutions, Jofebar has created a prototype of 8m-high sliding windows, with sliding guides modified to allow them to slide with any inclination. This solution was developed for the project “The Sanctuary” by Zaha Hadid, a house in Uccle (Belgium, 2013) that did not come to fruition. However, the work done gave rise to the skylight or inclined window solution, included in the panoramah!® series PH54 Skylight, ideal for sloping roofs. First installed in a residential building in Stockholm, the product won the Red Dot Design: Best of the Best 2014.
“The Sanctuary”, project of a house with 8 metre-high sliding inclined windows. Uccle, Belgium, 2013. © Zaha Hadid Architects.
Inclined sliding window mockup for Zaha Hadid’s project. This solution later originated PH54 Skylight. Photo: Jofebar.
The relatively recent innovations in glass manufacturing described in the previous chapter are part of a long process of technological and architectural evolution that runs through the entire history of the 20th century.
2.1. The ‘fenêtre en longueur’
The “column-beam-slab” reinforced concrete structure, popularised from the first decades of the century, decomposed the traditional façade wall by replacing it with lightweight, flexible structures. This innovation paved the way for a new transparency between interior and exterior space, consequently leading to a radical change in the design of the frame and in the concept of the opening itself. Moreover, the period coincides with the industrialisation of glassmaking – boosted by the shop window market, which was demanding ever-larger and more transparent glazed surfaces – finally turning it into a standard, affordable product.
At the beginning of the 20th century, the industrialisation of plate glass was based mainly on two methods, rolled plate glass and flat drawn sheet glass. The first method consisted of pressing molten glass with rollers until the desired thickness was obtained. This process easily produced translucent glass but, in order to obtain an acceptable transparent glass, required the faces of the sheets to be ground and polished. The second method, that of flat drawn sheet glass – or the Fourcault process – consisted in the production of a continuous glass ribbon drawn vertically by lifting it upward through cooled tubular rollers from inside the pit. Despite the good fire-finish, this type of glass had greater limitations in size and inevitably presented a wavy or striped surface. The two methods were perfected in the 1920s, consolidating the path of the Modern Movement. In the case of rolled plate glass, Pilkington developed a system in 1925 that guaranteed continuous feeding and finishing. The flat drawn sheet glass technique was perfected by the Libbey-Owens and Pittsburgh processes which, from 1925, allowed for larger manufacturing dimensions and greater thermal and thickness homogeneity.
Casting plate glass (the Bicheroux Process). Published in “The Making of a Sheet of Glass”, transcription of a talk by Major R. M. Weeks, Royal Institution of Great Britain, 1933, p. 20.
Flat drawn sheet glass production (the Fourcault Process). Excerpt from an advertisement for Union des Verreries Mécaniques Belges – Univerbel, La Maison, 1955, vol. 11, n.12. © Union des Verreries Mécaniques Belges
Thus, once the dimensional limit imposed by the lintel was overcome and the plate glass technique was perfected, conditions were created so that the glazed surfaces could extend to the whole extent of the façade, like Le Corbusier’s ‘fenêtre en longueur’. The panoramic window with sliding sashes thus became an icon of Modern architecture:
“Plate glass replaces window panes. The sashes run horizontally, unhampered by the clumsy accessories of the sash windows. They make possible the lengthwise window the source of an architectural motive of great significance.”
The horizontal window, as codified in Les Cinq points d’une architecture nouvelle (conséquence des techniques modernes) (1926-27), was opposed to the traditional porte-fenêtre or fenêtre en hauteur, originating a heated debate with Auguste Perret, from 1923 onwards, on the appropriate form of the modern window and the reorganisation of the visual field. The main objective was to maximise the entrance of natural light , but also to open the landscape: the window became an optical device, a large screen.
By removing the dependency between the openings and the supporting structure, reinforced concrete created the possibility of the façade being entirely constituted by a continuous glass frame: the ‘pan de verre’. This even more complex and radical proposal gave way to glass houses – which embody a different notion of interiority, maximising the visual relationship with the outside world – and the curtain wall. This formal transfiguration of buildings led to the gradual loss of autonomy of the window, transformed into the skin of the façade. Or else, the façade wall was transformed into a window.
2.2. The development of the horizontal sliding window
Modern Movement architects, who took these experiences to the limit, always took advantage of the materials, assembly techniques and new possibilities that the industry offered to architecture. Little wonder, therefore, at the comparison between the metallic profile of an industrialised window and a Bugatti engine on the pages of Vers une Architecture . They accepted and incorporated innovative products as new components of their architecture and, in cases where there was a lack of response from the industrial market, studied and developed new systems with relevant architectural features. This is precisely what happened to the horizontal window, for which Le Corbusier and Pierre Jeanneret engineered and promoted châssis coulissants, sliding frames. In July 1926, they patented a sliding window with an unlimited number of free moving sashes , which was applied at Villa Cook in Boulogne-sur-Seine (France, 1926-27). And they also developed a set of more than twenty technical solutions for sliding windows, among which a frame in anticorodal, an aluminium alloy, manufactured by Ernst Koller with two parallel Saint-Gobain glasses (1928-29). In 1927, the Parisian architects even signed a commercial contract with Saint-Gobain relating their patent. And they were not the only ones developing sliding metal windows, as in the same period, companies like Artaria & Schmidt in Basel, and Wanner in Geneva were also pursuing that path. Years later, many were the architects and companies doing the same.
This fact testifies the close proximity between the architect’s design work and the development of industrial products in the beginning of the century, a time when science was beginning to allow for properties of the frames to be engineered. Swedish architect Sigurd Lewerentz is perhaps the most vivid example of this close collaboration, having spent his life sharing his work with the development and manufacture of metal window frames. The slender steel profiles and ingenious fittings produced by Idesta, the company he founded in 1929, display a rigour and desire for innovation evident in the number of registered patents, and which paradoxically culminated in the use of frameless glass on mural supports in his latest works.
Le Corbusier and Pierre Jeanneret’s double-glazed sliding window in anticorodal, manufactured by Sutter & Koller in Basel, 1928-29. FLC29855. © FLC.
Years later, many architects and companies developed metal horizontal sliding windows. Details for a house at Chipperfield, Hertfordshire, by Maxwell Fry, 1935. Published in Mildred W. White, Working Details I – Domestic, London, The Architectural Press, 1939.
However, horizontal windows also reflect modern architecture’s preference for sliding sashes. Indeed, modern windows’ horizontal shape and lengthwise dimensions matched the reduced thickness of the new walls, imposing a solution in which the opening of the door or window did not subtract useful space in the interior. In addition, because they do not extend beyond the plane of action, sliding solutions are “designed to fill a long-felt want, eliminating the projecting frames of pivoted ventilators, which interfere with shades, screens, etc.” as reported in a 1912 catalogue. An affordable and relatively little used 17th century variant of a type of sash window used in the Netherland and in Yorkshire was therefore recovered and perfected. Sliding doors were an old invention – with examples in ancient Greece and Rome – and quite popular in Great Britain by the end of the 19th century and the beginning of 1900 – especially in salons of Victorian houses –, but its performance in terms of watertightness, acoustics and air permeability were always inferior to those of casement doors and windows (side-hung and top-hung). This fact limited its application towards the exterior, except in industrial buildings and vehicles. The inevitably reduced opening area compared to the traditional fenêtre en hauteur also seems to justify this fact.
Curiously, decades before Le Corbusier, Dr. Karl Turban and the architect Jacques Gros had already felt the need to adopt a new type of window in their Sanatoriums for the cure of tuberculosis from 1902. Folding and sliding doors and windows thus appeared (Fensterkonstruktionsvorschlag), with the particularity of allowing the complete opening of window frames composed of several sashes.
In any case, the longitudinal shape and large dimensions of the Modern Movement lights, supported by the availability of new materials and their progressive improvement – such as bearings, seals and, especially, larger sheets of glass – legitimised the use of the horizontal sliding window as an invariant throughout Europe in the ensuing decades, particularly in the post-war period.
Villa Savoye, 1928-31. Long sliding windows and a large steel-framed glass slider operable by crank. Photo: Marius Gravot, 1930 © FLC.
The window frames developed by Le Corbusier and Pierre Jeanneret consisted of systems with an unlimited number of sashes and could also include a mechanism for operating the window by a crank, as used in the large steel-framed glass slider of Villa Savoye (Poissy, France, 1928-31) to move a huge 4.65 x 3.5m sash, composed of two panes of 2.3 x 3.5m glass. In fact, and additionally, the automation of sliding windows was a fundamental aspect of the Corbusian fenêtre mechanique. This function was already offered for the windows of automobiles, of which Le Corbusier was a great enthusiast. It is not surprising, therefore, his call for a car industry’s contribution to the modernisation of building technology, underlining the possibility of mechanical window opening:
“Que Renault, Peugeot, Citroën, que le Creusot ou l’un des grands métallurgistes organisent l’industrie dans le bâtiment! La fenêtre considérée comme une mécanique. Glissement automatique, herméticité. Nous doter d’une fenêtre mécanique! […] Attention! les fenêtres ne doivent plus ouvrir à battants à l’intérieur des chambres qu’elles encombrent, ou à l’extérieur des façades. Elles doivent glisser latéralement (la première seule peut pivoter). […] La fenêtre est l’élément mécanique-type de la maison. On presse un bouton, ou plus simplement, on tourne une manivelle, et les fenêtres glissent doucement, s’ouvrant ou se refermant…”
Curiously, in his residence at Herqueville in Normandy (France, 1906-39) – where Jofebar intervened in 2014 – Louis Renault himself installed a steel window with two sliding sashes operated mechanically by rack, chain and crank, just like Le Corbusier’s call for action.
Le Corbusier and Pierre Jeanneret’s details of a sliding window with a crank mechanism, manufactured by Société Barriaux in Paris, 1928. FLC29854. © FLC.
Sliding window operable by crank at Louis Renault’s Château d’Herqueville (France) during restoration. Photo: Emanuel Gonçalves / Jofebar.
2.3 Introduction of aluminium in the building industry
This development was accompanied by an increasing use of aluminium in construction. As early as 1865, Jules Verne devised a rocket in this material in “De la Terre à la Lune” (From the Earth to the Moon). However, its use in construction dates back to the late 1920s. It was used in the Empire State Building (New York, 1930-32), the first building with aluminium structural components, the Aluminaire House (1931) by Albert Frey and Lawrence Kocher, the first all-metal American home – an experiment later repeated in the Frey House 1 (Palm Springs, 1940-43) also in aluminium – as well as the celebrated Dymaxion House (1930) by Buckminster Fuller. However, it was its widespread use during World War II in the aviation and military industries that boosted the application of this material. In North America, this process was handled by Alcoa – Aluminum Company of America and Alcan – Aluminum Company of Canada, followed by other major companies that emerged during the post-war period, notably Reynolds and Kaiser. This period, commonly described as “the aluminium industry in search of a market” , is marked by huge developments in aluminium due to its suitability for the most varied industries. With good strength but a much lower weight than steel and excellent durability – if properly protected – aluminium isn’t subject to atmospheric corrosion and has simple maintenance. Its alloys are especially suitable for structures, façades and supporting large panes of glass.
Shōji panels at Takamatsu Castle (Kagawa, Japan). Photo: Fg2, 2005.
2.4. From the Japanese Shoji to the sliding glass doors in Southern California
If the horizontal sliding window was relatively little used in Western architecture up to then, sliding elements are a constant of Traditional Japanese Architecture. In fact, the Japanese timber post-and-beam system provides great freedom for the organisation and relationship of spaces, enhanced by the use of partitions made up of removable sliding panels. As a consequence the various interior compartments easily become a large undifferentiated and fluid space. Their application in the outer perimeter, on the other hand, allows the façade to open promoting a fusion between interior and exterior. The use of sliding panels composed of a wooden structure – shōji and fusuma – whose filling was made of rice paper or cloth – allowing people to see out when open and only to admit light when closed – became therefore very popular. Glass panes only began to be introduced at the end of the 19th century.
Frank Lloyd Wright, deeply influenced by Japanese architecture , became interested in these elements, which he described in his Autobiography In his work he adopted the use of long ribbons of windows, as early as the Prairie Houses in Chicago (1893-1910), since this layout allowed a great relationship between interior spaces and nature outside. This architectural approach, then uncommon in Europe, was especially well received in Southern California because of its particularly temperate weather. In fact, glazing was still a long way from providing a suitable thermal response, so it was mild climate that opened the way to the “picture windows” and “glass walls” of ranch houses.
It was also in California that, from 1919, Wright designed and built a series of houses with courtyards, large glazed surfaces and mitred glass windows. Although Wright did not favour sliding solutions – preferring the coplanarity of casement windows – the same was not true for his Viennese collaborator Rudolph M. Schindler. Indeed, since his very first works, Schindler made an extensive use of sliding glass doors. At Kings Road House in West Hollywood (California, 1921-22), he adopted a single-story structure with a direct relationship with the exterior through sliding patio doors composed of a timber frame filled with glass or canvas.
Sliding doors at Schindler’s residence in West Hollywood (1921-22). Originally the sliding frames were filled with canvas. Above: Photo by Julius Shulman, 1953. Below: Photo by Julius Shulman, 1991. © J. Paul Getty Trust. Getty Research Institute, Los Angeles (2004.R.10).
2.5. The decisive contribution of Richard Neutra
However, it was Richard Neutra, a friend and then rival of Schindler , who was most notable for the intensive use of this type of glass doors, its development and dissemination. Neutra began to introduce sliding doors in the design of various buildings with prefabricated elements from the mid-1930s. In 1935, at Beard House in Altadena (California), an experimental house in many ways, Neutra did what is probably the first domestic application of commercial ball-bearing sliding glass-and-steel doors in the United States. These window/walls are installed in a protruding way on the outside and the sashes, despite their weight, are top-hung rather than being supported from beneath.
All his subsequent work, especially in the post-war period, focused on the elimination of the conventional boundary between interior and exterior, promoting the penetration of the outside within the space of the dwelling. It was also during this period that, with suburban development and the proliferation of single-family housing, the option arose for bigger windows and for a shared life between the interior and the exterior. Contrary to the initial postulates of the International Style – which promoted houses raised from the ground and encouraged the eminently visual role of the openings, through windows that framed the landscape and glass walls that created glass houses – the association of glass with the function of opening (generally sliding) made it possible to emphasise the fluidity of spatial movement between inside and outside.
Technical details of Richard Neutra’s sliding steel-and-glass doors. Published in L’Architecture d’Aujourd’hui, in. 6, Mai-Juin 1946.
Richard Neutra working by Beard House’s large window/walls, Altadena, California, 1935. © College of Environmental Design, California State University Pomona.
In the case of Neutra, the architectural option of extending the flat roof beyond the glass window/wall, significantly changes the perception of glazing and the relationship between interior and exterior space. This wide overhang creates, on the one hand, an area of shade that simultaneously attenuates and extends the boundary. On the other, it reduces the reflection from the glass, increasing the exposure to the exterior while paradoxically improving the visual experience from the inside out, framing the view and reinforcing the sense of interior protection. In the words of Sylvia Lavin, “the glass becomes not transparent but invisible, leaving the house unbounded”.
By the second decade of the century, glazing had already assumed the role of an important architectural element to demonstrate the new possibilities of modern architecture, both through the visual reconfiguration of the window and the structural implications of that choice. Wright, in particular, saw in this solution a way to architecturally transform a box into a free plan. It is though Neutra who for the very first time introduces the possibility of opening the glazed corner, transforming the window/wall from a screen into a device that promotes spatial movement and the complete dilution between interior and exterior. His most powerful and surprising use of glass walls is indubitably the corner formed by two top-hung sliders, which we find for example in the Kaufmann “Desert” House in Palm Springs (California, 1946).
Openable top-hung window walls corner at Richard Neutra’s Kaufmann House in Palm Springs (1946). Photos by Julius Shulman, 1947. © J. Paul Getty Trust. Getty Research Institute, Los Angeles (2004.R.10).
2.6 Glazing and interior/exterior continuity in the Modern Movement
Modern Movement always valued and exploited the power of glass in the definition of architecture, symbol of clarity and transparency. For Le Corbusier, “Glass is the most miraculous means of restoring the law of the sun”. However, glazing has not been always an element of access to the exterior, playing an eminently visually role. Many iconic buildings testify it. The Barcelona Pavilion (1928-29) by Mies van der Rohe employs fixed glass walls, despite the innovative way of revealing its interior. The large sash windows of the living room of Villa Tugendhat (1928-30) slide down to the basement disappearing completely into the flooring, but do not allow access to the garden, located at a lower level. The façade of Pierre Chareau’s Maison de Verre (1928-32) is made of frosted glass brick. And Mies’ Farnsworth House (1945-51), raised from the ground due to the frequent floodings of the Fox River, is equipped with fixed glass walls and awning windows. Even Philip Johnson’s Glass House (1949), though slightly elevated, solely opens to the garden through four casement glass doors. In these cases and others, glazing works primarily as a source of natural light or an element of contemplation of the landscape, but blocks the illusion of physical continuity promised by glass. Even the horizontal sliding windows of Le Corbusier are configured according to the distant skyline and an important part of his technical research is oriented towards the immobility and airtightness of the glazing, as we will analyse later. However, there are also examples to the contrary. The window frames in the chapel at Erik Gunnar Asplund’s Woodland Cemetery Crematorium (1935-40) slide vertically into the floor, opening the chapel completely. And Alvar Aalto, well versed in Japanese architecture, used wooden sliding windows at Villa Mairea (1937-39) and a large horizontal sliding wall so that “the house can be completely opened to the garden”. The Finnish climate, however, meant that it was almost never used. In Neutra’s Kaufmann House (1946) – designed by independent and movable glass walls and in a favourable climate – the extension of the floor and ceiling to the exterior, associated with the operability of the window/wall, promoted a physical and spatial continuity between inside and outside. It was this possibility that led to the flourishing of this type of sliding window, particularly from the 1950s, becoming ubiquitous in the housing of the last decades of the century.
Sliding top-hung window/walls at Richard Neutra’s Kaufmann House in Palm Springs (1946). Photo by Julius Shulman, 1947. © J. Paul Getty Trust. Getty Research Institute, Los Angeles (2004.R.10).
2.7 The industrialisation of metallic sliding window frames
Until the mid-1940s, sliding doors and windows were custom-made. But in the context of the progressive industrialisation of construction products, prefabricated sliding doors in wood and steel began to appear. A significant part of this success was due to companies like Steelbilt and Arcadia. They developed and industrialised metal sliding frames, perfected fittings, used roller bearings that made panels much easier to operate and producing hardware that made the product affordable, convenient, popular and easily available in standard sizes. Such components included steel or aluminium overhead tracks; pendant brackets; steel, fibre, plastic, rubber, brass or bronze bearings; head guides and floor guides; cupped ‘sheaves’ holding bottom-rolling wheels; weather strips for air and water tightness. In addition, these companies also introduced important product innovations, such as the introduction of bottom rollers, which allowed larger and heavier glazing to be moved. This change, which places the mechanism responsible for the opening of the door underneath the pane and not top-hung rails and bearings, allowed the bulk of the frame to be reduced and overloading of the supporting beam or lintel to be avoided. In addition, the lower cost and more efficient performance of the window in terms of air- and water-tightness led to widespread consensus around this solution.
The fact that most sliding door and window companies were based in Southern California led this type of window to become a transversal element for all modern Californian developments of that time and to begin appearing regularly in the publicity on the pages of architecture magazines. Joseph Eichler, one of the most important and influential real estate developers of modern homes in the United States , became one of Arcadia’s main customers and a major driver of this type of frame.
Arcadia sliding glass doors advertisement featuring Eichler Homes. From Arts & Architecture magazine, August 1954 © David Travers. Used with permission.
Steelbilt technical details for top-hung and bottom rollers doorwalls, single pane or thermo-glaze. © Steelbilt Inc. / Louis Danzinger. Courtesy of the Graphic Arts Collection, Rochester Institute of Technology.
2.8 The Case Study House Program
But perhaps more important was the trend created by the Case Study House Program, for the use of large sliding glazing. This experimental programme of modern residential design went on uninterruptedly from 1945 to 1966 and was sponsored by Arts & Architecture, a Los Angeles magazine edited by John Entenza and dedicated to Modernity in all its forms. Out of a total of 36 homes designed, the programme built 26, mostly in the Los Angeles metropolitan area, prototypical housing designed to be economical, efficient, and with the intention of proposing a new lifestyle model for society. The programme sought to open up modernity to new post-war developments, such as the impact of consumer culture, interior design, decoration and fashion, as well as to reach the non-specialist public. Among the architects who participated in the programme were Richard Neutra, Craig Ellwood, Charles Eames, Raphael Soriano, A. Quincy Jones , Ralph Rapson, Eero Saarinen and Pierre Koenig.
Bellevue steel-and-glass sliding doors at Pierre Koenig’s Case Study House No. 22, Los Angeles (1959-60). Glass panes of 3.2×2.7m were close to maximum dimension available by then. Photo by Julius Shulman, 1960. © J. Paul Getty Trust. Getty Research Institute, Los Angeles (2004.R.10).
Steelbilt sliding doors advertisement featuring Case Study House no. 1 by J. R. Davidson. From Arts & Architecture magazine, May 1948 © David Travers. Used with permission.
One of the key aspects of the programme was to stimulate close collaboration between the architect and the manufacturer of construction products, an investigation that proved crucial to its success and economic viability. This industrial and commercial side led to the promotion of certain materials, constructive solutions and commercial patents, such as in the case of the sliding window frames and Thermopane double glass insulating, which we will refer to later. The role of the company and the trade name thus gained unprecedented importance in the design of the architectural object. Advertising was based on the media coverage for houses and architects – something that resonates directly with today’s advertising – and the manufacturers sought to enhance the characteristics of their products on the basis of modern architecture and the modernity that they wanted to be associated with. The level of technical information on window frames was generally sparse, and producers of glass and window systems engaged in defending the dissolution between interior and exterior, transparency, natural lighting, the “exciting beauty of big Picture Windows” and the “distinctively modern appeal” of full-height, slender frames. They used slogans like: “For more cheerful, more interesting rooms ‘Open’ your walls with Glass” , “The Spaciousness of the Outdoors becomes part of the indoor living” , “Transparent walls” , “Bring the outdoors, indoors” , “Fine structural lines express design freedom” , etc. And, in 1947, the Libbey-Owens-Ford Glass Company published a small book entitled The Meaning and Magic of Windows, by Matthew Luckiesh, a lighting specialist, appealing to the sense and necessity of large glazed surfaces for reasons of comfort and health. Alongside this defence, proposals for new insulation and air conditioning systems arose as a way of mitigating the problems created by the new generous expanses of glazed surfaces.
Steelbilt advertisement featuring Case Study House no.1 (J. R. Davidson), no.2 (Wurster, Bernardi and Emmons), no. 3 (Raphael Soriano) and no. 4 (Craig Ellwood). From Arts & Architecture magazine, May 1948 © David Travers. Used with permission.
“Bring the outdoors, indoors… with Steelbilt window walls”; “see it through Steelbilt horizontal sliding window walls”. Advertisements from Arts & Architecture magazine, May and June 1950 © David Travers. Used with permission.
2.9 Industrial aluminium sliding window frames
In the late 1940s in California, Steelbilt, Arcadia and Miller were among the makers of “extra thin, extra strong rolled steel frames” . But with the advent of aluminium window frames in the 1950s, the product truly achieved unparalleled success. The Korean War (1950-1953) led the US government to concentrate heavily on the development of this raw material , and aluminium extrusion offered enormous savings compared to steel, in terms of labour.
The extrusion process is a mechanical method of manufacturing profiles, begun in the 1920s, where the material is forced through a cross section designed for the piece shape. While the hot extrusion of aluminium is done at 300°C-600°C (575°F-1100°F), steel requires much higher temperatures, above 1000°C (1825°F). The great flexibility offered by the extrusion process allowed the development of the curtain wall system – according to the pioneering experiences of Jean Prouvé in the late 1930s, developed in the early 1950s – and the window frame sector in which the material gained prominence through an infinite range of profiles. The surface treatment of the profiles was obtained by anodising – or anodic or electrolytic oxidation – or by powder coating. Anodising was a process also developed during the 1920s, which became extremely popular in the post-WWII period. The system consisted of the application of a protective anodised coating by submersion of the profile in tanks electrified by low voltage direct current, after degreasing and a chemical stripping treatment. This process resulted in a hard, wear-resistant surface with better resistance to corrosion. The surface of the aluminium profiles was then subjected to numerous mechanical and chemical sub-treatments. Among the most popular until 1960 were polished, sand-blasted and satin finish. In addition to the natural colour of aluminium, the anodising process allowed the electrolytic staining of the profiles. However, powder coating – a thermosetting paint process which coats the aluminium with a layer of polyester powder paint by an electrostatic process and subsequent polymerization – offers a wider range of chromatic possibilities.
Advertisement for Miller’s new aluminum framed sliding glass doors with interchangeable glazing mold for single or dual glass. From Arts & Architecture magazine, August 1954 © David Travers. Used with permission.
Advertisements for Panaview featuring Craig Ellwood’s Case Study House No. 17, with 21 aluminium-framed sliding glass doors. From Arts & Architecture magazine, January and March 1956 © David Travers. Used with permission.
On the basis of these possibilities the companies, from 1953 onwards, turned to the production of aluminium sliding windows, and new ones arose, such as Panaview, Ador and Glide. At the same time, important companies emerged in the early 1950s, such as Schüco in Germany (specialising in aluminium after 1970), Alusud in France in 1960, Technal from 1970 onwards, Sapa in Norway in 1963, Reynaers in Belgium in 1965, and many others. It is significant to note that while in other sectors most companies choose to specialise in a specific raw material, in the case of metals we find the same companies selling steel, aluminium and even bronze window frames.
2.10 Thermal insulation of glass, from the ‘mur neutralisant’ to the Thermopane
At the level of the thermal insulation of glass, it was also from the post-WWII period that more efficient performances were found. Until then, the large-scale glazing of the Modern Movement lacked an adequate response from a technical and environmental perspective, being only justified constructively for the magnificent transparency they provided. This choice implied that glass would respond to a series of questions that concerned architects – such as thermal inertia, soundproofing and neutralisation of radiation and the effects of ultraviolet rays – and which were solved in the traditional window by the use of an interior shutter and an exterior blind. During a phase of his career, Le Corbusier deliberately focused on the window as an immovable and totally hermetic element. Pierre Jeanneret once said “une fenêtre est faite pour éclairer, non pour ventiler!” and the pair sought to develop a thermally active glazed façade, a ‘pan de verre’ as a ‘mur neutralisant’. This stabilising solution, consisting of a hermetic double glass membrane, turned the interior environment independent of the outside air and humidity and at a constant temperature of 18ºC (64.4ºF), balancing the thermal gains and losses to the exterior through the mechanically forced introduction of hot or cold air between the two panes. This system consisted in the improvement of a technique used in dwellings with large windows since the end of the 20th century, with the introduction of an electric radiator inside a double frame to combat cooling and condensation on the glass, but brought to a very ambitious level. The research of Le Corbusier and Pierre Jeanneret was technically supported by Gustave Lyon’s research for Saint-Gobain. However, after being poorly tested in the Cité de Refuge de l’Armée du Salut in Paris (1929-339) and in the Centrosoyuz Building in Moscow (1928-33), due to technical and economic constraints, and excessively energy-demanding, the system did not take off. Le Corbusier then embarked on the use of the brise-soleil, an element of solar control and simultaneously of formal composition that became a constant in his architecture in the post-WWII period. But in post-war America, Mies van der Rohe’s glass curtain wall solutions, technically less complex than Le Corbusier’s, were more viable. However, the Corbusian ‘mur neutralisant’ can be regarded as a legitimate predecessor to the ventilated double glass skin façade, used – with natural or mechanised ventilation – since the mid-1980s, as well as other passive or active solutions for environmental control. The system also has a certain resonance with 21st century smart glass (see chapter 4). At that time, the heated glass obtained by the incorporation of an electric element was taking its first steps, being created for the automobile industry in 1931 by the Protes Glass Company and used intensively only during WWII, to avoid the formation of ice on the windscreens of aeroplanes.
However, the challenge of using glass as a neutraliser for the exchanges between interior and exterior was met with the appearance of the first commercial double-layer glass with a cavity for insulation, marketed with great success since 1952. Although the invention dates back to 1865, when the North American Thomas D. Stetson patented a system composed of two panes of glass separated by a rope and joined by tar, modern double glazing was developed in 1930 by C. D. Haven. It was transformed into an industrial product in 1941 by the Libbey-Owens-Ford Glass Company, which registered it under the name of Thermopane and launched it in 1944. The thermal conductivity of the glass and its sound insulation could finally come close to the values of light walls. Heat losses were reduced to 50% compared to single glazing, as well as condensation, thanks to the double transition between cold and hot. In acoustic terms, while single glazing retained up to 20dB, double glazing could reach 40dB. The publicity, widely present in architectural magazines, proposed to “Give your houses transparent insulation” .
The great success of Thermopane – to the extent that it became a generic term for double-glazed units worldwide – led other companies, such as Pittsburgh Plate Glass in the US, Schott in Germany, Pilkington in the UK, and Glaver in Belgium (later Glaverbel and now part of AGC), to rush launching their own products. The system was further adapted to incorporate more layers of glass – triple from the early 1950s; quadruple in the second half of the decade and fivefold in the late 1950s – and to replace the pocket of air with low thermal conductivity gases or a vacuum , in order to increase its performance. In addition to double glazing, other glass solutions with enhanced thermal properties were also developed, such as the Italian Thermolux system, which Le Corbusier referred to as early as 1935. This was a product consisting of the use of a thin layer of translucent fibreglass between two sheets of transparent glass, reducing heat losses, improving acoustic insulation and creating a diffused neutral light.
Advertisings for Libbey-Owers-Ford Glass Co.’s Thermopane. “Thermopane glass utilizes the sun’s rays for both heat and light” creating a “Solar House, intrinsically Californian”. From Arts & Architecture magazine, October 1947 and December 1945 © David Travers. Used with permission.
2.11 The float glass manufacturing process
In addition to the appearance of an insulating glass, the introduction of the float glass manufacturing process during the 1950s marked a major technological change, replacing the previous plate glass industrial processes that could not achieve a perfectly smooth and uniform final product.
The float manufacturing process gives the glass a uniform thickness, and a precise and perfectly transparent planimetry. Although developed at the turn of the century in the United States it was only from 1952 that the British company Pilkington Brothers turned it into a commercial application, patenting the manufacturing process in 1959. This process consists in floating a molten glass plate over a bath of molten tin. The glass is self-levelled through its own weight, flat and uniform, then lifted on to rollers and cut into pieces. It doesn’t require any post-processing finishing and guarantees a high quality product, and thus this manufacturing process successfully spread from 1959 onwards and became the norm in construction corresponding, at the end of the 20th century, to 90% of the flat glass used all around the world.
Pilkington Patent for float glass, 1959.
2.12 The thermal break section and other improvements in the frame
Combined with industrial aluminium sliding tracks, popularised by the same time, we were assisting the apotheosis of this type of solution. Thus, technical and architectural research during the ensuing decades concentrated no longer on the frame, but on the development of curtain wall systems. However, the technological evolution in glass and the sophistication of the technical properties of the frames allowed doors and windows to progressively improve their response from thermal, water tightness and functional points of view.
In fact, the high thermal conductivity of aluminium was still an obvious limitation to wood or even to composite materials, which were then set to become a viable option. The emergence of these alternative materials to wood, in particular polyvinyl chloride (PVC) in the 1940s – first in Germany and then in Britain and the United States – represented an important technological achievement because they did not require regular maintenance and their production process allowed unprecedented economies of scale.
As to aluminium, its thermal response was improved thanks to the high level of precision of its production processes that, by allowing mechanical profiles to be combined through a mechanical crimping process without compromising the water tightness, plastic sealant profiles have been created as a thermal break section, from the late 1960s onwards. Still, it has been especially in most recent decades that the system could be significantly improved thanks to the use of polymers – in particular polyamide connections – that allow the creation of high efficiency thermal barriers in aluminium profiles, retaining internal heat and minimising condensation and energy losses.
In addition, other door and window hardware have been developed and incorporated by specialized companies such as Roto, Sobinco and Gretasch-Unitas. The Lift & Slide system was elaborated from the elevation device patented by Viktor Gretash at the early 1940. Extensively used in railway carriage doors, this system of levers and wheels lifts the panel off the sill, enabling movement of large, heavy doors and windows and ensuring a better seal, with less draught and better soundproofing. Other variants have also been developed, such as the Tilt & Slide hardware – which allows the panes to slide and tilt – and the Parallel Slide – which allows the coplanarity of the sashes when closed.
José Marques da Silva’s sliding “pocket” doors and windows at Casa Allen, Porto, Portugal (1927). Photos: Carlos Machado e Moura © Direcção Regional de Cultura do Norte.
Also, from the late 1960s onwards, “pocket” sliding systems were perfected and popularised, in which the sashes slide into a cavity in the wall, thus allowing them to open to their full width. Although this solution is as old as the sliding door itself – used in Japanese temples and Victorian houses – and has spread widely in interior doors, the difficulties involved in maintaining and replacing hidden parts and hardware, which may involve the opening up of the wall, have limited its adoption in exterior window frames. However, improvements in the development of components, counterframes and cassette structures have led many frame systems, especially in recent decades, to include outdoor pocket solutions. Interestingly, Casa Allen in Porto (1927-1935), by José Marques da Silva, proposed sliding bottom-roller pocket doors and windows composed by three layers: wood-framed glass window, a steel shutter for security and wooden blinds for shade.
2.13 Composite materials
Since the 1980s, due to the oil shock of 1973 and the consequent rise in energy costs, uPVC systems (an unplasticised form of Polyvinyl Chloride) nearly monopolised the window and door market, accounting for almost 65% of the global market. In addition to the qualities already mentioned, these systems ensure optimum thermal performance and therefore become a natural choice for a large majority of projects. However, their cheap finish and structural limitations never allowed this material to replace metal alloys in certain segments of the market, particularly the so-called ‘structural glass’. It was the gradual overcoming of performance deficiencies in these composite systems that allowed the rebalancing between rival technologies. Despite their very strong presence in some countries, the global market share held by uPVC systems is currently 35%, being concentrated in emerging markets and under the pressure of standards that favour the use of recyclable materials with less environmental footprint.
Another composite material that must be referred to is GFRP (Glass Fibre Reinforced Polymer
2.14 From athermic glass to high-performance glass
Regarding glass, the implementation of double glazing was then complemented by yet another series of commercial products during the 1970s and ‘80s. This was the case of athermic glass, namely smoked glass (also known as parsol) which is coloured throughout its mass by the addition of metal oxides, and reflective (or mirrored) glass obtained by the application of a metal surface film on one side. Both came to promote the reduction of light, solar and thermal transmission, consequently achieving a more efficient energy balance by the reduction of solar gains. Strongly fuelled by the energy crisis of the 1970s, the technology developed for reflective glass evolved and gave way to selective energy response glass since the 1980s, geared towards solar and thermal control. Thanks to the composition of the metal layers used, these glass panes allow for the selectiveness of light spectrum to be selected to which the glass is transparent and the degree of transparency against a certain wavelength, depending on the luminosity or temperature desired. Among these, we highlight the selective glass with low emissive effect, called low-E, specially adapted to cold climates. It consists of one or more layers of silver (initially gold) that reflected ultraviolet and infra-red solar radiation, maintaining a comfortable interior temperature while allowing visible light transmission. In winter, it allows the thermal gains produced in the interior to be taken advantage of, reflecting them inwards.
It was only during the 1990s that the magnetronic sputtering technology was mature and ready to enhance the comfort and aesthetic requirements demanded by architects and users. Glass with mono-functional coatings – low emissive or solar control – were already widely used at the time, although there was no glass that juxtaposed both attributes. The development of dynamic coatings has also begun, capable of changing the state of transparency or of being able to capture the solar energy that could be reused. Finally, in 2002, the first layers that overlap a double silver layer could be produced, allowing the production of the first high-performance coatings, that is, with a high selectivity index. [footnote index="86"]86. The selectivity index is the ratio resulting from the division of the light transmission of glass and its solar factor. Despite the unequivocal advantages this brought, so-called soft coatings still had to wait for a few more improvements. On the one hand, there was an enormous probability of oxidation of the layers, which it proved necessary to reduce until its handling was similar to the pyrolytic hard coatings. On the other hand, the very suitability of tempering furnaces to the speed of controlled convective heating glass surfaces proved to be a fundamental development. The use of very efficient glass that could not be tempered was not scalable – the cost of the magnetron deposition equipment made it impossible for it to be made after the tempering process, since it was not economically feasible for glass processors. Once the difficulties of implementing a technology that had enormous benefits has been overcome, there was a very rapid spread of high-performance glass.
2.15 Structural Bonding
The various possible combinations between these types of glass and the use of laminating (which allows different types of glass to be stratified), tempering or heat-strengthening (which give the glass a much higher resistance) constitute the wide range that the market offers architects to work with, both aesthetically and in terms of energy efficiency. Furthermore, since the mid-1980s, development in the field of chemicals has brought the possibility of joining the glass by adhesion thanks to structural silicones and modified silyl-based polymers. We therefore see the appearance of curtain walls with the external face completely composed of glass, without any external fixing elements, such as metal fittings or neoprene gaskets. Concerning frames, glass-aluminium bonding has improved the thermal response and opened the way for profile size reduction, while glass-glass bonding has dispensed with vertical profiles at fixed angles and in other special architectural situations, dematerialising the joint without creating thermally weak spots.