The possibilities of glass as a load-bearing element, associated with structural bonding with new materials, brought about a true architectural revolution, allowing the construction of structures and pavilions entirely made of glass. This possibility of radical transparency seemed, finally, to achieve the desires of transparency and immateriality advocated by the Modern Movement. However, the critique of modernity brought about an important paradigm shift in the architectural perception of the window and the glass façade at both the formal and functional levels.

4.1 Between the formal versatility of glass and the reconsideration of the brise-soleil

As Juhani Pallasmaa points out in the text “Filters – Catching the Eye” [pp. 275-277] (88)88.  Regarding the way a vision-centred design and the consideration of the window exclusively as a source of light and of panorama have altered the very ontology of Modernity, see also: Juhani Pallasmaa, The Eyes of the Skin: Architecture and the Senses, Chichester, John Wiley & Sons, 2005, pp. 26-29., Postmodernity called into question positivism and the dogma of transparency, opening the way to the richer and more complex formal explorations of buildings. The evolution of this perception manifested itself progressively throughout the 20th century and and can be perceived in the sequence of essays compiled in The Light Construction Reader (2002) (89)89. The book brings together 28 essays that explore themes and issues emanating from the 1995 “Light Construction” exhibition at the Museum of Modern Art in New York. Todd Gannon (ed.), The Light Construction Reader, New York, Monacelli Press, 2002.: between Paul Scheerbart’s “Glass Architecture” manifesto (1914), the first questions raised in the influential “Transparency: Literal and Phenomenal” (1964/1971) by Colin Rowe and Robert Slutzky and the disenchanted visions of Antony Vidler and Gianni Vattimo (1992). Today, we could add to the list some other texts, such as Treacherous Transparencies (2016) by Jacques Herzog and Pierre DeMeuron.

These theoretical concerns found a practical response at the turn of the century, mainly through two processes. On the one hand, glass acquired a new formal versatility, being able to absorb a series of treatments that allow it to change its degree of transparency and colour, and to be the object of graphic printing, a process that the work of Herzog & DeMeuron introduced in an iconic and paradigmatic way. (1)90. Among the first examples, we consider the Ricola-Europe SA Product and Storage Building (Mulhouse, France, 1998), whose façade is composed of printed polycarbonate panels; the Institute for Hospital Pharmaceuticals (Basel, Switzerland, 1998) and the Information, Communications and Media Centre of IKMZ BTU Cottbus (Cottbus, Germany, 2001). On the other hand, buildings once again incorporated new exterior layers or “skins”, recovering a sense of materiality and textural variety lost with the extension of the glass to the entire façade. The works that Jofebar has carried out and which are partly illustrated in this book – including doorways, louvres and façades in different translucent materials – testify to the material and formal wealth of these elements in contemporary architecture, as well as the graphic possibilities offered by the evolution of digital printing processes. (1)91. Today, the development of digital printing allows us to use glass as an element capable of filtering, blocking and differentiating. This effect is enhanced by dual point technology, with overlapping impressions, which allow the glass two different perceptions, one on the outside and the other on the inside.

4.2 From the diaphragm window to responsive glazing

Parallel to these eminently formal achievements, there have also been important technological advances in the technical response of glass. Glazing has become an active and changeable element, capable of responding dynamically to changing environmental conditions, away from the inert and hermetic lamina hitherto advocated. Finally, Le Corbusier’s fenêtre diaphragmée, was achievable. He had argued as early as 1935 that “the glass wall can be, and should be, controlled with adjustable shutters inside the glass envelope”. (92)92. Le Corbusier, “Glass, the Fundamental Material of Modern Architecture”, West 86th: A Journal of Decorative Arts, Design History, and Material Culture, Vol. 19, No.2 (Fall-Winter 2012), p. 304.

But while in Jean Nouvel’s Institut du Monde Arabe (Paris, 1987) all the mechanics of the façade are exposed, the contemporary trend is precisely the opposite, making this entire process imperceptible and assimilated by the glass itself. Chambers between glasses have been explored for the introduction of filter elements – such as internalised meshes or in-built shading systems – and currently it is the glass itself that provides responsive glazing. The development of smart glass – switchable and dimmable glazing – began in the 1990s, but the first applications took place mainly over the last decade and are expected to reach a larger market share in the near future. (93)93. http://www.navigantresearch.com/newsroom/smart-glass-will-reach-nearly-890-million-in-annual-market-value-by-2022 Among the various types, electrochromatic glazing – only suitable for tempered glass – consists of a very thin coating (less than 1 micron) formed by ceramic-based layers and electrically switchable metal oxides. With the application of low voltage current, the movement of ions (lithium or hydrogen) changes the transparency of the surface and consequently allows privacy control and limits the transmission of heat and sunlight. It is therefore also known as privacy glass. The response of the glass varies by controlling the intensity of the applied electrical signal, and can be programmed and connected to air conditioning and lighting systems through sensors. The glass thus becomes a device that allows the user to regulate the thermal gains and the luminosity, depending on the external environment and the needs of the interior. Alternatively, thermochromatic and photochromatic glasses are dynamic systems that do not require connections and electrical components. The properties of their coatings change due to exposure to heat (thermocromatic) or direct solar radiation (photochromatic), reducing the transparency of the glass. These technologies can be integrated with other solutions, such as low-E glasses, which do, however, have lesser versatility than electrochromatic glasses. Among Jofebar’s works with responsive glazing, Contour House (Peak District, United Kingdom, 2016) by Sanei Hopkins Architects uses glass from Sage Electrochromics, a subsidiary of Saint Gobain.

 

 

 

 

4.3 Glass as energy generator and information transmitter

In addition to solutions that reduce the energy consumption of a building through the behaviour of glass coatings, the recent development of thin film photovoltaics allows the glass to be transformed into a solar energy generator through coatings composed of transparent cells, imperceptible to the human eye. Another possibility offered by metal coatings is the use of heated glass. Revisiting a technology initially developed for automotive glass, with the use of electric elements (already mentioned in chapter 2.9), this system was applied to conductive metal coatings invisible to the human eye from the end of the 1950s and applied to architecture to combat the formation of ice and condensation. While its applications to normal glass presented great heat losses, making it impossible to use as a source of heat for the interior of the building, with the advent of low-E coatings since the end of the 1980s – energy efficient and low emissive – the scenario changed. The glass can thus function as a space heater – Komfort Glass – a possibility that further enhances its capacity as a dynamic element of mediation between the interior and the exterior, able to regulate thermal comfort and even replace other forms of heating. (94)94. In spite of their feasibility, large panes of heated glass are, however, still relatively unused, because of their high price. Among the various uses of heated glass implemented by Jofebar in minimalist frames are, for example, Smådalarö Houses (Sweden, 2014) and the Turkish Airlines Hotel in Ortaköy (Istanbul, Turkey, 2015-17), the latter designed by David Chipperfield. For these projects, Jofebar and its partner Architectural Solutions developed a solution of electric power supply through dry contact, which makes the use of electrical components in sliding windows much less complex and particularly safe.

In addition to all these technological advances, glass has even become capable of conveying digitized information. While, in principle, the architectural applications of media communication technologies in contemporary society are more suitable to interior spaces, numerous functions can also be applied to windows. These, therefore, cease to be the optical screen of the Modern Movement to become a computer screen, tactile and interactive, capable of opening up a virtual world. These and other technological advances in glass are described in the text “Glass and Windows: the last frontier for smart and functional materials integration” [pp. 145-150] by Senentxu Lanceros-Méndez.

4.4 The frame: the paths of perfect seal and new materials

The main challenge with regard to frames has been to improve their response to ever higher technical requirements. Most curtain wall systems recommend the non-operability of glazing, separating its functions and relegating ventilation entirely to mechanical systems – revisiting some of the Corbusian mur neutralisant and respiration exacte principles. In this sense, as an element in the interaction between mechanical and passive energy management systems, the glazing sometimes sacrifices the feeling of connection to the outside, refocusing its role on the mere visual relationship and the source of natural light (possibly controlled). However, the glass door or window are elements that, in addition to the view, should offer the possibility of an effective connection with the outside, through air circulation, sound and spatial continuity. (95)95. With regard to airtight façades and the effect of the operability of windows on the feeling of comfort, see the interview: “Opening the un-openable window: Matthias Schuler” in Rem Koolhaas, AMO, Harvard Graduate School of Design, James Westcott (ed.), Elements, Marsilio, 2014, pp. 721-729.

In order to meet the increasing requirements of greater efficiency, research has focused on improving the performance of thermal and acoustic insulation, and air/water tightness, namely through the incorporation of more composite components. The life cycle of these solutions is much greater today, mostly due to a better mastery of manufacturing techniques, but also to the introduction of components that are more resistant to corrosion or even to the improvement of surface treatments. In particular, polyvinylidene fluoride (PVDF) paints, capable of maintaining their chromatic and protective properties for 20 years, have offered metal one of the advantages of uPVC frame systems: longevity without the need for regular maintenance. There is also notable progress in the field of engineered woods, which allow minimalist systems the possibility of using this raw material, with the same guarantees and benefits as metallic systems. We also highlight the efforts of some brands, such as AIR-LUX, in developing frame solutions with pneumatic sealing systems that offer a significant improvement in acoustic and air/water performance. Despite its complexity and price – which prevents its scalability of the solution – the solution points in the right direction, constituting an effort that is worthy of note. Like the lifting system hardware introduced in the 1960s, these innovations often correspond to intermediate – and necessary – steps in the development of more definitive solutions.

4.5 Between the suspension of gravity and invisibility

In addition to responding to ever more demanding technical requirements, the frame has the added challenge of ensuring a very easy and efficient operation despite the progressive and constant increase in the dimensions of the sashes and the weight of the glazing. In this sense, gravity-defying systems have been explored,eliminating the high wear shown on rolling systems subject to heavy loads and the consequently reducing their need for maintenance. Together with the wish for maximum transparency, the suspension of gravity is, moreover, the other great architectural axiom evoked by Eduardo Souto de Moura in ”Let us go back to the past; it will be a step forward” [pp. 156-166].

But if transparency – as we have seen – is now perceived more critically than it was in the past, the architectural desire for maximum formal purity that has always been associated with it remains in full swing. In fact, it is pervasive to all the achievements we have analysed and it is summarised, for better or worse, in the term ‘minimalist’. Although many of the changes made to frames and windows are mere exercises of redesign and formal mannerisms, there is a constant and guiding logic that consists in hiding the elements that today are intuitively perceived and will be obsolete tomorrow. On the contrary, the elements that have already reached their developmental climax are assumed and revealed. In this balance, glass has incorporated all mechanics imperceptibly and the accoutrements and engineering tend increasingly to be hidden in the profile. One of the most important and sophisticated elements of the building thus tends to become invisible.

 

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. (6)6. For an analysis of the evolution of the curtain wall during the 20th century, we refer to the following texts, to which the present text is indebted: Chapter 3 of Iñaki Abalos, Juan Herreros, Tower and Office: From Modernist Theory to Contemporary Practice, Cambridge MA: MIT Press, 2003 (original version: Iñaki Abalos, Juan Herreros, Tecnica y Arquitectura en la Ciudad Contemporanea,  1950-1990, Madrid: Nerea, 1992), and from Chapter I of Scott Murray, Contemporary Curtain Wall Architecture, New York: Princeton Architectural Press, 2009.

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 (7)7. Flat glass process was patented in 1848 by the British engineer Henry Bessemer. For the first time, a continuous ribbon of plate glass was produced, formed between rollers but it was not a commercial success. 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. (8)8. It could be poured on a metal surface or directly on two rollers. This second technique, the 1920s Bicheroux process, allowed better control. 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 (9)9. This process was invented in 1906 by the Belgian engineer Émile Fourcault. Independently, Irving Colburn developed a similar technique in the United States of America at around the same time.– 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. (10)10. After the invention and widespread diffusion of the float process and despite its inferior quality, flat drawn sheet glass continued to be used during the post-WWII period until the mid-1960s.

 

 

 

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.” (11)11. Le Corbusier, “Twentieth-century Building and Twentieth-century Living”,  The Studio Year Book on Decorative Art, London, 1930.

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. (12)12. See in this regard: Le Corbusier, Petite Contribution à l’étude d’une fenêtre moderne, in Almanach d’Architecture Moderne, “Collection de l’Esprit Nouveau”, Paris: Éditions G. Crès, 1925, pp. 94-101; Bruno Reichlin: “The Pros and Cons of Horizontal Windows: The Perret-Le Corbusier Controversy”, Daidalos 13, 15 September 1984, pp. 65-78; Beatriz Colomina: Privacy and Publicity: Modern Architecture as Mass Media, Cambridge: MIT Press, 1994. The main objective was to maximise the entrance of natural light (13)13. “Through the use of the horizontal window reinforced concrete suddenly provides the possibility of maximum illumination.” Le Corbusier/Pierre Jeanneret, Five points towards a new architecture 1926. “L’architecture c’est des planchers éclairés.” Le Corbusier, Précisions sur un état présent de l’architecture et de l’urbanisme, Paris, Éditions G. Crès, 1930, p. 53. For Le Corbusier, natural lighting was a preponderant factor, writing as early as 1920, that “architecture is the masterful, correct and magnificent play of volumes brought together in light.” (“Trois rappels à Mm. les architectes,” in L’Esprit Nouveau, no. 1, October 1920, p. 92; later included in Vers une Architecture, Paris, G. Crés et C.ie, 1923, p. 16)., but also to open the landscape: (14)14. “Le paysage entre tout entier dans votre chambre.” François De Pierrelefeu, Le Corbusier: La Maison des hommes, Paris, Éditions Plon, 1942, p. 69. 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. (15)15. Suggested in the proposals for skyscrapers in Berlin by Mies van der Rohe (1919 and 1921) and the cruciform towers of Le Corbusier (1920-22), it was used by Gropius on the façade of the Bauhaus in Dessau (1926) and since then adopted mainly in office designs. 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. (16)16. The anti-window character – or the window as “absence of walls” – of the Le Corbusier and Mies projects in the 1920s and ‘30s, serves as a counterpoint to Venturi’s nostalgic essay on windows. Robert Venturi, “Windows–c. ‘65” in Iconography and Electronics upon a Generic Architecture: A View from the Drafting Room, Cambridge, MIT Press, 1996, pp. 255-258.

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 (17)17. Le Corbusier, “Architecture ou Révolution”, in Vers une Architecture, Paris, G. Crés et C.ie, 1923, p. 238.. 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 (18)18.  Patent FR619254 (A) – 1927-03-30, “Châssis de fenêtre à coulissement horizontal”., 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. (19)19.  Artaria & Schmidt’s were top-hung to avoid visible wearing off of the inferior tracks, while the windows patented by Le Corbusier and Pierre Jeanneret were bottom rollers. Wanner’s windows were also bottom rollers and used ball-bearings designed by the Parisian architects. Arthur Ruëgg, “Châssis Coulissants, 1931”, in Le Corbusier Plans, Paris, CodexImages International, Fondation Le Corbusier, 2005. 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, (20)20. In 1929, Lewerentz registered Idesta, a trade name for the development of construction details. In 1930, he founded the construction company BLOKK and, in 1933, IDESTA Inc., which he kept until 1956, when he passed it on to his son Carl. In 1940, he bought a factory, to have complete control over the various stages of production. His research into innovations for doors, windows and glass partitions led to the registration of numerous patents. See: Janne Ahlin, Sigurd Lewerentz architect 1885-1975, Massachussets, MIT Press, 1987. 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.

 

 

 

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. (21)21. This reference is made in an American catalogue as early as 1912, which also mentioned that“Horizontal Sliding Sash are particularly adapted for use in sidewalls and monitors of rolling mills”. United Steel Sash, Trussed Concrete Steel Co., Detroit, 1912, p. 41. 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. (22)22. Sliding windows in wood have mainly been used in rural constructions (cottages). Maurice Wilmore Barney identified the first example at Moss Farm, near Doncaster (UK), circa 1705. M. W. Barney, The English Farm House and Cottage, London, 1961, pp. 263-64,  quoted in M. Tutton, E. Hirst (eds.), Windows: History, Repair and Conservation, London and New York, Routledge, 2007, p. 56. However, it is debatable whether its origin would have been in the Netherlands or  the UK, not least because this type of frame is also known  as the Yorkshire sash or Yorkshire slider, presumably due to its widespread use in that county’s vernacular architecture during the  18th and 19th centuries. See in this regard: Lisa Jardine, Going Dutch: How England Plundered Holland’s Glory, Harper Press, 2008. Sliding doors were an old invention – with examples in ancient Greece and Rome (23)23. Roman sliding door tracks can be seen at Pompeii, testifying the presence of sliding systems in the 1^st century AD. – and quite popular in Great Britain by the end of the 19th century and the beginning of 1900 (24)24. It is interesting to note that, in the last decades of the 19th century, there were innumerable patents related to fittings and systems for sliding windows, which points to a growing interest in this type of window. See, for example: GB189401724A – “Improvements in or relating to Sliding Windows, Skylights, Doors, and the like”, by Peter Shippobottam in 1894. – 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. (25)25. Several authors point out that the ideas developed for the architecture of the Sanatoriums – focusing on generous glazing for natural lighting and large balconies for outdoor living – were incorporated by the architects of the Modern Movement, namely Le Corbusier and Pierre Jeanneret at the end of the 1920s. See: Quintus Miller, “Das Sanatorium Schatzalp. Ein Beispiel zwischen Klassizismus und englischer Wohnlichkeit”, Archithese, no. 2, 1988, pp. 50-56 and Margaret Campbell, “Strange Bedfellows: Modernism and Tuberculosis”, in Giovanna Borasi and Mirko Zardini (eds.), Imperfect Health – The Medicalization of Architecture, CCA/Lars Müller Publishers, 2012, pp. 133-151. Folding and sliding doors and windows thus appeared (Fensterkonstruktionsvorschlag), with the particularity of allowing the complete opening of window frames composed of several sashes. (26)26. Authorship of this type of window is credited to Turban and Gros in: A. Corboz and G. Mörsch, Espoir: Sanatorien, PhD thesis, ETH Zürich, 1984, p. 421, quoted in Margaret Campbell, “Strange Bedfellows: Modernism and Tuberculosis”, in Giovanna Borasi and Mirko Zardini (eds.), Imperfect Health – The Medicalization of Architecture, CCA/Lars Müller Publishers, 2012, p. 144. A more developed version was patented in 1923 (US1559544 A) by Charles Bock for the Andrew Hoffman Mfg. Co.

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. (27)27. An interesting compilation  of technical details of sliding doors and windows for buildings, industrial warehouses and vehicles can be found in: Adolf Schneck, Fenster aus Holz und Metall, Stuttgart, J. Hoffmann, 1932; Adolf Schneck, Türen aus Holz und Metall, Stuttgart, J. Hoffmann, 1933.

 

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. (28)28. Glass was by then typically limited to US Standard of 100 in x 144 in, 2540 mm x 3658 mm. 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. (29)29. Le Corbusier directly honoured the factory of André Citröen – for the mass production of vehicles (1919) – by attributing the name “Maison Citrohan” (1920) to his prototype for standardised houses. He also collaborated with brothers Gabriel and Charles Voisin, producers of automobiles and airplanes, for whom he designed the door handles of the first vehicles. In addition, Le Corbusier’s friend, Gabriel Voisin, also financed his “Voisin Plan” (1922-25), an urban-oriented study for Paris based on the automobile. Le Corbusier later designed the project for a Voiture Minimum (1936), a small, economical car, precursor of the Volkswagen Beetle and the Citröen 2CV. See in this regard: Antonio Amado, Voiture Minimum: Le Corbusier and the Automobile, MIT Press, 2011. 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…” (30)30. Le Corbusier: “Appel aux Industriels,” in Almanach d’Architecture Moderne, “Collection de l’Esprit Nouveau”, Paris, Éditions Crès et C.ie, 1925, pp. 102-3 (the italics are L-C’s).

Curiously, in his residence at Herqueville in Normandy (France, 1906-39) – where Jofebar intervened in 2014 (31)31. Jofebar installed a panoramah!® 38 frame with three sliding sashes in the service zone of Chateau d’Herqueville. The sliding window with the crank, located in the main house, was recovered and kept in place since it is protected as an element of heritage. – 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.

 

 

 

 

 

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” (32)32. M. Bowley, Innovations in Building Materials; an Economic Study (1960), p. 308; quoted in James Ashby: The Aluminium Legacy: the History of the Metal and its Role in Architecture, Construction History Vol. 15, 1999, p. 85., 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 (33)33. Even with less tensile strength and hardness than steel (Young’s aluminium modulus is 70,000 N/mm2 versus the 210,000 N/mm2 of steel), aluminium offers an incomparably lower weight (2,700 kg/m3 versus 7,800 kg/m3). Stephanie Van de Voode, Inge Bertels, Ine Wouters, Post-war building materials in Housing in Brussels 1945-1975, Vrije Universiteit Brusel, 2015, p. 266. 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.

 

 

 

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 (34)34. Shōji are translucent panels of sliding or, later, folding screens. Fusuma are opaque sliding panels, sometimes decorated with paintings. – 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. (35)35. See on this subject “Absorbing Modernity: Japan’s Glassmaking Tradition” in Rem Koolhaas, AMO, Harvard Graduate School of Design, James Westcott (ed.), Elements, Marsilio, 2014, pp. 682-683.

Frank Lloyd Wright, deeply influenced by Japanese architecture (36)36. Frank Lloyd Wright had contact with Japanese architecture at the age of 26, through the Pavilion Ho-o-den (Phoenix) Pavilion at the World’s Fair Exposition of 1893 in Chicago. He later visited Japan in 1905, became a collector and dealer of Japanese art and built the Imperial Hotel in Tokyo (1919-23). All these encounters led him to revolutionise domestic architecture according to Japanese principles, not only in the adoption of naturalistic motifs and in the use of screens, but also in the organisation of open plan interior spaces and, particularly, in the promotion of an intense internal-external relationship in his architecture., became interested in these elements, which he described in his Autobiography (37)37. “The sliding paper shoji, or outside screens that serve in place of windows and enclose the interior room spaces (they are actually the outside walls), all slide back into a recess in the walls. They too are removable.” Frank Lloyd Wright, An Autobiography, Pomegranate Communications, p. 196 (first published in 1943 by Duel, Sloan and Pearce). 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. (38)38. These elements also merged  with public opinion on the easy-going lifestyle associated with the Southwest and the West Coast. Clifford E. Clark Jr., “Ranch-House Suburbia: Ideals and Realities”, in Lary May (ed.), Recasting America: Culture and Politics in the Age of Cold War, The University of Chicago Press, 1989, p. 174.

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. (39)39. In Schindler’s response to the benign climate of California, he was inclined to ignore the environmental differences between interior and exterior, often neglecting the effects of heat transfer, thermal expansion, and water migration. He abandoned the idea of walls and windows as membranes to keep heat in or out as well as the traditional details for doing so. Edward R. Ford, The Details of Modern Architecture, Volume 1, MIT Press, 1990, pp. 293-295. 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.

 

   

 

2.5. The decisive contribution of Richard Neutra

However, it was Richard Neutra, a friend and then rival of Schindler (40)40. Like Schindler, Neutra was a native of Vienna who emigrated to California in the 1920s. For a brief period, he was also a collaborator of Frank Lloyd Wright., 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. (41)41. Among the built work, see for example the Corona Avenue Experimental Garden School (Bell, California, 1935). 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. (42)42. Track and roller are concealed and waterproofed with a 18 gauge housing. These windows were also entitled to a special glass to reduce solar gain and the frames, like the entire structure of the house, were covered with a bright aluminium paint to reflect the heat. See Peter Gossel (ed.), Neutra: Complete Works, Taschen, 2010, p. 108 / Edward R. Ford, The details of modern architecture, volume 2: 1928 to 1988, Cambridge: MIT Press, 2003, pp. 93-95.

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. (43)43. In the case of Neutra, this focus on continuity was also reinforced by other devices, such as the interior floor heating and cooling system that extends to the outdoor terrace and pool area of Kaufmann’s “Desert” House (Palm Springs, California, 1946), who was also the owner of Frank Lloyd Wright’s Fallingwater.

 

 

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”. (44)44. Sylvia Lavin, Form Follows Libido: Architecture and Richard Neutra in a Psychoanalytic Culture, MIT Press, 2005, p. 59

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. (45)45. “I knew enough of engineering to know that (…) a certain distance in each way from the corner is where the economic support of a box-building is invariably found. You see? Now, when you put support at those points you have created a short cantileverage to the corners that lessens actual spans and sets the corner free or open for whatever distance you choose. The corners disappear altogether if you chose to let space come in there, or let it go out. Instead of post-and-beam construction, the usual box building, you now have a new sense of building construction by way of the cantilever and continuity. Both are new structural elements as they now enter into architecture… [in] this simple change of thought lies the essential of the architectural change from box to free plan and the new reality that is space instead of matter.” Frank Lloyd Wright, Writings and buildings / selected by Edgar Kaufmann and Ben Raeburn, New York: Horizon Press, 1960, p. 285 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).

 

   

 

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”. (46)46. Le Corbusier, “Glass, the Fundamental Material of Modern Architecture”, Tchéco-Verre, vol. 2, nos. 1–4, Prague, 1935, reprinted in English in West 86th: A Journal of Decorative Arts, Design History, and Material Culture, Vol. 19, No. 2, Fall-Winter 2012, p. 292. 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. (47)47. These are the Nevada glass tiles developed by Saint-Gobain from 1928, which allowed diffracted illumination, guaranteeing the total protection and privacy of the interior. 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) (48)48. Villa Mairea has two large sliding windows on the southwest façade and a large sliding glass wall that connects the living room to the garden. Aalto used a book about traditional Japanese architecture to design this sash window and, a few years later, was one of the founders of the Finnish-Japanese Society and used to work in a kimono. Juhani Pallasmaa, “Villa Mairea – Fusion of Utopia and Tradition,” in Yukio Futagawa and Juhani Pallasmaa (eds.), GA: Villa Mairea, Noormarkku, Finland, 1937−1939, Tokyo: A.D.A. Edita, 1985. and a large horizontal sliding wall so that “the house can be completely opened to the garden”. (49)49. Aino and Alvar Aalto, “Mairea,” Architectural Description, Arkkitehti, no. 9, 1939, quoted in Dean Hawkes, The Environmental Imagination – Technics and poetics of the architectural environment, London and New York: Routledge, 2007, p. 75. The Finnish climate, however, meant that it was almost never used. (50)50. Juhani Pallasmaa, “Villa Mairea – Fusion of Utopia and Tradition,” in Yukio Futagawa and Juhani Pallasmaa (eds.), GA: Villa Mairea, Noormarkku, Finland, 1937−1939, Tokyo: A.D.A. Edita, 1985. 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.

 

 

2.7 The industrialisation of metallic sliding window frames

Until the mid-1940s, sliding doors and windows were custom-made. (51)51. Larry Schaffer – current owner of The McAlmon Guest House, designed by Schindler and completed in 1935 – refers to the operational problems of these handcrafted window frames and the way found for their contemporary restoration. ”Schindler devised a mechanism to make it slide, but it was so primitive that the slightest settling made the windows not operate. We just removed all the glass, made the window frames slightly smaller to fit the current size and shape of the opening and replaced all the glass with new laminate.” In David Plick, Preserving the Legacy of RM Schindler, The Value of Architecture, https://www.thevalueofarchitecture.com/blog/preserving-legacy-rm-schindler/ 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. (52)52. Dave Weisenstein, On the Threshold of Discovery – When the aluminum sliding glass door made its groundbreaking debut—then ushered in the classic look of mid-century modern, CA-Modern magazine, Summer 2015, p. 10. 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 (53)53. Steelbilt in Gardena, Arcadia in Arcadia city, Panaview in North Hollywood, Frank B. Miller Manufacturing Co. in Burbank, Malibu Manufacturing Corp. in Los Angeles and, from 1957, Blomberg Windows in Sacramento. Dave Weisenstein, op.cit., p. 8. 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 (54)54. Between 1949 and 1966, Eichler Homes built more than 11,000 single-family homes in various regions of California for the middle class. Paul Adamson, Eichler: Modernism rebuilds the American Dream, Gibbs Smith, 2002, p. 22. These houses are today known as “Eichlers”., became one of Arcadia’s main customers and a major driver of this type of frame. (55)55. Dave Weisenstein, op.cit., p. 6.

 

 

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. (56)56. See on this subject Elizabeth A. T. Smith (ed.), Case Study Houses: The Complete CSH Program 1945-1966, Taschen, 2002. Among the architects who participated in the programme were Richard Neutra, Craig Ellwood, Charles Eames, Raphael Soriano, A. Quincy Jones (57)57. Architect A. Quincy Jones, head of the Jones & Emmons office who, in addition to the Case Study Program, worked regularly with Joseph Eichler, became a major promoter of steel and aluminium sliding frames. The Arcadia company was one of their best customers and his wife, Elaine Sewell Jones, was an Arcadia sales rep when they first met. In 1955, Jones even designed the Arcadia headquarters in Fullerton, CA. Dave Weisenstein, op.cit., Ralph Rapson, Eero Saarinen and Pierre Koenig.

 

 

 

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. (58)58. The programme had a complex operation that employed the balanced participation of four agents – industry, architects, clients and the magazine – all of which benefited from the process and in a context of great residential demand in California.
The companies presented their products to the programme’s architects, who tested them and designed the architectural projects according to the possibilities offered by the new systems. Each product was then offered at cost to the client, individuals who participated in the programme, in exchange for the opening to the public of their home for a stipulated period and its publication in the magazine. The programme was hugely successful and the first six homes built were visited by almost 400,000 people. Beatriz Colomina, Domesticity at War, MIT Press, 2007. 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. (59)59. Advertising, which was the main source of income for Arts & Architecture magazine, was eminently programme-oriented, accounting for about half of the total number of pages. The magazine prepared its own advertising campaign for the programme and sought to maintain the perfect graphic and thematic harmony between the contents and the advertising for the various products. Ethel Buisson y Thomas Billard, The Presence of the Case Study Houses, Basel, Birkhäuser, 2004, p. 242-246. 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” (60)60. Thermopane, advertisement in Arts & Architecture, March 1945., “The Spaciousness of the Outdoors becomes part of the indoor living” (61)61. Thermopane, advertisement in Arts & Architecture, July 1945., “Transparent walls” (62)62. Thermopane, advertisement in Arts & Architecture, October 1945., “Bring the outdoors, indoors” (63)63. Steelbilt, advertisement in Arts & Architecture, May 1950., “Fine structural lines express design freedom” (64)64. Steelbilt, advertisement in Arts & Architecture, July 1950., 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.

 

 

 

   

 

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” (65)65. Steelbilt, advertisement in Arts & Architecture, January 1954.. 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 (66)66. ”Even without the Korean War the demand for aluminum would have grown (…) But it is accurate to assign to the government expansion program at least partial responsibility for the excess smelting capacity that marked the industry from 1959 to 1963” Merton J. Peck (ed): The World Aluminum Industry in a Changing Energy Era, New York: Routledge, 2016, p. 36 (first published in 1988 by Resources for the Future Inc.), 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. (67)67. Numerous emblematic buildings of the Modern Movement used aluminium window frames. These include some of the windows of the Unité d’Habitation de Marseille (1952) by Le Corbusier, supplied by S.C.A.N., cf. Jacques Sbriglio, Le Corbusier – L’unité d’habitation de Marseille / The Unité d’Habitation in Marseilles, Basel Boston Berlin: Birkhäuser, 2004, p. 241. 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. (68)68. Xsusha Flandro and Helen M. Thomas-Haney, “A Survey of Historic Finishes for Architectural Aluminum, 1920 –1960”, APT Bulletin Journal of Preservation Technology, vol. 46, No. 1, 2015, pp. 13-21. 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.

 

 

   

 

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 (69)69. Despite surpassing the technology available at the time, Le Corbusier included all these parameters in his work on the concept of “respiration exacte” as well as in a survey for physicians, cold and hot installers, physicists in 1930. Cf. Le Corbusier, ‘Questionnaire International’, in La Ville Radieuse, Paris, Vincent, Fréal, & Cie, 1935, pp. 47-50. – 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!” (70)70. Le Corbusier, Précisions sur un état présent de l’architecture et de l’urbanisme, Paris, Éditions G. Crès, 1930, p. 56. 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. (71)71. Rosa Urbano, “Le pan de verre scientifique: Le Corbusier and the Saint-Gobain experiments (1931-32)”, Architectural Research Quarterly, vol. 17, no. 1, March 2013, Cambridge University Press, pp. 68-69. 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) (72)72. The hermetic glass façade in flat drawn sheet glass was replaced, in 1952, by a new façade composed of sliding frames and exterior brise-soleil. 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, (73)73.  In 1935, Le Corbusier presented these elements of shading in reinforced concrete, placed in front of the glazed façade, in the book: La Ville Radieuse, Paris, Vincent, Fréal, & Cie, 1935. 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. (74)74.  Like the Lake Shore Drive apartments (Chicago, 1948-51), Commonwealth Promenade and Esplanade apartments (Chicago, 1954-58) and the Seagram Building (New York, 1954-58). For an analysis of the Miesian curtain wall, see: Juan Herreros, Tower and Office, op.cit., pp. 106-116. 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” (75)75. Thermopane, advertisement in Arts & Architecture, September 1945..

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 (76)76.  Stephanie Van de Voode, Inge Bertels, Ine Wouters, Post-war building materials in Housing in Brussels 1945-1975, Vrije Universiteit Brusel, 2015, p. 146. – and to replace the pocket of air with low thermal conductivity gases (77)77.  Monoatomic gases like argon, krypton and xenon. or a vacuum (78)78. Despite its excellent thermal performance, the evacuation of air from the cavity always faced difficulties in maintaining the vacuum and guaranteeing the resistance of the glass to the deflection resulting from the differences in pressure. After several attempts in the 1970s, this technology was only re-implemented in recent years, with vacuum chambers of a few tenths of a millimetre and microspacers between the two panes., 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. (79)79.  Le Corbusier used the example of Thermolux to show that the development of the product ”demonstrates that architects can focus on the solution to the problem and provide an answer”. Le Corbusier, “Glass, the Fundamental Material of Modern Architecture”, Tchéco-Verre, vol. 2, nos. 1–4, Prague, 1935, reprinted in English in West 86th: A Journal of Decorative Arts, Design History, and Material Culture, Vol. 19, No. 2 (Fall-Winter 2012), p. 305. Tim Benton, in his introduction to this article, notes however that despite this reference Le Corbusier only used this product from 1950. Ibid., p. 285. 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. (80)80. Stephanie Van de Voode, Inge Bertels, Ine Wouters, op.cit., pp. 176, 180.

 

  

 

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 (81)81. The process was invented in 1843 by Henry Bessemer and developed and patented in 1902 by William E. Heal and in 1925 by Halbert Hitchcock. At the time, it was a very expensive process so it was only marketed much later. it was only from 1952 that the British company Pilkington Brothers (82)82. Through the developments carried out by Sir Alastair Pilkington and Kenneth Bickerstaff. The float process is also known as the Pilkington process. 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.

 

 

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. (83)83. Importantly enough, amongst the mixed opening systems, the Tilt & Turn hardware, first produced industrially in 1935 by the Wilhelm Frank company (currently Roto Frank AG), Stuttgart (Germany). The Lift & Slide system was elaborated from the elevation device patented by Viktor Gretash at the early 1940. (84)84. Patent DE000000729646 “Device for lifting of doors and windows”, published in 19.12.1942. 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.

 

   

 

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 (85)85. http://authors.library.caltech.edu/5456/1/hrst.mit.edu/hrs/materials/public/composites/Composites_Overview.htm[/index]), developed from the 1930s, but only presented in 1977 – by Ensinger – as a component for doors and windows. Being recyclable, but not offering the economic advantages of uPVC, it guarantees similar thermal performances and better mechanical properties than the latter, and in recent years has increased its use as a complement to aluminium frame systems. Several manufacturers specialised in this material that develop specific components for door and window manufacturers, some of which – Soreg Glide, Home of Horizon – have promising systems that are fully developed using this material. Limitations of various kinds however remain, namely the quality and limited range of final finishes and the coarse calibre of its pultrusion process. But it is two of their intrinsic properties in particular – brittleness and inelasticity – that suggest a selective use of this raw material, especially in the case of critical system components – such as structural mullions – that cannot take the risk of ultimate failure. Significant developments using more sophisticated resins and stronger fabrics and filaments – carbon, for example – appear promising but await a more favourable environment.

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 (87)87. A protective and sacrificial layer of nickel-chromium oxide (NiCr oxyde) was added. 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.

The glass used in the minimalist window, commonly referred to as ‘structural glass’, is the result of the combination of different manufacturing and post-processing methods that allow it to reach large dimensions with a strength and behaviour capable of offering total security to the user. Although they date from an earlier time, the industrial development of these processes was generalised from 1980 onwards.

The mechanical properties of glass are always structural. However, it is only when the mechanical strength is calculated to play the role of the main component of the door or window that its importance becomes paramount. And a normal float glass – the process created in the 1950s, which provides the glass with a uniform thickness, absolutely flat and perfectly transparent – although structural, becomes extremely dangerous if it is broken, if not subjected to a post-processing that gives it greater mechanical strength and safety. This is achieved by lamination, tempering or both simultaneously.

Laminated glass combines the rigidity and durability of glass with the elastic properties of plastic. Generally, it is obtained through two or more layers of glass combined with a plastic intermediate sheet – generally PVB (2)2. Polyvinyl butyral (PVB) was invented in 1927 by Canadian chemists Howard Matheson and Frederick Skirrow. or EVA (3)3. Ethylene-vinyl acetate, a satin vynil type of foam. –, which does not allow disintegration in the event of breakage, keeping all the pieces of glass together. This process was invented in 1903, inspired by a laboratory accident. (4)4. By the French chemist Édouard Bénédictus, who patented it in 1909 and called it “Triplex”. It was used immediately in the automotive industry, where the product was very successful and became widespread, especially from the 1930s, with the invention of PVB, which improved its efficiency. At present, its composition can extend to complex multilayer systems, combining different types of glass and plastic. In the field of architecture, laminated glass offers not only a safety response but also performances in acoustic insulation, elimination of UV solar radiation and even shock resistance, depending on the type of lamination and the composition used.

Tempered glass or toughened glass, according to the American or British designations, is a heat-treated glass that modifies its characteristics, greatly increasing its hardness and strength, allowing it to be safely used in unframed assemblies. If broken, it shatters into small fragments, which is an important safety measure. Its manufacture consists of the controlled heating of the raw material in a furnace (650°C to 700°C), and then rapid cooling (the tempering) that causes a thermal shock. The mismatch between the solidification of the outer surfaces and the interior of the glass triggers stress and compression processes that improve the mechanical properties of the product remarkably in terms of resistance to physical shocks and thermal variations. It is also possible for the tempering to be done chemically, ensuring a better result in the planimetry of the glass and further increasing the tensile strength of the glass. Alternatively, heat-strengthened glass consists of a similar process, but with a slower cooling, which gives it a lower mechanical strength, three times that of ordinary float glass, while that of tempered glass is five to seven times higher. The tempering of glass dates back to a 19th century French invention, (5)5. The invention is attributed to the Frenchman François Barthelemy Alfred Royer de la Bastie, who registered in the United Kingdom the first patent on the development of tempering in 1874. The process was later improved and obtained by a different method in 1877 by the German Frederick Siemens. It was then developed by Saint-Gobain and patented in France in 1932. The first patent for the global process of tempering was registered in 1935 by the Austrian Rudolph A. Seiden, in the United States of America. adapted by Saint-Gobain in 1929 for the production of Securit glass, applied in the automotive and aeronautical industry. However, between the 1930s and the 1970s the expansion of the use of tempered glass in the construction field was rather slow, and the process only became widespread in the 1980s. In fact, it was at this point that independent glass processors started using tempering furnaces, to meet the growing demand for this type of glass with increased mechanical strength.

These processes were, in short, the starting point for the decision to use larger tempered glass in doors and windows, taking advantage of its structural properties, and with minimal aluminium profiles reduced to mere finishing components.