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] , 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) : 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. 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.
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”.
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. 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.
Mechanics of the façade exposed in Jean Nouvel’s Institut du Monde Arabe sunshades, Paris, France (1987). Photo: Serge Melki.
Installation of electrochromic glazing in Sanei Hopkins’ Contour House, Peak District, United Kingdom. Photo: Pedro Gama / Jofebar.
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. 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.
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 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.
Toute l’histoire de l’architecture tourne autour de la fenêtre pour donner de la lumière.
Le Corbusier / Pierre Jeanneret, Cinq points vers une nouvelle architecture (1926)
Windows constitute a more important element in modern architecture than they have in any architecture since that of the Gothic cathedrals. They are the most conspicuous features of modern exterior design. Their handling is therefore an aesthetic problem of the greatest importance. (…) Light simple frames, preferably of durable non-corroding metal in standardized units, are to be desired as much aesthetically as practically. (…) the general development in this direction is undeniable and one of happy augury for the contemporary style.
Henry-Russel Hitchcock / Philip Johnson, The International Style: Architecture since 1922, New York/London: W. W. Norton & Company, 1996, p. 61 (originally published 1932)
Here is the value of a wide sliding door opening pleasantly onto a garden. It cannot be measured by counting how often and how steadily the door is used or how many hours it stays open. The decisive thing may be a first deep breath of liberation when one is in the almost ritual act of opening it before breakfast or on the first warm and scented spring day. The memories of one’s youth and of the landscape in which it was spent seem composed, to a considerable degree, of this sort of vital recollection. There are in each life certain scattered quanta of experience that may have been of small number or dimension statistically but were so intense as to provide impacts, forever essential.
Richard Neutra, Survival Through Design, New York: Oxford University Press, 1954, p. 229.
Although their name refers to a formal achievement, minimalist doors and windows result mainly from a technical prowess. It consists of the optimisation of a series of raw materials and manufacturing processes, which maximise glass surfaces and reduce the frames to the bare minimum.
The main innovation is that the glass, instead of simply filling the frame, becomes the structural element of the door or window, thereby becoming load-bearing. The frame – composed of aluminium profiles with thermal break section – is thus restricted to the function of sliding guide and ensuring the tightness of the window. This new expression for the frame enables the peripheral elements of the frame to be embedded into the floor, the ceiling and the walls, taking to the extreme the notion of transparency and immateriality that modern and contemporary architecture advocate and that was given great expression in the designs of Tadao Ando, John Pawson and Souto de Moura during the 1980s.
This text is composed of four chapters, which can be read sequentially or independently, and seek to revisit the technical and architectural history of this type of doors and windows. In the first part, we define what is meant by structural glass – a fundamental element of the minimalist window – and briefly describe the technical processes of its manufacture. In the second, much more extensive part, we give an historical overview of horizontal sliding doors and windows throughout the 20th century, identifying the main stages of their technological and architectural development from the Modern Movement to the present day. In the third, we trace the history of the minimalist window, through a brief journey through the projects and processes that have driven the wide variety of solutions currently available, tailored to the specific needs of customers. Finally, we point out the technical developments achieved in the 21st century and outline some perspectives for the future.
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 or EVA –, 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. 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, 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.
Tempered glass used for frameless doors. Advertisement for Securit glass, c. 1950. © AAM/Fondation CIVA Stichting, Brussels.
In the late 1800s, the widespread availability of plate glass allowed for large windows spanning the lengths of shops. Small children gazing through Macy’s shop window, New York City.Photo: George Grantham Bain, ca. 1908-1917. © Library of Congress Prints and Photographs Division Washington D.C.