Transparent glass
Thanks to its high degree of transparency, durability and versatility, glass continues to be the first choice when a view through a material is required. Yet the standard transparency value of 92% can be increased to 98% by drastically reducing the reflectivity. Anti-reflective coatings are employed to increase the efficiency of photovoltaic panel’s covering, eliminate reflections in storefront displays, and ensure that glass does not cause unwanted effects that would disturb a sensitive setting, e.g. a city’s historic centre. To prevent birds from colliding with glass, a translucent, patterned coating has been developed that – in contrast to the bird-shaped stickers – is barely perceptible to those inside the building. In past years, applications of structural glazing – including all-glass facades, stairs and even bridges – have demonstrated what glass is capable of. And the size in which laminated safety glass can be now produced is astonishing. For the new branch of the Louvre in Lens, for example, Sanaa determined the glass skin’s proportions based on the largest format available when the project was in the design development phase. However, by 2012, the year the building was completed, much larger formats – 17.0 ≈ 4.5 m – had become available. This leap in scale was made possible by the incredible demand for large formats: this made it appear feasible to a manufacturer to acquire the world’s largest laminating machine – an autoglaph – in which toughened glass is made into laminated safety glass. This also requires adapting the entire process chain to these dimensions: the oven in which float glass is heated to 650° C, then quickly chilled, resulting in pre-stressed, toughened glass; the chamber in which the subsequent heat soak test is administered; and the entire transportation logistics. Screens for 17-metre-long imprints are not yet available, but the degree of translucence can be customised by employing coatings or frosted interlayers. Curves can be achieved through cold-forming or by laminating pre-curved panes of toughened glass. Metres and metres of translucent or transparent railings – free of distracting posts or caps – can be produced, as can 10-metre-long units for the facade and roof of the trendy Apple glass cube in New York City. The latter were installed just recently to replace the much smaller formats used in the original design. In January 2012, a new laminated product – Glascobond – in which the bonding can also be exploited structurally, became available: now the pane thickness can be as much as 120 mm (the previous maximum was 20 mm).
Thin sheet glass
But glass panes are not just becoming thicker – they are also becoming thinner. High-strength, chemically hardened thin sheet glass can be produced with a thickness as little as 0.5 mm, but it has, to date, been used primarily as covers for smartphones, tablets, or photovoltaic modules. In contrast to conventional, thermally hardened glass, thin sheet glass is not heated, but is instead dipped in a salt bath in which an ion exchange process hardens the layer on the surface. The result is unusually resistant to breakage and scratching, and, on top of that, can be shaped in curves with small radii. Currently, the largest available format of thin sheet glass is a mere 2.0 ≈ 1.0 metres, due simply to the fact that the main application – electronic products – does not require larger panes. There are no real obstacles, however, to larger formats for future architectural applications. The advantages of thin sheet glass – which, like other types of glass, can be processed, coated, laminated, and cold- and hot-formed – lie above all in the lower weight per unit surface area. In this context, with triple glazing in mind, when one takes into account the considerable weight of conventional 4 mm panes, the future prospects of thin sheet glass look good. This not only causes problems for load-bearing systems, but also with regard to transportation and installation procedure.
Glass tubing
Glass tubing also holds potential for new applications. It is made of borosilicate glass and is produced in a wide range of symmetrical or asymmetrical cross-sections (ills. 5, 7). A tube’s outer surface can be either smooth or textured, with a diameter of up to 150 mm, and a length of up to 10 m. Glass tubing lends itself primarily to use in interiors, where, when employed for example as partition wall, can produce unexpected effects. The curves in the corners of the completely transparent material cause the background to become blurred.
Artistic techniques with glass
Colourful glass surfaces can be attained by applying coats of ceramic enamel colours. A colour powder consisting of ground glass and colour pigments is mixed with screen-printing oil or gum arabic and then applied to the surface manually with a brush or airbrush, or in a silk-screen process (ill. 8). A vulnerable, “dusty” colour layer forms as the solvent evaporates: this is baked in an oven at 620˚ C, bonding the layer to the glass. This process can be repeated as many as three times to apply colours in multiple separate layers atop one another – but more than four fires will cause a noticeable deterioration of the glass’s stability. Not until all of the layers of colour have been applied and the finishing – for example, drilling, sanding, or sandblasting – has been completed is the glass fired to attain toughened glass. It is then ready to be installed.
Areas of application for this type of colourful transparent glazing are bespoke glass design, and in buildings in which colourful, transparent or translucent surfaces or effects are desired. Although enamel colours are lightfast, they are not acid-resistant, which is why, when used in facades, they should be positioned on the side facing the interior.
Switchable Glass
Glass is normally either transparent or translucent. But a number of years ago, materials scientists developed switchable glass, which alternates between these two states. Liquid crystals are embedded in a laminate; when no voltage is applied, the crystals are randomly arranged and diffuse light. When the electricity is switched on, the crystals align themselves, and the pane suddenly becomes transparent.
One new way to control transparency: self-regulating thermotropic solar glazing that makes solar-controlled opacification possible. Depending on the composition of the glass – which has a special resin laminate – the degree of transparency changes automatically; no outside energy is required. It is simply a function of the outdoor temperature. When the sun shines, i.e. when the temperature of the outer pane’s surface is high, the glass becomes cloudy and light-diffusing; when temperatures fall below the switching point, it becomes transparent again. Both the intensity of the opacification and the switching temperature can be customised during glass fabrication.
Translucent glass
In glass assemblies with multiple layers, there are a number of options regarding the positioning of the translucent layer (ill. 3). With high-quality laminating techniques – instead of applying coatings and imprints to the glass surface – sensual textiles, metal meshes, perforated metal, or interlayers can be bonded between two glass panes (ill. 2). To achieve bubble-free results, an autoglaph is essential: in it the layers are heated – at a temperature of up to 140° C – under high pressure for twenty-four hours. By incorporating LED lamps with transparent electrical wiring not visible to the human eye, laminated safety glass can be transformed into a luminaire or a media panel. If organic inlays such as reeds or wood veneer are employed for long-term solutions, it is of course inevitable that the appearance will change. In double glazed units, the cavity between the panes may contain mini-blinds or perforated interlayers; such products have been on the market for many years. Particularly in administrative buildings and high-rises, box-type windows with baffle plates that shield the solar protection components from the wind are increasingly being implemented. In closed cavity facades (CCF), the facade cavity is sealed, keeping exterior air out: dried air circulates in the cavity so that the inner faces of the glass panes and the integrated louvers cannot soil or become dusty.
Acrylic solid surface material
Silica sand, used to produce inorganic glass, has a lattice structure that, at the molecular level, refracts rays of light and is therefore not transparent. Not until the fusing process, when this lattice is deformed, can light pass through the material. The situation is similar for transparent plastics with an irregular arrangement of long macromolecule chains. If they are tangled like a piece of felt, they are referred to as amorphous: they are glass-like, transparent and usually brittle.
Acrylic solid surface material is a composite material containing aluminium-based minerals not penetrable to light (Gippsit, about 70%), and PMMA, a transparent plastic (about 30%). On the market it is referred to by its trade names, including Corian and LG Himacs.
The design team at the Swiss firm Marty Architektur took advantage of its special characteristics for a counter at a branch of the bank Stadtsparkasse Schwyz (ills. 10–12). CNC-milling was employed to give the boards – which are in a special shade of white with enhanced translucence – a serrated surface. The curves were then thermoformed and, in the final step, bonded together, giving the bank counter a monolithic appearance. Thanks to the different thicknesses attained by structuring the surface, the glowing effect that comes about when the counter is backlit is gradated, and, correspondingly, soft and subtly differentiated.
Plastics
Thin sheets of different types of plastics are available in cases in which a material with low weight and a high degree of translucence is needed. PMMA’s sparkle and translucence most closely approaches that of mineral glass; polycarbonate, on the other hand, is a better choice when impact resistance is a requirement. Both materials are thermoplastics, which means that they can be formed and bonded through application of heat. PMMA and polycarbonate’s different characteristics can be combined in a honeycomb composite panel; these panels are characterised by high bending stiffness. The panel’s honeycomb core is of extruded transparent polycarbonate, creating 19 mm honeycomb holes. These holes allow the observer to perceive the material’s depth. The cores are then laminated with different layers of polycarbonate or PMMA (ill. 13).
Membranes
As was demonstrated during the light show of the International Song Contest, the Crystal Hall, a multi-functional sports and entertainment complex in Baku, is a convincing example of the implementation of a coarse-meshed translucent building envelope made of flat surfaces with integrated LED lamps. A total of 180 rhombus-shaped and triangular membrane panels of dark-grey, silvery shimmering PVC-PES fabric mesh and pebble grey PVC-coated polyester fabric are employed in the 20 000 m2 facade. With their Textile Hybrid M1 project, architects and engineers of the University at the University of Stuttgart’s ICD and ITKE have demonstrated that membrane surfaces can also be employed as translucent, free-standing, load-bearing structural members and a new formal vocabulary can come into being (ills. 16–18). The design is based on studies of the integration of bending-active elements and multi-dimensional membranes (deep surface). In these studies, the aim is to create systems of equilibrium by interlinking elastically deformed GRP rods and membrane surfaces. In addition to functioning as envelope, it optimally diffuses and manages light and sound. Beginning at the upper edge of the foundation, only textile materials were used. The connections were executed in traditional tying techniques using UV-stable polyester rope.
Translucent energy generation
Solar collectors and photovoltaic modules are increasingly being used not only to generate energy, but also as shading devices and as solar protection for transparent roofs and facades. First-generation PV modules are made of brittle mono- or polycrystalline silicon, are square in shape, have standardised dimensions, and are laminated between – but at a distance to – glass panes. The second generation is, in contrast, flexible. As laminates with plastic backing film as the basis, thin-film solar modules of amorphous silicon can be encapsulated in EVA, and then laminated from below to the structurally effective membrane of ETFE roofing. In comparison to glazed units, the reduced weight is significantly reduced – a major advantage (ill. 23). And now the third generation is in the works. From the designer’s viewpoint, OPV organic solar cells– a thin colourful foil – offer the most options. When integrated in handbags, umbrellas or tents, they can provide energy “on the road” (ill. 19). OPV are still in the development phase and will soon be economically mass-produced. Dye-sensitised solar cells (DSSC) have nano-crystalline electrodes of titanium dioxide in which a layer of organic dyes – on a ruthenium basis – are embedded (ills. 20, 22). In this way, the light production is enhanced; the electron transfer from light absorber to electrode is improved, as well. They are semi-transparent and available in different colours and are more economical in production than panels using silicon technology because greatly simplified processes (silkscreen techniques) are employed. Further advantages are: high efficiency (even when the light is diffuse, e.g. when the energy is required indoors for smartphones and tablets). At the Swiss Tech Convention Center in Lausanne, a 350 m2 DSSC facade, made up of 30 x 40 cm modules, will begin operation in 2013. Last year, the Fraunhofer ISE introduced a prototype of a larger module: 100 x 60 cm.
Metal meshes
Metal nets, curtains or latticework offer a relatively simple way to achieve different degrees of transparency – they serve as temporary visual partitions indoors and as cladding for entire facades. Generally speaking, there are a number of natural materials to which colour coatings can be durably applied (e.g. stainless steel, aluminium, brass, copper and bronze) for planar or single or double-axis curved surfaces. These may be nets, woven wire cloth, or ring or shingle meshes (ill. 24, see p. 16). Woven wire cloth is receiving increased attention: it can be used to control views in or out of a space, as solar protection, or as multi-media screen (woven wire cloth requires only one sixth of the electricity of conventional displays for this purpose). Recent innovations in this area have more to do with optimising technology – e.g. energy consumption, light intensity or image resolution – than with the mesh itself. Another innovation is the exterior use of RGB-SMD-LED lamps. These enable not only optimal legibility from the side and colourfastness for images and videos, but also make it possible to reduce the pixel grid to 40 x 40 mm – and thereby decrease the necessary distance from the image. A great variety of materials can be woven into the wire cloth, for example bamboo or translucent strips of a particularly durable, paper-like spunbond that is also used in protective clothing and in envelopes. This material is largely rip- and scrub-resistant, permeable to water vapour yet water-repellent, and is woven – in place of the weft wire – into stainless steel wire (ill. 25).
Translucent concrete
In contrast to glass, plastic and metal mesh, concrete does not, at first glance, appear to be a material that is suited to translucent construction (ill. 29). The material has first been employed on a broader basis at the beginning of the twenty-first century – once the appropriate concrete mix, economical glass fibre, and optimised production processes developed. Because it has no reinforcement, translucent concrete is not suited to structural applications, and at present its main areas of application are in designer products and interiors, for example, as backlit wall cladding. The reason this material, with its paradoxical aura as a translucent heavyweight building material, has not been used more extensively is probably related to its cost. On the one hand, the optical fibres are relatively expensive. On the other hand, individual panels must always be cut out of larger blocks, which consist of multiple layers of glass fibres and concrete. Initially these steps were executed manually. In the meantime the process has been automated, and correspondingly, simplified and expedited. And the costs have been reduced, as well: the price per square metre is now roughly equivalent to that of stone.
