Translucent Materials

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•1 -~ - Glass as a building material Qualities of glass 2 Solar radiation through a pane of gtass: reflection, absorption, transmission and overatt energy trans mission (g-value) 3 tnternal stresses building up in toughened safety gtass 4 Tension in the gtass surface under toad 5 Event venue and congress centre n San Se bastiãn, Spain Mater at curved aminated safely gtass onsist ing of transparent figured glass and sand-blasted float glass Architect Rafae Moneo, Madrid 13
Glass as a build ng material The glass facade Glass panes The glass facade The radiation permeability of a pane of glass determines both the amount of heat and daylight entering a building and the amount leaving it. In summer, the intention is to make the best use of daylight, but avoid undesira ble heat gains. In winter, however, heat gains are wel come, as they partially compensate for heat losses via the glass facade. These partially conflicting requirements require a glass with changeable proper ties; this is still being developed. Up to now, glazing has had to be comple mented with additional protection against sunlight and heat. For glazing, glass is available with and without coating, and in combinations like laminated and insulat ing glass. Glass panes The properties of a pane of glass can be varied by changing the composition of the body of the glass and/or the pane’s surface properties. Float glass Quartz sand, the most important raw material for glass manufacture, always contains minor impurities that cause dis coloration. The slightly green cast in nor mal float glass comes from iron oxide (Fe 0). Typical values for a 4 mm thick float pane are a t value of 0.9 and a g value of 0.87, but they vary according to the thickness of the glass (ill. 6). White glass Purer glass panes with a lower iron con tent, so-called “white glass”, are created by chemically purifying the basic material or by using particularly pure raw materi als. 4mm thick “white glass” has at value of 0.92 and a g value of 0.9 (ill. 7). Coloured glass Adding metal oxides produces stronger coloration, with an associated reduction of light transmission. The colour range is currently restricted to green, blue, bronze and grey. Green glass is produced by adding iron oxide and is the most common, 4 mm thick green glass has a t value of 0.78 and a g value of 0.67. Grey-tinted glass is made with nickel oxide, bronze glass with selenium; both are used to diminish glare. Grey glass has a t value of 0 54 and a g value of 0.67, bronze glass has a t value of 0.55 and a g value of 0.65 (ill. 7). Photosensitive glass Photosensitive glass is a special develop ment in colouring the body of the glass. Exposure to UV light and subsequent heat treatment create textures or patterns in the body of the glass. Corning Glass developed their Louverre ® product in 1983; its louvre-shaped structure is an aid to sunscreening. Phototropic glass Brown or grey phototropic glass is used to make spectacle lenses. It is a self-reg ulating product whose light permeability changes automatically with UV radiation. They have a large transmission range, for example, lenses can darken from 0.91 to 0.25 (ill.8). 5 100 reflection S 80 absorption a V 60 _100 ~6o 40 20 n 500 1000 1500 2500 wavelength 0 20 40 60 80 100 7 Diagram of spectral transmission in various tinted glasses 1 Cl ear, 2 green, 3 blue, 4 bronze, 5 grey unexo~ed 60 ~ 60 /l 40 fl j exposed 20 0 transmission 100 1000 1500 2000 — transmission 6 Diagram of spectral transmission in float glass C 0 E L0 500 1000 1500 2500 wavelength 8 Diagram of spectral transmission in light-sensitive glass C 100 reflection 0 ~60 20 C 60 absorption ~ 40 60 ~ 0 40 20 transmission 80 .~ 0 100 0 C 100 1000 1500 2000 a — wavelength 9 Diagram of spectral transmission in glass with a reflective coating C .2 S 100 1000 1500 2500 wavelength 10 Diagram of spectral transmission in a low-E coate.d glass 0
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