Physical Properties of Glass
Glass is a most astounding building material. A typical piece of 6mm-thick clear float glass is 87 percent transparent to visible light and yet strong enough to fulfill a number of building roles that provide protection, security and comfort. This GlassTalks post explores the general physical characteristics of glass. Glass strength is highly variable and deliberately modified for differing applications. This information is for general interest and not to be used for design purposes. If you are seeking specific properties, please refer to local standards for glass design or contact Viridian Glass.
Glass will resist most acids with the exception of hydrofluoric, and at high temperatures phosphoric acid. Alkalis will attack the surface of unprotected glass. General water-born materials from surrounding surfaces and the atmosphere may leave deposits on glass, these should be removed for longevity and optimal performance.
Glass, like water, can be deceptively heavy even in relatively small physical sizes. Glass has a density of 2,500 kilograms per cubic meter, making it approximately 2.5 times heavier than the equivalent volume of water and heavier for its size than many other building materials. The weight aspect of glass means that window frames and other structural elements need to be specifically designed for their glazing role. Specialist products, laminate units and double glazing can be exceptionally heavy in large units and need specific safety, handling, mounting and engineering consideration.
Glass is a strong building material with greater capacity to resist compression than stretching or sudden impact. Specialty products, outlined in detail on the Viridian Glass website, are produced with enhancements that add to the natural strength of float glass. Typical float glass may have the following properties:
– Compressive strength – 248 Mpa for a 25mm cube.
– Tensile strength – 20 Mpa as a modulus of rupture (highly variable depending upon the glass).
– Impact strength – Highly variable depending upon shape, hardness and velocity of impacting object.
– Hardness scale – Around 6.0 on Moh’s scale of hardness and 575 Knoop hardness.
Glass is generally a poor conductor of electricity, with volume electrical resistivity of 310,000,000,000 Ωm. Glass is a better conductor of heat, a typical thermal conductivity measure (U Value) of 5.9 W/m2°K for 6mm thick float glass (determined under AFRC 100-2001 environmental conditions and varying slightly with thickness). The thermal conductivity of standard glass is a potential problem for energy efficient building that is addressed by coating technology and double glazing (see our posts on Energy Management with Glass).
Clear glass is not completely transparent, a 6mm-thick piece of clear float glass will capture around 13-percent of light within the visible spectrum, allowing 87-percent of the visible light to pass through it. As the wave-length of light moves away from the visible range, the transmission changes and for many frequencies, glass is quite opaque. Almost as though it was created for our viewing pleasure and natural light transmission. Glass is relatively transparent to short wave infra-red but opaque to long-wave infra-red. Float glass transmits very little in the short-wave length of the ultraviolet band but transmission increases as the boundary with the visible light spectrum is approached (in all cases, transmission varies with glass type and thickness).
Glass is created at high temperature (see our post on Manufacturing Float Glass) and will return to liquid form if heated sufficiently. This can be a problem for fire-resistance. Glass products made for fire protection are enhanced with the addition of substrates, laminates and other technologies to maintain rigidity at high temperature. The most common temperature issue with glass is not ‘high temperature’ but ‘thermal endurance’. Normal 6mm-thick float glass will rupture if heated to 75-degrees Celsius and plunged into 20-degree Celsius water (a temperature differential of 55 degrees). For this reason many glass products are toughened. Toughened float glass has a temperature differential of around 250 degrees. Other temperature properties of standard float glass include:
– Thermal conductivity – (K value) 1.05W/m°C.
– Softening point – 737°C.
– Annealing range – 480°C to 560°C.
– Strain point – 523°C.
– Mean specific heat – 1162 J/kg°C (25°C to 850°C).
– Coefficient of linear thermal expansion – 88 x 10-7/°C (lower than most metals).
Other Physical Properties
Other measures include elasticity, resonating frequencies, dielectric constant and a range of other performance properties required for complex engineering and construction purposes. Please contact Viridian Glass if you need detailed data on a specific product or for a particular project requirement.
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