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Physical Properties of Glass

March 16, 2011

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 Wall

Chemical Resistance
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).

Light Transmission
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).

Temperature Performance
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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9 Comments leave one →
  1. September 20, 2011 12:45 pm

    Thanks for the information!! i’ve been looking for the complete information for the physical properties of glass for my class project!!! =D

  2. December 22, 2012 7:51 pm

    I want to build some thermal acumulators using glass of 8 mm thickness and Bison glass silicone and lots of water!The dimensions would be 200cm by 20cmx20cm and at least 5 “units” for a total of 400 liters of water!
    If the water is heated to 70 degrees Celcius and the room temperature is 20 degrees Celcius could there be any problems ,like cracked glass because of this difference?
    I want a longer heating time from a wood stove and water has a lot of thermal storage capacity,actually the biggest from the known “normal” liquids!
    And the storage capability in kwh for 400 liters of water at 70 degrees Celcius(initial temperature will be 15 degrees Celcius) , is 25,5kwh!
    4,18×55(degrees C)=229,9×400 liters=91960Kj/3600Kj(1kwh equivalent)=25,5kwh.
    Thank you kindly and I hope you can answer me soon!

    • January 14, 2013 8:31 am

      Thanks for your question!

      What is being referred to here is the risk of thermal fracture. The potential relates to the thermal gradient (temperature difference) across the glass. In Viridian’s physical properties data we say that a temperature difference of 50 degC across the glass, if applied suddenly, is expected to cause breakage. In this case the temperature difference between air and water is 50 degC but this does not mean that the temperature difference across the glass is 50 degC.

      It seems that the intention is to connect these glass panels together using silicone therefore the full face of the glass will be in contact with the water so will be evenly heated by the water. There will be heat loss to the surrounding air and this should be at a constant rate because the air has access to the full surface of the glass. In a steady state situation such as this there will be very little in the way of thermal gradient.

      If the tank was empty and the glass was at the 20 degC temperature of the surrounding air and water of 70 degC temperature was introduced to the tank then the glass would be subject to a thermal shock, because of the 50 degC temperature difference, and the glass may well crack because of this thermal shock.

      The capacity of glass to withstand thermal shock is reliant on its edge and surface strength. Glass which has shelled or chipped edges has a much reduced resistance to thermal shock (or any other applied stress). So glass which has damaged edges should not be used.

      This silicone manufacturer (not a supplier) should be consulted as to the suitability of using silicone to bond glass edges together in these circumstances. Considering the silicone will be subject to long term exposure to water, especially hot water, as well as the constant load applied by water pressure. The warranty conditions associated with the silicone should be investigated as the proposed use might void any warranty.

      The suitability of the proposed glass thickness should be verified as sufficient for the purpose.

  3. singh permalink
    February 17, 2013 9:57 am

    Hello, I want to know which side and up down, location, of tempered mark go on Glass?

    • March 1, 2013 4:33 pm

      Hi, it SHOULD be in the bottom right corner of the glass (although sometimes it can be in a different corner).

  4. Rehan permalink
    October 27, 2013 7:37 am

    hi, I am looking for the properties of 3mm clear tempered glass. Basically this glass is the front face of solar panel which means it is covering the solar cells. So for my research anaylsis i want to know the reflectance, transmittance and thermal conductivity of the mentioned glass.

    I would appreciate if you tell me the reference as well if you write those properties here so that i can use that reference in my paper.

    Thanks for your help

    • October 29, 2013 2:42 pm

      Hi Rehan,

      The minimum thickness for Viridian toughened glass is 4mm.

      If you’re after energy performance data for 3mm float glass, including the visible light transmittance this can be found in the Knowledge section of our Architectural Glass Specifiers’ Guide which can be downloaded here.

      The thermal conductivity (K Value) for glass can also be found in the Knowledge section on the Glass Physical Properties page.


  5. wanieos permalink
    June 10, 2014 12:42 am

    hi there ,

    for my final year project i must make a testing by using waste material in ceramic body to study the effect, suitable temperature e.t.c . then for that i choose waste glass to mix it with ceramic body.

    i want to ask is it glass can replace feldspar in the ceramic body ? i just want to use 10% or 20% of glass in ceramic body without change ceramic body composition so i just want to reduce the original material use in ceramic body which is ball clay, feldspar, kaolin and silica.

    • June 19, 2014 2:38 pm

      Hi Wanieos,

      Your core question seems to be “replace feldspar w glass in a ceramic makeup”. Without knowing what sort of ceramic it is, what material properties it needs, etc. this is of course hard to answer. But in general terms, here are some things to think about…

      Feldspar is a class of minerals found abundantly in nature. Aluminium and Silicon and Oxygen are its key signature elements, with other metal oxides as the variable part. Feldspar is crystalline in its normal form and often has a distinctive cleavage, which may contribute to structural weakness if any remains intact through the ceramic furnacing process. However its hardness is similar to glass (which may be important to materials processing prior to preparing the ceramic materials batch).

      Feldspar is used as a flux material in ceramics – i.e. it effectively lowers the melt temperature of the mix to allow the components to bind in a “glassy” fashion. It relies on the “other metal oxides” component of its makeup to achieve this. The “silicate” (metal silicates and any free silica) components would assumedly participate in the ceramic process like the silicate-bearing materials already present in the existing components (kaolin, clay, and of course the silica)

      Our glass (soda lime silicate) is essentially a mix of Sodium and Calcium oxides and silicates plus some traces of other stuff. So a bit similar in components to feldspar, in different proportions, but importantly glass has far less Aluminium in its makeup as compared to feldspar. Aluminium (perhaps as alumina) may need to be supplemented in the mix.

      I certainly can’t say for sure that replacing feldspar w glass would absolutely work, but on the face of it there seems to be a good case for giving it a go, perhaps to a certain critical ratio, and perhaps in combination with added alumina. You’d be employing the (ample) metal oxide parts of the glass composition as the flux agents in the ceramics process to assist in the formation of a glassy matrix. It seems a bit of a round-about way to get there, but if the assumption is that the waste glass was cheap / accessible compared to feldspar, then why not.

      I understand from our friends in PGH (CSR’s brick business) that waste glass (and other glass-like stuff) has been used in brick manufacture for this same flux purpose to allow the other constituents to bind together at a lower input energy. Not quite ceramics as you are dealing with, but this may be a useful precedent to build from.

      Hope this assists. Good luck with the project, and if you get a good result please let us know.

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