Annealed, Heat Strengthened, Tempered and Insulated Glass

Annealed glass, heat strengthened glass, tempered glass and insulated glass are the main types of glass used on buildings. Each has unique properties and applications. Strength, breakage pattern and thermal properties are key variables.

Buildings with glass reflected through glass

This article will focus on:

  • Annealed glass
  • Heat strengthened glass
  • Tempered glass
  • Insulated glass

Annealed glass

Float glass plant, where silica sand and other powders are melted and floated on a bed of molten tin to create flat glass
Float glass plant, where silica sand and other powders are melted and floated onto a pool of molten tin to create float glass

Annealed glass refers to glass which comes from the primary factory, the “float” plant. Most architectural glass is made by melting silica sand, soda ash, dolomite, limestone, and salt cake to 2700 degrees F,and then floating it out atop a pool of molten tin in a continuous process. As it is pulled onto a long conveyer, it passes through an annealing lehr, a furnace that controls the cooling process. The flat, cooled, solidified clear stuff at the end of the line is annealed glass. This is cut into large sheets and shipped to other facilities where secondary processes are applied, including cutting-to-size, heat strengthening, tempering and insulating.

Heat strengthened glass

Heat strengthened and tempered glass are heated to 1200 degrees F, then rapidly cooled
Heat strengthened and tempered glass are heated to 1200 degrees F, then rapidly cooled

Annealed glass is heat strengthened so that it can tolerate more rapid and uneven temperature swings during its service life, for instance where the glass is backed with insulation, back-painted, is in the presence of certain Low-E coatings, or uneven shading. To heat strengthen glass, first cut to size, place in tempering until it reaches 1100-1500 degrees F, then cool rapidly. Season to taste. Heat strengthened glass is about twice as strong as annealed glass.

Tempered glass

Heat strengthening and tempering produce permanent tension & compression within glass, producing correspondingly greater strength.
Heat strengthening and tempering produce permanent tension & compression within glass, producing correspondingly greater strength.

Tempered glass is tempered in order to make the glass significantly more resistant to blunt impact, such as a distracted pedestrian walking into a glass door. Tempering has an additional benefit: if it does break, it does so somewhat explosively in a zillion little pieces, making it less of a cutting hazard. This is why tempered glass is also called “safety glass” and is mandated in doors, sidelites, shower doors, glass handrails and often where people can walk nearby. To make tempered glass, cut to size, heat it in a tempering oven, then cool even faster than heat strengthened glass. This creates a permanent equilibrium of tension and compression within the glass, where the surface layers are stretched tightly across the inner layer, creating strength 4-5 times greater than annealed glass.

Insulated glass

Composition of typical insulated glass with metal spacer
Composition of typical insulated glass with metal spacer

Glass is insulated in order to give it improved thermal properties. Today most architectural glass is insulated, even in warm climates where it helps with cooling loads. To insulate glass, cut two pieces the same size, clean them, place a dessicant-filled spacer between them, and use one or two layers of sealant to hold the glass together. This seals the airspace between the pieces of glass, creating an insulating layer of dry air. Sometimes this airspace is filled with an inert gas such as argon for better R value. Insulating glass can also be fitted with newer “warm-edge” technology, where the spacer, dessicant and sealant are combined into one material, a thermoplastic which is applied to the glass in a near-liquid state. This improves thermal properties at the glass edge, improving overall window thermal performance.

Much more could be said about all of these technologies. Indeed your Chicago Window Expert has enthralled scores of cocktail party attendees with much more lengthy discourses on these subjects (“Hey come back! Where you going? I’m not done yet!”), so I will stop here for now. But if you want more…

Click below to listen to more about annealed, heat strengthened, tempered and insulated glass:

Mark Meshulam surveys curtainwall leak
Mark Meshulam stares thoughtfully at glass


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9 thoughts on “Annealed Glass, Heat Strengthened Glass, Tempered Glass and Insulated Glass”

  1. Dear Sir or Madam,

    Good morning and have a nice day.

    This is Peter watson from Zibo Natergy.

    Kindly allow me to introduce detail information about my company:

    Our company, Zibo Natergy Chemical Industry Co.,Ltd. is specialized in producing and selling molecular sieve 3A(insulating glass desiccant).Until now our products have been approved by IFT according to EN1279-2. We produce the goods under ISO9001:2000 quality control system. In 2008 our daily output is more than 30 metric tons.

    We supply the products with good quality and most competitive price to our customers coming from all over the world.

    If your company is interested in our products — molecular sieve 3A (insulating glass desiccant), please contact me directly.

    Best regards

    Peter watson
    Zibo Natergy Chemical Industry Co., Ltd.
    Tel: 86-533-2318602

  2. I read quite a bit on the web site and it is very interesting. I learned
    a lot from it.

    I have a question. I saw a few times, one on a glass table top. The
    glass was tempered, and appeared to be broken in to all of those little
    cubes, but then it looked like it was melted back together and was
    solid. What is this glass called and how is it made?

    It wasn’t like the blanket but it was a solid piece of glass with just
    the broken look.


  3. Hi Robert,
    The broken table top look (known as “cracked glass’) is created by laminating three pieces of glass, then breaking the center one which is tempered. You can break the center one by hammering a sharp object like a steel ice pick into the edge.

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  5. Dear Sirs/ Madams I would like to shortly introduce my company Nedex A.S

    Company NEDEX AS is multinational company concentrate his production of desiccant, polysulfide, butyl and German spacer in four countries Turkey, Russia, Ukraine and Germany.
    I would like to acknowledge you that NEDEX AS is the largest producer of Molecular Sieve for IG units application worldwide.

    The total production of IG (Insulated Glass) units worldwide is reaching today 480,000,000 m2’s, including the Chinese market with 120,000,000 m2’s. The world demand for molecular sieves as bead is about 64,000 tons

    Today 12 major producers are offering desiccant for the IG applications. NEDEX supplied in 2009 approximately 35% of the world market reaching to appr. 25,000 tons.

    130.000.000 m2 IG units in the area of Western Europe, Middle East, CIS countries, North Africa and Mexico were produced with NEDEX desiccant in 2009.

    Major IG unit producers having production capacities between 1,000,000 m2/year and 6,000,000 m2/year are directly supplied by NEDEX and smaller producers get their needs from NEDEX dealers, which are chosen by NEDEX carefully. They are specialized for IG producers’ needs, give the best service, keep the product available in the right quality and quantity and they are professionals of their business.

    NEDEX is producing molecular sieves designed ONLY for the needs of IG industry.

    NEDEX products are certified according EN 1279, GOST, and this allows its customers importation and application without any authorization problems.

    NEDEX defined as minimum critical stock levels 350 tons for Turkey and 200 tons for Ukraine. NEDEX has in its warehouses always product ready to deliver between 550 tons and 2,000 tons, in different labels, sizes, packages. The fluctuations of demands between winter months and summer months reaching up to 4 times can be compensated only with such stock capacity.

    NEDEX offers logistics, on site storage, delayed payment terms according to customer needs. NEDEX Logistic department finds the lowest prices and costs for DDU/CIF deliveries. With its production capacity of 25,000 tons/year NEDEX can get better rates for transportation. NEDEX can help its customers to cover their peak demands with a material storage by customer on NEDEX costs. Only used products will be paid and NEDEX follows the storage level by customer by its sales department.

    NEDEX as the only desiccant producer, who produces also Polysulfite based Sealants , Butyl, Hotmelt and EPDM and silicone soft and rigid spacer for IG industry with its nominated dealers, partners in Europe, CIS, Middle East and Mexico guarantee the best service and cosultancy to IG producers. NEDEX supports technical trainings of IG producers and dealers, NEDEX and its partners keep the IG producers always very well informed about the technical and administrative developements.
    Do not hesitate to contact with us

    Yours Sincerely,

    Aslan BILMEN
    Export Manager

    Nedex Kimya Sanayi Ticaret A.Åž
    Esentepe mah. Milangaz Cad. Unluer Sitesi A Blok D: 6
    34870 Kartal / Istanbul TURKEY

    Tel : +90216 488 01 55 ext. 121
    Fax : +90216 488 52 96


  7. Mark,

    I am installing a glass wall and a glass railing around a rooftop patio.

    The glass wall will have glass panels 4 foot wide and 6 1/2 foot tall. The framing has been determined by a licensed structural engineer. 4 inch by 4 inch HSS tube will be used for vertical and horizontal pieces. I need the glass to meet the following capabilies.

    1) Withstand a wind load of 90 miles per hour for 3 seconds.
    2) Withstand a line load of 200 lb

    The glass railing will have glass panels 5 1/2 foot wide and 4 foot tall. The framing has also been determined by licensed structural engineer. I need the glass to meet the following capabilities.

    1) Withstand a line load of 200 lb

    1) Can tempered glass work for both scenario’s? If so, what thickness?
    2) Can laminate glass work for both scenario’s? If so, what thickness?

    Thank you in advance for your assistance.


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  9. Annealed glass is glass produced without internal stresses imparted by heat treatment, rapid cooling, or by toughening or heat strengthening. Good read. Cheers!

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