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Jun18
Annealed, Heat Strengthened, Tempered and Insulated Glass
Filed under: About windows & glass; Tagged as: annealed glass, heat strengthened glass, insulated glass, tempered glass4 CommentsAudio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.
The glass you see as you drive down the street has been subjected to a variety of processes that impart desirable properties to the glass. Today we will briefly touch upon:
- Annealing
- Heat strengthening
- Tempering
- Insulating
Annealed 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
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.
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 and sidelites. 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
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. There is new “warm-edge” technology now available, 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.
Need glass or information about it?Contact me, Mark Meshulam,
the Chicago Window Expert
for the expert attention you deserve.My phone: 847-945-9200 ext 229
My email: mark@chicagowindowexpert.com
4 Responses to “Annealed, Heat Strengthened, Tempered and Insulated Glass”
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Dear Sir or Madam,
Good morning and have a nice day.
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msn: natergysieve@hotmail.com
Skype:natergy
ICQ:586391978 -
Robert Romasko January 7th, 2010 at 11:26 pm
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.Thanks!
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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. -
Another January 14th, 2010 at 1:56 pm
Title…
Very interesting post. I would like to link back to it….
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