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Heat-Treated and Spandrel Glass

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Heat-treated glass provides both the durability and safety found in many of the world’s most visually breathtaking buildings.

But manufacturing heat-treated glass is a complex and multi-step process. This video provides an explanation of how insulating glass units (IGUs), spandrel glass, tempered glass and heat-strengthened glass are manufactured, courtesy of United Plate Glass in West Butler, PA.

Typical of most glass fabrication, large glass sheets are initially scored to their end-use sizes. Automatic edge deletion may also occur on the cutting table if the individual lite of glass has an MSVD or soft low-e coating, before the glass is “snapped” along the score lines prior to being sent to a large industrial washer. The washer thoroughly cleans the glass, while protecting the MSVD or soft low-e coatings.

Tempered glass and heat-strengthened glass are the two types of heat-treated glass. If glass needs to be tempered or heat-strengthened, it is moved to the tempering line immediately after the washer. Surface compression measurements and break tests are then conducted to ensure that the heat-strengthened glass meets industry specifications, including ASTM C1048, the standard specification for heat-strengthened and fully tempered flat glass.

When glass is delivered to the spandrel line, it is run through a second washer before a ceramic frit is spread evenly over the glass surface. The ceramic-coated glass is then rolled through a drying oven set to 400 degrees Fahrenheit. From there, the glass is moved to the heat-treating line for either heat strengthening (where glass is heated and allowed to cool slowly) or tempering (where the ceramic frit is fired and fused to the glass, then cooled quickly).

Whether annealed, heat-strengthened or tempered, glass lites are placed on an insulating glass assembly line and washed yet again, before a pre-sized spacer with the primary sealant is added to the first lite. The second lite is then added to the now-assembled first lite and spacer. Finally, a secondary seal is added, and the completed insulated glass unit is ready to be inspected and packaged for shipping.

Click here for more information on Vitro Architectural Glass’s (formerly PPG glass) heat-strengthened and tempered glass products.

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Resources. Information. Support. From engaging videos on challenging technical issues to common industry terms, learn more about one of today’s most popular building envelope products.

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    Fun Glass Facts


    The first insulating glass units were developed in 1945 for the Pullman Car Company in Butler, PA.


    A standard float tank can produce glass 24 hours/7 days a week for up to 15 years before it needs to be rebuilt.