STRUCTURAL GLASS FACADES - USC School of Architecture

to as annealed glass. Subsequent processing, such as bending during which the glass is
heated, may require that the glass be annealed again.
2.4.2.3 Tempering
Tempering or toughening are terms used interchangeably in the glass industry. Tempering is
a secondary process whereby annealed glass is subject to a cycle of carefully controlled
heating and subsequent rapid cooling. After all cutting and machining work have been
completed on a piece of annealed glass it is run over rollers through a tempering oven,
heating it to approximately 1,150° F. On reaching this temperature the glass exits the
furnace and is rapidly cooled by airflow over both surfaces simultaneously. The glass first
cools and contracts at the surface, but as the interior glass cools and contracts more
gradually it pulls the contracted outer surface into high compression. The end result is the
core in tension and the surface in compression. (Wigginton 1996, pp.262-263)
Improved strength and resistance to thermal stress result from the tempering process. Fully
tempered glass is up to four to five times stronger than annealed glass. Tempered glass also
possesses a unique behavior when broken; the glass shatters into rounded kernel size
pieces without sharp edges. Because of this attribute, tempered glass is sometimes referred
to as safety glass, and building codes require its use in doors and other public areas.
Tempered glass cannot be worked, cut or drilled; all such working must be completed prior
to tempering.
Modern day glass produced by the float process is a remarkably flat material of high surface
quality. The tempering process involves moving these flat glass panels through a specially
designed oven. These ovens are custom in design and can vary substantially in width
between fabricators. The tempering oven can be the limiting factor in the maximum glass
dimension, and must be considered during façade design, especially if the intent is to use
very large pieces of glass.
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