Guide To Avoiding Glass Failure
Glass Type
For purposes of this discussion, glass type is defined as either
annealed glass or heat-treated glass as discussed below.
Annealed Glass
The basic flat glass product that is available is commonly referred
to as annealed float glass or simply annealed glass. Annealed glass is manufactured in a
process where molten glass is poured continuously onto a bed of molten tin.
The molten
glass tends to seek a level configuration as it floats on the surface of the molten tin.
Because the melting point of the tin is much less than that for glass, the glass
solidifies as it slowly cools on top of the molten tin. Once the glass solidifies, it is
fed into an annealing lehr where it is slowly cooled so that the residual stresses are
minimized. This process results in the production of an annealed glass product which is
very flat and with nearly parallel surfaces. Because annealed glass has a minimum amount
of residual surface compression, care must exercised in the use of this product to
minimize the potential for thermal stress fractures.
Heat-Treated Glass
The strength of annealed glass can be greatly increased by
subjecting it to a heat-treatment process. In this process, annealed glass is heated to a
temperature which is near its softening point and then it is quenched in a controlled
manner. During the quenching operation the surface of the glass cools quicker than the
interior of the glass so that a residual compression stress is locked into the surface of
the glass. These residual compression stresses must be overcome before the glass can
fracture due to tensile stresses. The level of the residual compression stress is
controlled by the rate of quenching. Glass that has a high level of residual surface
compression is referred to as tempered glass, and glass that has a medium level of
residual surface compression is referred to as heat-strengthened glass.
Both
heat-strengthened and tempered glass have sufficient residual surface compression to make
failure due to typical thermal stress conditions unlikely.
The level of residual surface compression induced in tempered
glass is sufficient to assure that this product qualifies as safety glazing.
As such, most
tempered glass products are marked with a clearly visible corner etching stating that the
glass complies with safety glazing standards. The presence of this marking is intended to
assure that the glass is fully tempered.
If there is no corner etching, the most direct way to tell the
difference between annealed glass and heat-treated glass in a field situation is through
the use of two sheets of polarized film. One sheet should be positioned on each side of
the glass plate and the character of the light that shines through the glass is examined.
Annealed glass will exhibit a neutral appearance, while heat-treated glass will exhibit a
mottled display of residual stress patterns.
Guide
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