Guide To Avoiding Glass Failure
Glass Color
Window units are commonly fabricated with clear, tinted, or coated
glass. Variations in the tinting and coating processes influence the solar, optical, and
heat absorption properties of the glass. Of most importance in this discussion is the
influence of the total solar absorbance properties on the development of high thermal
stresses. If the total solar absorbance of filmed annealed glass exceeds
50%-60%, thermal
stresses may become critical. The only reliable method to estimate the total solar
absorbance of filmed glass is through laboratory testing or by computer modeling in
conjunction with lab test data.
Clear safety film can be applied to coated or tinted glass
without substantially changing the risk of thermal fracture. However, as with insulating
glass, the glass may be in a state which is near failure prior to the application of the
film. In such a case, failures that occur after the application of the film may wrongly be
associated with the film application. Therefore, if the solar
absorbance of the glass
without the film is near the critical point, it may be best to not apply window film.
Clear Glass
Clear glass is manufactured with a minimum amount of
non-glass
constituents so that it is colorless with a high light transmission which ranges from 75%
to 92%. The total solar absorbance of clear glass is dependent on its thickness and
generally varies from 3% to 41%.
Tinted/Coated Glass
Tinted glass is manufactured by adding colorants to a clear glass
batch to create the desired color so that the colorants become an integral part of the
glass matrix. The visible light transmission of tinted glass ranges from 14% to 83%.
The
total solar absorbance of tinted glass depends upon the thickness of the glass, and the
amount and type of colorants that are added to the glass. The total solar
absorbance of
tinted glass generally varies from 30% to 74 %.
Coated glass is manufactured by chemical vapor deposition
during the on-line float glass process or through various vacuum deposition technologies
in an off-line process. Coated glass is often referred to as reflective glass.
This type
of glass would include low-e coatings which reflect primarily long-wave infrared
radiation. The coatings affect both the solar/optical properties of the glass as well as
the aesthetics of the installation.
The value of the total solar absorbance of a particular piece
of tinted or coated glass should be determined from manufacturer's literature.
If the
total absorbance is unknown it is not recommended that window film be applied to annealed
glass.
Guide
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