Focus on Anisotropy
b y E l l e n R o g e r s
n the span of just a few years, glass
has gotten thinner, bigger and more
transparent. In many cases, these
changes were driven by architects
and their desire for glass perfection.
But nothing is perfect—not even glass. stages and there are a lot of unan- and cooling will create stress differ-
Consider the term anisotropy. If swered questions.
you’re not familiar, it basically de-
entials in the glass, and based on the
photo-elastic theory,this will create the
appearance of anisotropy.”
Äppelqvist adds, though, that tem-
Simply put, anisotropy is the vi- pering isn’t the only reason for anisot-
scribes the appearance of glass; in par- Getting To The
ticular, attributes such as color, haze Root Cause
and gray patches in the glass. You may
have heard it called iridescence, which sual unevenness of a given material, ropy. He says it can also be a result of
refers to stress differences in tempered including glass. But what causes it? the interlayers used in the laminating
glass that are visible in certain condi- Äppelqvist says there can be several processes,as well as the tempering line
tions, such as the viewing angle or in- reasons, but perhaps the most com- design and how it’s being operated.
According to Miika Äppelqvist,busi-
mon is thermal tempering.
“Thermal tempering creates stresses What Do Architects
ness unit director with Glaston,anisot- in the glass, and anisotropy is a visual See In Your Glass?
ropy has been an issue since thermal phenomenon that is caused by different
tempering began. light retardation between different stress chitects and designers,as it can detract
Recently, however, the growing levels in the tempered glass,”he says.“In from overall quality and appearance.
complexity of glazing solutions has essence, anisotropy is caused by stress “There is an increasing recognition
This can be a major concern for ar-
increased the probability for seeing differences in tempered glass. Stresses, that glass defines the image—the
anisotropy effects in glass. Increases in in turn,are caused by uneven heating or spirit of a building,”says Louis Moreau,
laminated glass, triple-insulating glass cooling during the tempering process.”
senior technical specialist with Archi-
units (and other multi-layer glazings), Francis Serruys, director of techni- tectural Glass North America (AG-
as well as glass coatings have made this cal sales support and business devel- NORA). “Architects are more sensitive
phenomenon more visible, so more opment with Saint-Gobain Building to the building envelope performance
people are talking about it,” he says.
Glass Europe, adds that during heat and uniformity, and building owners
While many in the industry agree treatment the glass is never heated and will not accept a patchwork of coating
anisotropy needs to be addressed, cooled down perfectly and evenly.
these discussions are still in the early
“Consequently, the unequal heating
shades or a high level of anisotropy.”
Serruys adds, “Architects prefer an
USGlass, Metal & Glazing | August 2017