If Walls  
Could Talk  
Industry Efforts  
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.  
stallation location.  
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  
Anisotropy, sometimes called iridescense, refers to visible stress differences in glass. These patterns are commonly  
seen in cases of significant polarized lighting.  
equal, homogeneous, uniform appear- strain pattern, also known as irides-  
As the architectural industry con-  
ance that can only be obtained with cence, is inherent in all heat-strength- tinues to request increasingly high-  
float glass where you don’t have stress ened and fully tempered glass. This er-quality glass for both performance  
differentials in the glass.”  
strain pattern may become visible under and aesthetics, suppliers will be chal-  
One challenge, though, is these re- certain lighting and other conditions. It lenged to step up and answer the call.  
sulting patterns aren’t visible in all in- is a characteristic of heat-treated glass  
stances,and can only be seen in certain and should not be mistaken as discol- Is This An Issue  
conditions. For example, significant oration, non-uniform tint or color, or That Can Be Fixed?  
polarized lighting increases the risk of a defect in the glass. The strain pattern  
While glass might always have some  
seeing anisotropy, as well as locations does not affect any physical properties or level of anisotropy,the important thing  
close to water, low angles, and the time performance values of the glass.” is getting to a non-discernible level.  
of day two to three hours before sunset. This is also becoming a concern for Serruys says this needs to start by im-  
According to Äppelqvist,it“becomes many glass companies. proving the heat-treatment process, so  
even more visible with thick glass (8 to “Currently, the glass industry does that heating and cooling are more ho-  
5 mm) and with heat-strengthened not consider anisotropy to be a de- mogeneous over the glass surface.  
glass … visible iridescence doesn’t fect as it is inherent in all heat-treated  
“Visualization equipment is needed  
make the glass bad from a structural glass,” adds Paul Bush, director, tech- to better optimize the process param-  
perspective, but it does make it look nical services with Vitro Architec- eters,” he says, adding that machinery  
like low quality.”  
tural Glass, noting that his company manufacturers will also need to im-  
According to Serruys,another issue is is interested in any new or improved prove their equipment. “The nozzles,  
that while official standards (i.e.,ASTM, tools, such as scanners, to allow mea- Kevlar, etc. are also heavily influencing  
EN) mention the appearance of anisot- surements and analysis of the anisot- the appearance of anisotropy,” he says.  
ropy under certain light conditions, ropy patterns,or even improved design  
Äppelqvist says it’s important to  
they explicitly state that this is not a de- and operation protocols for tempering remember this is an inherent part of  
fect. For example, he cites ASTM C1048 ovens that may help reduce anisotropy  
which says “7.4 Strain Pattern—A visibility.  
continued on page 50  
August 2017 | USGlass, Metal & Glazing  
IcofntWinuaedllfsromCpoague l4d9 Talk  
the tempering process, so the industry abilities of the tempering furnace are by the serious furnace manufacturers,”  
must recognize anisotropy throughout also important.  
the supply chain. “All thermally treated glass will ex- piped quench will need to be recycled  
This means the phenomenon can hibit anisotropy. Its level will be condi- or used only on products that are not  
he says.“Older radiation furnaces with  
be discussed and people who specify tioned by:  
building [materials] know that if the Furnace design;  
building is exposed to polarized light it Quench design;  
is a risk,” he says.“Then, the rest of the Operating recipe; and  
supply chain can pay attention to it in The operator’s skillset.  
critical (i.e., glass shelves).”  
Moreau suggests another solution:  
using annealed glass.  
“Annealed glass is by def nition iso-  
tropic (meaning it’s not anisotropic).  
each] particular project.”  
According to Moreau,the design and studied and ref ned in the last 20 years [distortion] and will not spontaneously  
burst,” he says. “European users and  
“The f rst three factors have been As a bonus, it doesn’t have roller wave  
manufacturers understood this 20  
years ago and chose lamination instead  
of tempering to provide ultimate safety.”  
However, he adds that since heat  

treatment (for structural or thermal  
issues) is inevitable, there are some ac-  
tions the glass industry needs to take,  
including the development of a stan-  
dard for measurement.  

 Precision Series Tiles  
for Roofs and Walls  
This will sensitize the specif er and  
user to the presence of the optical phe-  
nomenon,” he says.  
What Measures  
Should Be Taken?  
Moreau isn’t alone in thinking that a  
standard of measurement is important.  
Others agree this is a step that must be  
“Very soon we will need some sort  
of scale to measure this phenomenon,  
which will allow façade consultants to  
specify a class or a f gure, and for the  
glass industry to deliver against that  
specif cation,” says Äppelqvist.  
Seurrys agrees.  
In my opinion, we need a standard  
Four Unique Profiles  
unit of measure, a methodology and  
a common mathematical average,” he  
says. “I don’t see any reasons why it  
could not be worldwide. Then we can  
tack permissive values on the standard  
as a starting point, and for more sensi-  
tive projects, architects and fabricators  
can negotiate tighter values.” n  
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E l l e n R o g e r s is  
the editor of USGlass  
magazine. Follow  
her on Twitter @  
EllenGRogers and like  
her on Facebook at  
usgellenrogers to receive updates.  
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