Page 56 - Glass-Technology International no. 2/2018

TEMPERING

Glaston

TOWARDS BETTER ANISOTROPY

Riku Färm glass. Customers, however, terns. And everyone has MEASURING
sometimes refuse to accept their own opinion about ANISOTROPY
Product Manager
glass with anisotropy levels what looks good and what The main problem is that
Glaston Finland Oy
they feel are too high, mak- looks bad. So, acceptable anisotropy can be very dif-
ing the phenomenon an is- levels differ from project ferent in different condi-
sue for glass processors as to project. The prominence tions, such as geographi-
hen it comes to well. of anisotropy also depends cal location, time of year
the visual quality on visual conditions sur- and time of day. Also, the
W of heat-treated PHENOMENON WITH rounding the glass. “level of anisotropy” is very
NO STANDARDS
glass, anisotropy has be- Seeing anisotropy is easy. hard to quantify. It’s not
come more and more an To date, there are no stand- All you need is a polariza- like a roller wave, where a
acknowledged concern for ards when it comes to ani- tion filter, like the one in trained eye can evaluate the
glass processors, architects sotropy. The intensity of sunglasses, with polarized amount of roller wave just
and their clients. strain patterns may vary light coming through the by looking at the glass. An-
Since anisotropy is caused from slightly visible under glass. Measuring it, how- isotropy may present itself,
by stress differences in certain conditions to obvi- ever, is much harder. How for example, as vertical or
glass during the heating ous under most conditions. can you reliably measure diagonal lines in the glass,
and cooling phases, it is Anisotropy can also appear something that has no or as spots in the glass such
inevitable in heat-treated in different kinds of pat- standards? as seen in the rear windows
of cars. Then again, which
one of these is best? Or
worst? As long as a univer-
sal standard doesn’t exist,
there’s no way to answer
this question. Everyone has
their own opinion – which
means we may even need
more than one method to
evaluate anisotropy.
Having a unified evaluat-
ing method, or methods,
would certainly make our
lives easier. So, as we are
slowly moving towards an
anisotropy standard, what
can be done in the mean-
time?
Let’s consider roller wave

54 Glass-Technology International 2/2018