Taking control of anisotropy

By Miika Äppelqvist

One of the most controversial issues in the glass industry today (and one of the most read articles in Glastory) is the subject of anisotropy, and whether it is a quality defect or not. This is the term used to describe the phenomenon of patterns and colorful areas in heat-treated glass that become visible under certain light and viewing conditions. And sometimes, this effect can be disturbingly noticeable.

Glass anisotropy: Who to blame?

Anisotropy is increasingly the subject of dispute. Glass suppliers rely on standards that state anisotropy is not a defect but rather an inherent part of the tempering process. Designers and their clients, however, consider it a defect and refuse to accept glass with visible anisotropy.

Technically, anisotropy may not be a defect. Yet the fact that end customers inevitably perceive it as a major problem makes it a real issue. Visible anisotropy does not make the glass bad from a structural point of view, but the glass indeed looks as if it is of poor visual quality.

Instead of debating about what is acceptable or who is to blame, glass processors should shift their focus to find solutions that satisfy quality-conscious customers. The best response is to deliver the highest possible quality glass products without any irritating effects.

Light in the environment is crucial

A more scientific definition of anisotropy is that a material (in this case glass) has directionally dependent properties. Glass has number of properties but in this case the most valid are tensile stress, conductivity and refractive index.

The anisotropy phenomenon is more noticeable in certain conditions.  For example, two or three hours before sunset the effect is considerably amplified in glass installations close to the seaside. In practice this means the anisotropy effect is more visible when there is polarized light in the natural environment. Polarized light together with birefringent property of glass and mechanical stresses in the glass that are caused by tempering process. Ok, I know this goes now deeper into the physics.

Anisotropy can be always seen with polarization filter (like sun glasses) but sometimes it is visible with naked eye. It is important to understand that if the anisotropy can been seen with naked eye there is some light polarization happening already in the environment. Because of this the final installation place has a huge impact on the visibility of anisotropy. Polarization can happen through reflection (light beam is reflected from a dielectric medium and gets polarized) or through scattering (e.g. which can happen on a clear day in the atmosphere) Read more