by Riku Färm
Ever looked at a building’s glass facade through polEver looked at a building’s glass facade through polarized sunglasses? If so, you may have noticed patterning or spotting on it. This phenomenon is called anisotropy – or iridescence – and occurs in tempered glass as a result of heat treatment. Although there are still no standards for acceptable anisotropy levels in glass, some breakthrough in regard to the phenomenon has taken place recently.
When it comes to the visual quality of heat-treated glass, anisotropy has become more and more an acknowledged concern for glass processors, architects and their clients.
Since anisotropy is caused by stress differences in glass during the heating and cooling phases, it is inevitable in heat-treated glass. Customers, however, sometimes refuse to accept glass with anisotropy levels they feel are too high, making the phenomenon an issue for glass processors as well.
Phenomenon with no standards
To date, there are no standards when it comes to anisotropy. The intensity of strain patterns may vary from slightly visible under certain conditions to obvious under most conditions. Anisotropy can also appear in different kinds of patterns. And everyone has their own opinion about what looks good and what looks bad. So, acceptable levels differ from project to project. The prominence of anisotropy also depends on visual conditions surrounding the glass.