Building envelopes fulfill different roles and functions, from creating the image of the building to offering comfort and protection to the occupants. Facades have evolved to convey the concept of the architect and the inspiration of the owner. Today, transparency and neutrality dominate façade aesthetics.
These trends have not only resulted in an upsurge in the use of glass as one of the preferred materials for the façade of buildings, but also increased the expectations placed upon it, whether with regard to its dimension or its aesthetics. One question still remains pertinent to architects and engineers: How do we make sure that a thoughtful design does not get damaged by unwanted optical effects that distort the whole façade (i.e. anisotropy)?
This question and the resulting search for practical solutions has been evident in Saint-Gobain’s latest projects, most notably, the LAKTHA tower in Saint-Petersburg, and has pushed Saint-Gobain since 2015 to build up a solid knowledge base and develop skills to better understand visual effects and therefore offer higher quality glass for these remarkable projects.
What is anisotropy and what do we observe exactly?
Those who have worked with heat-treated glass have most probably observed some visual inhomogeneity on the glass surface, and are aware of the dreaded phenomenon of ‘anisotropy’. Anisotropy, also known as “leopard spots”, “iridescence”, “quench marks”, “Brewster marks” or “strain marks”, refer to the manifestation of patterns and colorful areas after the thermal heat-treatment of the glass.
To-date, there are no standard defining viewing conditions, methodologies or instruments to measure anisotropy. There is also no common value, definitive measure or scale system. The European and American standards (EN, resp. ASTM) still consider this phenomenon as part and parcel of the heat-strengthening or glass-tempering process and subsequently, impossible to eliminate.
To complicate matters further; to observe anisotropy on the glazing installed on a building, different factors must converge. On a façade, anisotropy will not be observed in the same way every day and by everyone because the phenomenon is perceived when different factors combine. For example:
- The presence of inhomogeneity induced by the thermal treatment of glass
- Weather conditions (the polarization of the natural light changes according to meteorological conditions and position of the sun)
- The optimal angle of observation toward the façade
Is anisotropy an intrinsic problem of heat-treated glass?
Toughened or tempered glass is processed by controlled thermal treatments to increase its strength compared to standard glass. This glass is used in a variety of demanding applications, for example, architectural glass to enhance safety or to avoid thermal breakage.
The thermal treatment process, called tempering, includes a heating phase and a cooling phase. Tempering puts the outer surfaces of the glass into compression and the interior into tension. Such stress causes the glass to crumble into small granular chunks instead of sharp shards if the glass breaks.
Depending on the speed of the cooling phase, the level of strength is determined (from heat strengthened to fully tempered glass for higher speed). Anisotropy is visible when unbalanced residual stress, that is induced by the process, reflects a polarized light. As daylight is naturally polarized, anisotropy is especially visible with blue skies or water reflections.
Can we control or minimize anisotropy?
Saint-Gobain manufactures both raw and coated glass as well as processed glazing units. Anisotropy is therefore well known but does not fit the quality standards of the company anymore. For a few years, Saint-Gobain’s R&D and production teams have been developing the skills, the knowledge and the right tools to minimize anisotropy.
Different parameters influence the magnitude of anisotropy such as the homogeneity of the glass and the thermal evolution along the heating and cooling phases. The significance of anisotropy also depends on the glass substrate used (i.e. glass type, presence of a coating or not and the thickness, etc.). The scale of the phenomenon is only managed by the ability to measure it and by controlling the tempering process.
In 2015, Saint-Gobain developed an on-line inspection tool in its R&D center in northern France which was implemented in one of the processing plants of the Saint-Gobain Glassolutions Network at the same time.
From the beginning, this system has been able to quantify a physical value related to anisotropy, the so called optical retardation, in a consistent way, allowing Saint-Gobain to deliver, high quality glass with reduced anisotropy.
Anisotropy and Saint-Gobain glass, today and in the future
Today, in Saint-Gobain’s production and transformation plants dedicated to the façade business, the anisotropy of all heat-treated glass is quantified, minimized and recorded just as any other quality control data should be.
In addition to this major processing improvement, EasyPro, a unique and innovative protective coating developed by Saint-Gobain, is a huge contributor to the reduction of anisotropy during the tempering process. EasyPro sets the heating of coated glass to the same level of ease as for clear glass. At the end of the heating phase, the glass is thermally homogeneous and anisotropy is minimized drastically.
The combination of Saint-Gobain’s R&D, its measuring device and skills in processing, as well as the unique contribution of EasyPro, gives Saint-Gobain a strong position in delivering the most efficient glass with the best aesthetics for demanding projects.
At the end of the day, the evaluation of such a complex phenomenon will be made by a unique judge: The human eye! Due to the necessity of measuring the physical value for the optimization of the quality of the delivered glazed units, Saint-Gobain thus keeps working on the perception of anisotropy. A second ranking scale may be needed to give more information about the actual appearance of the glass after installation.
Discover Saint-Gobain’s innovative solutions for façade and architectural glass at Glasstec 2018 in Dusseldorf from 23rd to 26th of October in Hall 11 /A24.
Article courtesy of Saint-Gobain