Glass Distortion and Anisotropy

With both heat-strengthening and tempering, it is possible that glass will develop either optical image distortion or anisotropy, which refers to patterns or visual effects in glass that are not considered defects.

Distortion occurs in glass for many reasons, including glazing pressure, wind load, temperature and barometric pressure changes—or even changes in altitude between where a glass is made and where it is installed. Because of its fluidity at high temperatures, glass is also susceptible to roller wave, bow, and warp while it is being heat treated.

• Produce all heat-treated glass for a given project on the same equipment, using the same processing parameters
• Use thicker glass, as it is less prone to distortion
• Orient heat-treated glass so that roller wave (the periodic wave imparted to glass during heat-treatment, measured by the peak-to-valley distance) is parallel to the windowsill/header
• Procure glass from manufacturers that use advanced automated optical inspection systems, which identify defects more precisely and efficiently than manual inspection.

Conversely, strain patterns in the glass from heat treatment can cause anisotropy, which is not considered a defect in the glass. Anisotropy is not entirely avoidable, but it can be quantified during production to ensure consistent results. That's why at Vitro Architectural Glass, we recommend doing full scale mock-ups under job site conditions to evaluate the optical aesthetics of a specific heat-treating process.

While there is no industry standard to quantify permissible heat-treated glass roller wave, the tolerance of five thousandths of an inch from peak to valley is often specified. Vitro recommends using a millidiopter specification if it is available instead.

To learn more about optical image distortion, please review TD-138. To learn more about strain patterns in tempered and heat strengthened glass, please review TD-115. Technical documents can be found at vitroglazings.com/TD.

For more information about heat-treating or any other glass question, visit vitroglazings.com or call Vitro at (855) VTRO-GLS or (855) 887-6457.