Heat-treated glass (heat-strengthened or tempered) can have an optical phenomenon that is called strain pattern or quench pattern. This phenomenon can appear as faint spots, blotches, or lines. This is the result of the air quenching (cooling) of the glass when it is heat-treated and should not be considered a glass defect.
The strain pattern or quench pattern is a result of heating the glass to its softening point and quenching the glass with high velocity air. Heat treatment of glass is required when safety glazing is required (tempered), or to reduce thermal breakage potential or to improve wind load resistance.
While not normally visible, a pattern of perceived faint light and/or dark spots or lines in heat-treated glass may become apparent under certain light and viewing conditions. The “quench pattern” is most apparent under polarized light with a visible horizon and viewed at an oblique viewing angle to the glass surface. The visibility of the pattern decreases as the viewing angle to the surface of the glass increases. When viewing from the interior of the building, the quench pattern may be visible from a 10° viewing angle and not apparent at a 90° viewing angle from the surface of the glass. When viewing the glass in reflectance from the exterior of the building, the quench pattern may be visible when looking at the glass surface at a 30-60° angle. Visibility of the quench pattern may be accentuated with thicker glass, tinted glass substrates, coated glass and multiple panels of heat-treated glass in laminated or insulating glass products. As a result of variations in fabrication systems (or tempering furnace systems), the quench pattern may vary from one fabricator to another.
As frequently seen in back and side panels of cars, the quench pattern in the fully tempered glass can become more visible when wearing polarized sunglasses. Polarizing filters or lens for cameras will create the same phenomena and may cause the pattern to become more visible.
This strain pattern is recognized by the European Standards as seen in the following extract from BS EN 12150 – 1
The toughening process produces areas of different stress in the cross section of the glass. These areas of stress produce a bi-refringent effect in the glass, which is visible in polarised light. When heat soaked thermally toughened soda lime silicate safety glass is viewed in polarised light, the areas of stress show up as coloured zones, sometimes known as ‘leopard spots’. Polarised light occurs in normal daylight. The amount of polarised light depends on the weather and the angle of the sun. The bi-refringent effect is more noticeable either at a glancing angle or through polarised spectacles.
Construction sites may yield viewing angles and conditions that cause the quench pattern to become visible. However, upon completion of construction; the presence of interior walls; finishes; furniture; and plants frequently results in the strain pattern being less visible or not visible at all.
The stresses introduced in the heat-treating of glass are an inherent part of the fabrication process, and while they may be affected or altered depending on the heating process, controls and/or quench design, they cannot be eliminated. Design professionals should be aware that quench patterns are not a defect in heat-treated glass and, therefore, are not a basis for product rejection.
The phenomenon of quench pattern may be visible in any heat-treated glass. While pre- or early-construction applications will not provide final project conditions, consultation with the glass supplier and viewing full size mock-ups under typical site conditions and surrounding landscape may be helpful in evaluating the potential for visibility of the quench pattern.