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Roller wave distortion is a condition found in heat-treated glass that causes the glass surface to have imperfections known as peaks and valleys. These alternating high and low points result in the glass exhibiting optical distortion. Most noticeably, images reflected in the glass appear to be rippled.
Roller wave distortion is very common in heat-treated glass used on construction projects as opposed to annealed glass, which is not treated. There is no ASTM or other standard that defines how much distortion is acceptable from a glass producers and fabricators. Consequently, it is important that building owners, architects and general contractors clearly express their expectations to glass companies well before glass production starts.
Roller wave distortion is a result of the heat-treatment process used by all architectural glass producers. To strengthen sheets of glass, they are moved into a furnace on a track lined with rollers and are heated and cooled to specified temperatures. During this heat soaking process, the glass becomes slightly more pliable and can sag in the space between the rollers. Thinner sheets of glass tend to sag more than thicker sheets, and as a result, they tend to have more distortion.
The valleys this produces are virtually imperceptible to the human eye in the factory. However, when the glass is installed in a building, the surroundings and the varying light conditions can make it so that even very small amounts of distortion can be seen.
Given that roller wave distortion is common and there is no defined standard in the industry for allowable peaks and valleys, building owners and contractors have little recourse if installed units have significant optical defects. But, if expectations are addressed upfront, it is more likely that a positive outcome ultimately can be reached. Three ways to do this are:
By using one or more of these approaches, stakeholders can help ensure that the product they receive meets their requirements.
In the past, the only testing commonly done for roller wave distortion in heat treated glass involved viewing the reflection of a “zebra board” in the glass as it rolled out of the furnace. If the pattern from this board that was hung above the production line was highly distorted in the operator’s opinion, the glass would be rejected, and steps would be taken to address the problem with subsequent pieces.
However, both glass production techniques that mitigate distortion and glass quality testing continue to improve. Today, cameras mounted at the end of the production line can capture images of every square inch of a sheet and produce a 3D representation that identifies any distortion that is present, from roller waves to pitting or other problems. Sheets not meeting specifications can be rejected based on a numerical value and not the operator’s opinion. There are also handheld devices called three-point contact gauges that can be used to measure peaks and valleys in a sheet of glass that is no longer located in the glass production facility.
With these advances in distortion detection and a standard for the measurement process (ASTM C1651) available, Lerch Bates can perform assessments and provide insights to owners and general contractors about the quality of their glass. Measurements can be taken after glass has been installed and those numbers can be used in subsequent conversations with the supplier.
A more effective use of Lerch Bates’ expertise, however, is to work with architects, builders, and owners to develop specifications for the purchase of heat-treated glass. Stakeholders who understand the origins and implications of roller wave distortion can be proactive in their decision making regarding acceptable tolerances.
Ultimately, setting expectations with glass suppliers before a project gets underway can help prevent costly and time-consuming conflicts. As the design aesthetic of today’s construction trends toward the inclusion of greater quantities of glass, this forward-looking approach becomes even more critical.