Understanding and Eliminating White Haze in Tempered Glass

By Ellen Rogers

Two decades ago, roller wave distortion in tempered glass was a big issue. The industry spent years debating how to measure, test and control it. That hard work was a success. In 2008 ASTM International published its Standard Test Method for Measurement of Roll Wave Optical Distortion in Heat-Treated Flat Glass, giving the industry a consensus-based method for testing roller wave. Thanks to that document and advances in tempering furnace technology, fabricators today have the tools and resources they need to monitor and control optical distortion in glass.

But the issues go beyond roller wave. Anisotropy, or iridescence in the glass, was the next big concern. Again, thanks to a collaborative industry effort, in 2021, ASTM published its Standard Test Method for Measuring Optical Retardation in Flat Architectural Glass. That document, combined with advances in inspection and measuring equipment, allows fabricators to minimize iridescence in their tempered glass.

With roller wave and anisotropy under control, what’s next? Lately, the industry is talking more about white haze. While these ghost-like markings aren’t a new phenomenon, the industry’s ability to detect and control them is new.

What is White Haze?

For years fabricators have pondered the mysterious white markings on glass. Sometimes they’re visible as soon as glass leaves the tempering oven. Other times, the marks are only noticeable in certain lighting conditions.

The causes can also vary. The hazy white marks can come from dust, roller residuals or actual deformations caused by a mechanical pressure that is too strong between the glass and the rollers.

According to Nate Huffman, director of sales and support for LiteSentry-Softsolution in Burnsville, Minn., white haze is a general term for several issues.

“White haze is one of those things where it’s been difficult to find the exact source. Where is it coming from? Is it the heating or something before that?”

Some issues are due to pre-processing before tempering. For example, grinding dust on the glass that wasn’t fully washed off might be baked on during tempering. The “skunk stripe,” white markings down the center of the glass, often happens during the tempering process.

“As the glass goes into the furnace, the heating can cause the edges of the glass to rise so the glass is riding on its center where it’s curved. That can cause fine abrasions in the glass,” says Huffman. “Sometimes, you can see it right away. Other times it’s [in a certain] lighting or angle that it shows up.”

Nancy Mammaro, CEO of the Italian machinery company Mappi, describes white haze as the scratches on the bottom of the glass in the middle.

“At the beginning of the heating cycle, the glass surface that touches the rollers [is heated] faster than the upper side. So the glass bends and touches the rollers only in the middle,” she says. “If the movement of the rollers is not smooth and has vibrations, it makes little scratches on the glass touching them.”

Raising Awareness

If white haze isn’t new, why is the industry talking about it now? Because technology improvements allow fabricators to detect it reliably. For example, Softsolution worked with Glaston to develop a White Haze Scanner that is an artificial-intelligence-based online solution that provides a visual indication of where the problem is on the processed glass. According to Glaston, “the neural network developed detects white haze in processed glasses and objectively categorizes the haze marks by strength into three intensity levels.”

Viprotron’s White Haze Scanner is another solution that can detect the areas where oven-related damage to the glass surfaces has occurred after tempering. According to the company, “these are analyzed and localized in
real-time with the first occurrence to polish out the defects before delivery to the customer.”

“In the past, we knew it was there, but it’s been since [the development of] the white haze scanners [that people are] spreading the word,” says Rainer Feuster, vice president of sales and authorized officer of Viprotron. “Today, fabricators know it can be avoided or repaired, so now [more] are talking about it.”

Process Improvements

According to industry experts, most white haze issues are caused during the processes before tempering. To avoid these issues, fabricators must be proactive in those early steps.

“Make sure you do everything you need to do prior to tempering to eliminate it,” says Huffman. “That means keeping the washer clean, water recycling, maintaining the grinders and seamers.” Step two is in the tempering furnace, and includes taking care of the rollers.

“Dirt on the rollers can also cause abrasions, so if the rollers are not clean, that can cause scuffs,” says Huffman.

Fabricators should check rollers to see if they are rough, cracked or brittle or if there are any solid particles on the surfaces. Issues such as these should be addressed and corrected. Also, make sure rollers are level. “If they are not properly adjusted, white haze can be caused by mechanical pressure that is too strong between the glass and the rollers,” according to Glaston.

Until the rollers can be thoroughly cleaned, some companies use a dry lubricant such as Sulphur dioxide (SO2) to prevent tempered glass marking. The dry lubricant is injected into the tempering furnace and acts as a buffer between the rollers and the bottom surface of the glass. It can help eliminate, minimize or reduce bottom surface markings.

Mammaro says Mappi furnaces do not need SO2, as the company developed what it calls Syncro transmission technology.

“The Syncro system uses a brushless motor and a rubber chain to move the rollers and eliminate the vibrations created from the transmission,” she says. “Plus, we use the convection to put the glass straight as soon as possible, so we can avoid this issue.”

There are other steps that can improve the actual tempering process. This includes being aware of and controlling the furnace temperature. As Feuster says, “Fabricators must better control their furnace. With older furnaces, everything is limited. Newer furnaces are far more sophisticated in their controls.”

For example, fabricators may be able to address specific issues by adjusting the furnace settings. This could involve decreasing the temperature of the bottom section and/or increasing the top convection, according to Glaston.

Quest for Quality

The glass tempering process has seen many changes over the past 20 years. Those changes have brought better and better glass quality. Many of those improvements are due to better equipment and machinery, along with the availability of standard test methods. There’s no white haze test method, but the possibility is there. As with roller wave and anisotropy, it takes a group of people to step up and work together to make a change. And as architects and specifiers continue to demand better and better glass, that might be the nudge to drive the industry forward.

Ellen Rogers is the editor of USGlass magazine. Follow her on Twitter @USGlass and like USGlass on Facebook to receive updates.

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