Volume 50, Issue 12 - December 2015

Exploring Evidence

Does Ceramic Frit Weaken Heat-Strengthened Spandrel?

by Megan Headley

Is ceramic frit—the all-too-familiar pigment fused to glass to create an opaque surface—weakening spandrel panels? That’s the issue under debate among members of ASTM E06.51.13, the task group working on updates to ASTM E1300, “Standard Practice for Determining Load Resistance of Glass in Buildings.” Ceramic frit, which has been used architecturally for many years, can be used to reduce reflection and enhance glass design.

Earlier this year, industry consultant Chris Barry noted that he’s become aware of more than 20 buildings that have had some sort of spandrel glass breakage in recent years, and the application of ceramic frit to heat-strengthened glass may be the culprit (see May 2015 USGlass, page 30).

Scott Norville, Ph.D., a professor at Texas Tech University, is among those testing the theory. He notes that the focus on ceramic frit has come from “observing several pieces of thermal fracture in heat-strengthened glass with ceramic frit.”

“Typically, if you’re worried about thermal fracture and use heat-strengthened glass, and you put a frit on it, it seems it starts to break again,” he says. “Actually, I think people have been aware of it for a long time, but they’ve attributed it to different causes than the frit.”

A Hypothesis with History

It’s a hypothesis that’s only recently been explored in the U.S. architectural market, but is fairly widely recognized in other regions and industries.

According to Norville, “European standards recognized for a long time a 30- to 40-percent reduction for heat-strengthened glass, but there was no basis for it. It’s like the two and four [times as strong as annealed glass] for heat-strengthened and fully tempered glass in U.S. codes—where did [those requirements] come from?”

This issue is also under discussion in other segments of the glass industry. David Maikowski, senior business manager, automotive, with SABIC Innovative Plastics, says, “From the automotive side, this very issue is a hot topic concerning tempered sunroofs and spontaneous breakage on certain OEM vehicles. The amount of surface area coverage of ceramic frit has been shown to weaken the overall glass structure after a certain point. This topic is under discussion under a proposed revision to Global Technical Regulation No. 6 (GTR6) and I’d suspect has some correlation to architectural glazing applications worth consideration.”

Matthew Tangeman, president of Custom Glass Machinery Ltd. in Columbus, Ohio, adds that the appliance industry has documented that

“ceramic frit screen-printed highly tempered glasses don’t temper successfully at 15,000-plus psi as compared to undecorated glasses.” He notes that since architectural spandrel applications require less strength than these appliance products, this has been less of an issue for spandrel than in appliance applications, until now.

“Roll- or spray-coated ceramic frits would exercise the same behavior [i.e. weakness and breakage] in appliances,” Tangeman says.

Seeking Evidence

The architectural glass fabrication community is still looking for hard evidence in updating guidance for their own products, however.

Barry has advised adding ceramic frit to the types of glass excluded from ASTM E1300.

As Norville puts it, “I don’t want to see it excluded from E1300, I just want to see it treated for what it is.” And what it is, he says, depends on the application for which it’s being used. “Typically, in spandrel glass, the glass is heat-strengthened to prevent thermal breakage. If you’re not relying on the heat-strengthened glass to resist windload or other lateral loads, then you really don’t have a problem. But if you put a piece of heat-strengthened, fritted glass in a window and you need it to resist windloads, you could have a problem.”

Norville’s solution is to include a caution in the standard referring readers to product fabricators or manufacturers for guidance. That and other solutions were discussed in October during an ASTM E06 committee meeting in which an E1300 ballot on ceramic enamel was discussed. A consensus has not yet been reached.

According to Urmilla Jokhu-Sowell, technical director of the Glass Association of North America (GANA) and co-chair of ASTM E06.51.13, “After much discussions and debate, it was decided that the Ceramic Enamel sub-task group within E1300 discuss this at its interim meetings in January. The ballot comments asked for more precise data before rendering any changes to the E1300 standard.”

Extensive testing will likely be needed to shift industry acceptance to tempering frit-covered spandrel in load-bearing applications.

“Efforts in E1300 to make it reflect the fact that frit reduces glass strength have been fought with great [tenacity] by people who don’t believe it,” Norville says.

Running the Tests

GANA is among those experts seeking to provide necessary data for validating or denying this proposed ASTM change.

“GANA’s members have discussed the subject and looked at published papers. So far, there is still much to be learned and researched, and many questions to be answered. Questions on how widespread the breakage and causes of breakage are still to be determined,” Jokhu-Sowell says.

Norville is conducting extensive testing on this issue as well, and points to several other groups testing solid and patterned frits. “It seems that the people who are doing the testing are all reaching the same conclusion that the frit reduces the strength of new glass by 40 to 50 percent,” he says.

Norville’s test process consists of two parts. So far he has completed 4-point bend tests on heat-strengthened and fully tempered glass samples with and without frits.

“That was about 150 or 200 pieces of glass, but these were small specimens,” he says. “Then we gathered four samples of full-size glass, 38 by 76 ¼-inch heat-strengthened. One of the samples was clear with no frit to serve as a baseline, one sample was fully covered frit, and then the other two were patterned so they weren’t fully covered. We’ve just finished breaking those and are reducing the data as we speak.”

Under his direction, a student has conducted an additional 4-point bending test of frits of different colors and different application methods.

“In every case there’s been a degree of strength reduction with the lowest being about 25 percent,” Norville says.
Norville also found that the amount of applied frit impacts the degree of weakness recorded. “Patterns reduce [strength], it seems, a little less than a solid frit. There are different methods of applying the frit and there are methods that show different strength reductions and, believe it or not, I’ve even seen differences in colors.”

Tangeman isn’t surprised at findings that indicate variation depending on type of frit applied. “A spandrel glass can gain significant amounts of heat depending on color,” he says. “A southward-facing black spandrel could get very hot from center to edge or in partial sunlight, creating large thermal stresses, which could lead to failure even in lesser heat-strengthened glasses (3,500 to 4,000psi),” he says.

Tangeman has a hypothesis about why this process weakens the glass:

“Elements of the ceramic frit (a combination of metal oxide pigments, glass powder, and a carrying medium) may not fully melt-fuse into the substrate glasses, leaving particles that are harder than glass sitting on the surface bonded by fused glass from the frit to the substrate glass. If impacted, these hard particles are the fracture source that initiates the flaw which grows into a catastrophic failure.”

But until research conclusively validates the need for tempered spandrel in specific applications, it will be difficult to draw conclusions.

Resistance to Theory

For many, heat-strengthening for frit-covered spandrel has been the standard in the U.S. for so long it’s surprising to hear it might not be the best option.

For example, Warren MacLean, business development manager with Superior Glass, a glass supplier and installer in Karate, British Columbia, says, “Ceramic frit paint is permanently fused to the glass once it’s put through the tempering furnace. The ceramic frit paint must be applied to the glass before it’s put through the oven. I have never heard of issues where the ceramic frit has weakened the glass, as the paint just fuses to the surface.”

Others, however, are accustomed to issues and questions from customers about spandrel glass, so they’re already advocating the use of tempered glass.

Troy Johnson, vice president of SIGCO Inc., a glass fabricator in Westbrook, Maine, has produced spandrel with ceramic frit as well as other coatings since 2011. He notes that unless fallout protection is required, his customers consistently prefer to use ceramic frit spandrel compared to other options—but that doesn’t mean they’re using it correctly.

“We are regularly faced with applications that are not appropriate for the product (i.e. vision areas), and installation details that are incorrect,” Johnson says. “We do our very best to try to educate owners, architects, contractors and glaziers on the appropriate use for the product and how it should be installed.”

The company advises that fully tempered glass be used in lieu of heat-strengthened glass, and at a minimum on the spandrel lite.

Innovative Structural Glass Inc. (ISG) in Three Rivers, Calif., has offered spandrel with ceramic frit for 17 years without seeing evidence that it can weaken heat-strengthened spandrel glass. ISG president Manuel Marinos, however, says the company encourages the use of tempered glass in any case.

“In our market niche it’s all about transparency, therefore we rarely utilize spandrel in our applications. If we did, however, the glass is always tempered for structural reasons. We stay away from heat-strengthened glass,” Marinos says.

the author

Megan Headley is special projects editor for USGlass magazine.
She can be reached at mheadley@glass.com.

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