Volume 43, Issue 6 - June 2008

Cracked Up
Experts Share Insights on the Heat-Soaking Process
by Ellen Rogers


Congratulations and thank you for taking this survey. Our goal was to prove, statistically and scientifically, once and for all that yes, glass does indeed break. And, yes, sometimes glass breaks for what seems like no reason.

In all Seriousness …
This survey may have taken a satirical look at glass breakage, but it’s still true that glass has been known to break spontaneously on occasion. Why? One cause for spontaneous breakage is a concentration of thermal stress, the physical characteristic causing the breakage. In some applications the thermal stress comes from poor glass edge quality. In tempered glass specifically, the stress is concentrated around nickel sulfide (NiS) inclusions, which can result from the float production process. The time and/or temperature change of the NiS can disrupt the surface compression/core tension layers of the glass at high levels of surface compression—typically greater than 10,000 pounds per square inch (psi) for tempered glass—and result in breakage. 

Since the origins of NiS stones can be traced to the float process, the stones also can be present in annealed and heat-strengthened glass. However, heat-strengthened glass with a surface compression of 3,500 to 7,500 psi and annealed glass with a surface compression of less than 3,500 psi have historically not experienced spontaneous breakage.

According to industry consultant Bill Coddington, the probability of NiS stones in glass is very low due to the efforts of float glass producers. 

“The primary glass plants in the United States do an excellent job selecting the raw materials and production equipment used in order to minimize the chances that nickel could be introduced into the float glass batch,” says Coddington. “When contamination occurs it is usually for a short period of time and often the periodic sample testing that the primary producers perform catches the problem before the glass is shipped. The chances of occurrence are volume-related and, therefore, directly proportional to the thickness of the glass.”

For float producers, careful material selection can help ensure their batch is free of nickel sulfide. But there are also methods that can be used on the tempering end to check for such inclusions. Heat-soak testing (HST) is one way fabricators can check for the presence of NiS in their tempered glass. HST is a destructive process through which the tempered glass is heated to a certain temperature, held for a certain amount of time and then cooled to room temperature. If NiS inclusions are present the glass will explode during the test (see a related article Temper Temper from the April 1998 USGlass by visiting the Only Online section of www.usglassmag.com). 

While HST is definitely a quality assurance measure, it’s not one that’s necessarily a given for every tempering operation. The test is not a guarantee that spontaneous breakage will not happen; it only reduces the chance of it happening. And experts agree, the process isn’t always right for every company or every application.

Soak it Up
Owatonna, Minn.-based Viracon has been using HST for more than 20 years.

Rich Voelker, Viracon’s vice president of technical services, says they installed their first heat-soaking oven because, at the time, they were supplying more tempered glass than heat-strengthened glass.

“So there was more potential for spontaneous breakage in the field and we had a desire to offer and provide a means to our customers that would help them avoid a situation like that,” says Voelker. 

Today, however, Voelker says a higher percentage of the glass his company provides is heat-strengthened versus fully tempered, but it does recommend HST any time tempered glass is specified.

J.E. Berkowitz (JEB) LP located in Pedricktown, N.J., has been heat-soaking tempered glass for more than ten years.

“We had an opportunity [to work] with an overseas fabricator who was supplying projects here in North America. The company also wanted to have a local fabricator provide the glass for smaller projects that required heat-soak testing,” says Arthur Berkowitz, president of JEB. “We felt there was enough opportunity ten years ago to invest in our initial chamber.” 

According to Berkowitz, probably less than 1 percent of their fully tempered glass undergoes HST, but he expects this number to grow. One reason for this anticipated demand is the increasing desire for structural glazing applications. 

“For certain unique products that we offer we make it a requirement to [put that glass] through heat-soak testing,” says Berkowitz. “For example, we have a complete engineered glass division that provides glass for canopies, point-supported, high-risk or other demanding applications. When we provide that [glass] we provide it with the requirement/recommendation that fully tempered glass be heat-soak tested.” 

Craftsman Fabricated Glass in Houston began HST last September after the company purchased a laminating line. Vice president Phillip Lawrence explains his company is able to heat-soak by modifying the autoclave cycle so it only adds heat and not pressure.

“We’ve been seeing more and more specifications for heat-soaking, so we added it because we bought the autoclave,” says Lawrence. “It’s not a complicated process, and if we continue to see the test specified we’ll most likely purchase an oven specifically for heat-soaking.”

As with Berkowitz, Lawrence says the increasing demand for structural applications has accelerated the demand. 

“[Spider wall systems] are designed to deal with some breakage, but they are not as forgiving as when the glass is in a frame,” Lawrence says.

Even more recently, Arch Aluminum & Glass began heat-soaking in early April.

“We’re doing it because it’s market-driven,” says Max Perilstein, vice president of marketing. “We’ve had requests for it for years, but we didn’t see it as necessary or beneficial. But now, as more glass that [could have questionable quality] in need of heat-treating is coming in from overseas we’re seeing a growing demand.”

Perilstein says the investment to begin heat-soaking isn’t too significant from a cost factor, but it is an important investment.

“It’s an important investment to make sure we continue to take care of our customers,” he says, explaining that before they began the process they even questioned whether it was something they really needed to do. “But we were getting so many requests for it that we could not ignore it,” says Perilstein. 

Perilstein adds that one of their greatest challenges in offering the test is overcoming the misconception that heat-soaking glass will eliminate the chance of spontaneous breakage completely. 

“It only reduces the chance,” says Perilstein. 

Attention to Detail
The process and investment to begin heat-soak testing may not be as significant as launching an insulating, laminating or tempering line, but it’s not without considerations. For starters, there are no North American standards or specifications that regulate the heat-soak test. While there have been industry discussions about developing a standard, nothing has yet come to fruition. 

“The tempering division of the Glass Association of North America (GANA) has been asked by its members to look into adopting a standard or possibly developing a North American standard,” says Greg Carney, GANA technical director. “The issue will be on the agenda for the division’s standards and engineering committee meeting during the GANA Fall Conference.”

Presently, though, most companies say they follow the European standard (EN 14179-1:2005).

“From an operating and manufacturing standpoint you have to know what the standards are. That’s been a challenge in our industry because there are no standards in North America,” says Berkowitz, who explains that when his company relocated to its new plant in Pedricktown last year they also bought a new heat-soaking oven to operate in accordance with the European Standard EN-14179-1:2005. “We had it built, certified and calibrated in Europe,” adds Berkowitz. “From a capital standpoint, it’s clearly nowhere near the expense of a tempering oven and operating it is not as sophisticated as a tempering oven, but, on the flip side, the documentation and the procedures if you follow the European standards are very precise.”

Voelker says Viracon also heat-soaks in accordance with the European standard.“

I think the Europeans have investigated this whole phenomenon of NiS breakage in tempered glass much more than we have in the United States,” says Voelker. “So, I would say there is a universal standard out there that people can reference if they want to perform a heat-soak test and from our perspective it is a reliable process that has worked wonderfully for us.”

According to Voelker, the European standard is very prescriptive. “It’s not so much how high the temperature is to which you subject the glass or to what extent, it’s really describing how you achieve that comfort level by making sure you’re heating the glass and not the air itself.” He continues, “It’s very descriptive as far as the process itself, including [taking into account] spacing between glass lites and certifying the equipment is achieving the temperatures uniformly in the oven.”

Craftsman and Arch also conduct heat-soak testing according to the same standards.

Not Just Yet
While some companies are fully supportive of HST, others have yet to jump on board with the process. It’s not because they don’t believe in it, but rather because the market they serve has yet to demand it.

In Charleston, S.C., for example, Jack Hoey, president of Coastal Glass Distributors, says they have not seen a real demand for the test. “If our ballgame was the high-rise market, it would be a different deal,” says Hoey. “One of the things that has really driven a lot of this is some of the high-profile buildings where you’ve got panes that break and over time they are a danger to pedestrians and they are expensive to replace.”

Trent Hartley, sales and marketing director at Coastal, adds that the growth of point-supported and spider wall applications is another reason that some companies are seeing the test specified more and more.

“From a safety standpoint, I fully understand [heat-soaking for spider wall applications],” says Hartley. “I would never put one of those systems together without a heat-soak test.” 

Hartley also says he has not seen a great demand for HST. “In our area, specifiers are usually not worried about the breakage and are more focused on the ASTM 1300 wind-load calculation.” He adds that spec writers in his area are not very familiar with NiS breakage issues.

“I’m still educating them on ASTM 1300. Plus, we’ve had a lot of code changes in the past few years and the architects here are still playing catch-up,” says Hartley. “Heat-soaking hasn’t come up with them, I think, because they are still learning about all of the impact codes and wind-load requirements. I see a lot of architects going back to the drawing board because their opening sizes with tempered glass will not meet our wind-load calculations.”

Barber Glass Industries in Guelph, Ontario, may not be HST currently but, according to sales and marketing manager Mike Wellman, it could very well be doing so at some point in the future.

“We’re expanding and, on September 1, we’ll begin manufacturing IG that will be as large as 130 by 200 inches and we’ll be able to marry that together with our laminating, tempering and CNC capabilities. That will change our focus with respect to heat-soaking as there will be more potential for it,” says Wellman.

As with other fabricators, Wellman says he’s started seeing more requests for HST from architects, especially in regards to structural glazing.

“Since we do not currently heat-soak our option is to see if the architect would take a tempered-laminated product,” Wellman says. “If you use a tempered-laminated product, even if it breaks it stays integral.”

Wellman also is in agreement with Hoey, saying heat-soaking is definitely not a test for every piece of tempered glass.

“It’s a process that has its place. You wouldn’t heat-soak a small interior wall partition,” says Wellman. “It would cost more to do the test than it would to replace the glass if it breaks.”

For Coastal, Hoey says he has no plans to begin heat-soaking at this time, but it’s certainly something he could consider in the future. “We don’t think it’s a waste of time, in particular in the liability environment,” says Hoey. “If it affords you some extra protection, if your personal assessment is that the benefits are not outweighed by the cost [then it’s right for that business] … from a safety point of view and also a replacement point of view, you are much better spending a little bit of money upfront rather than risking the cost of replacement later on.”

What Next?
With more architects specifying HST, the number of North American companies conducting the test has grown. But, will it continue to make its way into mainstream fabrication?

“When we started this there were probably more differences of opinions than there are now as to the validity of heat-soak testing,” says Voelker. “I think the European studies and papers that have been written and are available show a tremendous amount of research has gone into this notion of heat-soaking and they validated that a proper HST process will give you somewhere in the neighborhood of a 99 percent confidence level that spontaneous breakage due to NiS inclusions will not occur in tempered glass once it’s been heat-soak tested.” Voelker adds that Viracon has even gone so far as to offer a warranty on heat-soaked tempered glass. “That’s unheard of in our industry because no one warrants glass breakage. But, the only warranty we have for glass breakage is for glass that we have heat-soaked. It’s not only for the glass, but covers labor as well.”

Still, the number of spontaneous breakages due to NiS is relatively slim. Lawrence, for example, says he’s yet to lose a tempered lite in the heat-soak chamber, and Coddington is skeptical about some aspects of the test, as well.

“In my 20 years of tempering and heat-soaking glass I have seen only a handful of cases in the field where the cause of the spontaneous breakage proved to be NiS stones that could have been prevented by heat-soaking,” says Coddington. “I think in very critical, monumental, projects heat-soaking of the primary structural glass components may be justified, but if the need is that critical, tempered-laminated glass should be used to eliminate that concern completely.” He adds that sometimes fabricators with the capability to heat-soak recommend it and promote the process to the design community for applications where it’s not critical. “The extra cost due to the extra handling and energy required to heat-soak the glass needs to be considered. Heat-soaking is not 100-percent effective and most of the causes of spontaneous breakage of tempered glass are not reduced by heat-soaking.”

Still, many say the demand for heat-soaking will grow; by how much is anyone’s guess.

“It’s more of a wait and see, and part of that might depend on future events,” says Hoey. “If there is a failure resulting in loss of life that might have been prevented had the testing been performed … rightly or wrongly, that could push the industry in a certain direction.” 

Ellen Rogers is a contributing editor for USGlass magazine. 

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