In architectural glazing applications, the use of heat-treated glass reduces the breakage potential due to stress induced by temperature change and strain from wind and snow loads considerably. In most cases, safety glazing is required by applicable building codes, and fully tempered or laminated heat-strengthened glass often is specified.
The most commonly used type of safety glass is tempered glass. It is made by heating presized glass panels to approximately 1200°F (650°C), then rapidly cooling them through quench. The result of using a fast quench is a glass panel with the surfaces and edges in compression and the center of the glass in tension.
Tempered glass is four to five times stronger than conventional glass, and the tempering process dramatically changes the break characteristics of the glass. When tempered glass is broken, it shatters into tiny cube-shaped pieces rather than shards. This drastically reduces the potential for injury caused by sharp edges. Despite the high strength and desirable breakage characteristics, tempered glass is uniquely vulnerable to a catastrophic failure mode that is often referred to as spontaneous breakage.
During the production of float glass, small nickel-sulfide (NiS) inclusions can form due to minor contamination of the glass by nickel-bearing metal alloys such as stainless steel. Nickel-sulfide inclusions are very small, and their occurrence in the final glass product is covered under ASTM C1036, Standard Specification for Flat Glass. The standard permits blemishes — including nickel-sulfide particles — in float glass, depending on glass size and quality.
While nickel-sulfide inclusions may be present in annealed or heat-strengthened glass, the problems they cause are specific to tempered glass because of the tempering process. During the annealing and heat-strengthening processes, glass is cooled at slower, controlled temperatures. This allows the nickel-sulfide particles that are present to complete a phase transformation during which they fully expand to their final size and remain stable thereafter.
In the tempering process, this phase transformation is arrested during the rapid quench. This causes any nickel-sulfide particles present to be arrested in a compacted, pre-transformation state. An inclusion in this condition may be as small as 0.003 to 0.015” (0.07 to 0.38 mm) in diameter. Particles of this size are invisible to the naked eye. When that tempered glass is later installed and exposed to varying temperatures of an outdoor environment, solar radiation or other thermal stresses, nickel-sulfide particles can experience volumetric growth. If that expansion is large enough (typically around 4 percent), the nickel-sulfide inclusions located in the center tension zone of tempered glass can produce enough stress to shatter the glass.
In 2011, there were a series of highly publicized incidents involving spontaneous breakage of window glass falling from high-rise buildings in Toronto, Chicago, Las Vegas and Austin. Incidents of this type are rare, and they cannot be definitively attributed to nickel-sulfide inclusions. However, the danger posed by spontaneous breakage has forced building-code writers, architects and government officials to consider more widespread use of safety glass where increased strength and risk reduction to bystanders are required.
Heat Soaking Glass to Reduce Inclusions
As previously mentioned, nickel-sulfide inclusions are small, rare and random. This combination makes visual inspection for nickel-sulfide inclusions nearly impossible. As an answer to that problem, many glass fabricators offer heat soaking to reduce the risk of spontaneous glass breakage.
During heat soaking, tempered glass panels are loaded onto an A-frame cart, rolled into an oven and exposed to temperatures of 550 to 600°F (288 to 316°C). After the coldest glass panel reaches that temperature, a soak timer is started. Typically, the soak time is between two and eight hours. The objective of this process is to accelerate the phase change of any nickel-sulfide inclusions present in the tension zone and to cause the glass to break in the controlled environment of the oven rather than being shipped to a customer with the chance that it will spontaneously break after installation.
While this “break-it-now” technique does catch defective tempered-glass panels by shattering them prior to shipment, it cannot provide a 100 percent guarantee against spontaneous breakage. An informal survey of glass manufacturers indicates that the probability of spontaneous breakage of tempered float glass that is not heat soaked is nine out of every 1,000 panels. With heat soaking, that number drops to three out of every 1,000 panels.
Heat soaking provides a significant risk reduction, but it comes at a price. Among other aspects, adding heat soaking introduces:
- Additional handling of the glass.
- The need for personnel to load and unload glass from the cart.
- The utility expenses of operating the heat-soaking oven.
- Increased reject rates due to scratches, edge damage or breakage.
- Additional work-in-process inventory.
- Shipping delays.
Because heat soaking can represent a significant cost, when presented with a proposal request for tempered glass, many glass manufacturers break out the additional cost impact to heat soak the glass. It is then left to the architect and building owner to assess the risks against the increased price of the glass.
Improvements in Heat-Soaking Ovens
One of the challenges in a heat-soaking oven is to minimize the variance between the hottest and coldest part of the load. Remember, the entire load must be heated to the minimum threshold temperature prior to starting the soak timer.
Traditional heat-soak ovens impinge air on the outside sheets of the A-frame cart and attempt to heat the inner sheets by blowing air below the cart. This approach relies on natural convection that allows the hot air to flow up between the glass panes.
An alternate solution offered is to incorporate ductwork into the cart. This promotes substantial airflow between the glass panels, allowing faster heatup and improved temperature uniformity along the entire length and height of the glass. Faster heatup means a shorter overall cycle time and more product throughput in a day. Changes in load density or thickness have little effect on the airflow and efficiency of heat-soaking ovens.
Heat soaking is not a guaranteed practice to eliminate spontaneous breakage of tempered float glass. Yet, it is the only proven method to test and contain glass panels that may be subject to contamination and possible failure due to nickel-sulfide inclusions. When selecting thermal processing equipment for heat soaking glass, it is important to work with a manufacturer with experience and expertise in this application. This will help ensure proper airflow between the glass panels for efficient heatup times and better temperature uniformity.
For more information on spontaneous glass breakage and heat treating architectural glass, please see the following sources.