Heat Soaked Glass
At PFG Glass, we understand that certain architectural applications demand more than standard tempered glass can provide. When spontaneous breakage poses unacceptable safety or economic risks, heat soaking offers an additional layer of quality assurance. Our facility is certified to process heat soak testing for tempered glass to the European Standard EN 14179-1:2016-TC, providing architects, contractors, and building owners with enhanced confidence in glass performance.
Understanding the Heat Soaking Process
Our heat soaking procedure follows rigorous international standards to ensure consistent, reliable results. After tempering, glass panels are carefully loaded into our specialized heat soak oven where they undergo a controlled thermal cycle.
The heating phase begins at ambient temperature and gradually elevates the glass to 260±10°C, with maximum temperatures not exceeding 290°C. This temperature range is carefully calibrated. It is high enough to trigger the expansion of problematic nickel sulphide particles but controlled to avoid affecting the fundamental properties of the tempered glass.
Once the target temperature is reached throughout the glass, the holding phase begins. Glass maintains this elevated temperature for a minimum of two hours, allowing any latent nickel sulphide inclusions to undergo phase transformation and expand. Following the holding period, the glass slowly cools to approximately 70°C. The entire cycle typically lasts between 5.5 and 6 hours.
Any panels containing nickel sulphide particles large enough or positioned within the tension zone to cause failure will shatter inside the furnace during this process. Only intact glass continues through our production pipeline to your project site.
The Nickel Sulphide Challenge
Nickel sulphide inclusions originate during float glass manufacturing when nickel-bearing contaminants from stainless steel equipment combine with sulphur present in combustion fuels or fining agents. These microscopic impurities are extremely rare, estimated at approximately 1 in 4 metric tonnes of glass; however, their rarity does not eliminate the concern in critical applications.
The mechanism behind spontaneous breakage involves a phase transformation. During tempering, glass is heated to approximately 650°C and then rapidly cooled. This rapid cooling freezes nickel sulphide particles in their high-temperature crystalline structure. Over time, these particles attempt to revert to their natural low-temperature structure, which occupies approximately 2% to 4% more volume.
This expansion exerts localized pressure on the tensile zone at the centre of the glass. When that pressure exceeds the glass’s capacity to absorb it, the entire tempered panel suddenly explodes into thousands of small pieces without warning or external stress. This can happen weeks, months, or even years after installation.
Standards and Certification
Our heat soaking process complies with EN 14179-1:2016, the European standard that defines specifications for heat soaking fully tempered soda-lime silicate safety glass. This standard specifies temperature regimes, holding times, cooling rates, and fragmentation requirements after heat soaking.
Following heat soaking, glass must still meet fragmentation criteria per EN 12150, which are identical to the fragmentation standards applied to all tempered glass. This ensures that the additional thermal processing does not compromise the safety characteristics that made tempered glass the specified product.
Our certification provides architects and contractors with documented verification that our heat soaking procedures consistently meet recognized international standards.
Understanding Thermal Stress
While heat soaking addresses nickel sulphide-related spontaneous breakage, it does not affect thermal stress breakage, which represents a distinct failure mode. Tempered and heat-soaked glass can withstand temperature differentials of approximately 250°C before thermal stress fracture becomes likely, compared to only 40°C for annealed glass.
Thermal stress failures result from differential expansion when different areas of glass reach different temperatures simultaneously. Proper installation practices, appropriate frame design, consideration of solar gain patterns, and adequate edge clearances prevent most thermal stress failures. We provide technical guidance on installation requirements to help avoid these preventable failures.
Frequently Asked Questions About Heat-Soaked Glass Products
What is Heat-Soaked Glass?
Heat-soaked glass is tempered safety glass that has undergone an additional thermal processing step designed to identify and eliminate defective panels before installation. After standard tempering, glass is placed in a specialized heat soak oven where it is heated to 260±10°C and held at this temperature for a minimum of two hours. This process forces any panels containing problematic nickel sulphide inclusions to break in a controlled environment rather than after installation, substantially reducing the risk of spontaneous breakage in service.
Heat soaking does not alter the fundamental properties of tempered glass. Heat-soaked glass remains approximately four times stronger than annealed glass with the same characteristic safety breakage pattern of small, relatively harmless fragments. The process serves as quality assurance, removing defective panels before they reach project sites.
Why Heat Soak Glass?
The heat soaking process works to reduce the potential for glass to break spontaneously due to nickel sulphide inclusions. These are microscopic impurities formed during float glass manufacturing that can cause defects smaller than the human eye can detect. While these inclusions are rare, they represent an ever-present possibility that can expand over time, causing glass to break spontaneously without warning or external stress.
Since tempered safety glass is often installed in locations where safety is critical, such as balcony railings, overhead glazing, or structural applications, spontaneous breakage caused by nickel sulphide inclusions can create serious safety hazards. In high-rise installations, spontaneous breakage also poses significant economic challenges due to replacement complexity and access requirements.
Where is Heat-Soaked Glass Used?
Heat-soaked glass is optimal for applications where the risk of nickel sulphide-induced failure needs to be minimized along with a requirement for safety from glass fallout. The additional processing makes it more expensive than ordinary tempered safety glass; however, its use can help ensure safe and secure environments in areas where access to replacement panels may prove difficult.
Applications where heat-soaked glass adds the most value include projects where crane access is required to install glass, buildings with complicated or intricate glass designs, and installations where glass serves as a structural element supporting other building components. Oversized or specialized glass units benefit from heat soaking, as do balconies or mezzanine floors using frameless or infill glass balustrades.
What are the Advantages of Heat-Soaked Glass?
Heat soaking is a valuable process used to detect nickel sulphide inclusions within tempered glass before it reaches its installation destination. This thorough screening procedure ensures the highest quality, safety-tested glass products for our customers. It is a recognized, effective solution for preventing the costly consequences of nickel sulphide inclusions, protecting legal liability, and ensuring that building occupants remain safe.
The process reduces spontaneous breakage incidence from approximately 2 in every 10,000 square metres of untreated tempered glass to approximately 1 in every 1 million square metres. This dramatic risk reduction provides architects and building owners with substantially greater confidence in glass performance, particularly in applications where replacement would be complex or expensive.
Is Heat-Soaked Glass Safer Than Tempered Glass?
Heat-soaked glass has already been tempered, so it maintains all the strength and safety characteristics of standard tempered glass. The heat soaking process forces the occurrence of spontaneous breakage to occur in a controlled manufacturing environment rather than after construction has been completed.
This means heat soaking minimizes the risk of failure due to nickel sulphide inclusions and, in turn, reduces the legal liability and associated replacement, maintenance, and disruption costs. Heat-soaked glass is not inherently stronger than tempered glass, but it does provide substantially greater assurance against spontaneous breakage from nickel sulphide inclusions specifically.
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