Heat soak testing explained
Share this blog:
What is heat soak testing and why is it important in glass manufacturing? Get the answers in our guide.
Some things in life are negotiable – but the quality of building materials never is.
This applies to everything from bricks to plastic, from stone to steel. But it's especially apparent in the case of glass.
Manufactured and installed to a high standard, glass is durable, safe and long-lasting. Cut corners, however, and it could injure or even kill somebody.
We're not scaremongering. Most of the time, you can be satisfied with your glass products just by getting them from a reputable and accredited glass processor.
There are, however, some aspects of glass manufacturing that still give installers pause for thought. Take
nickel sulfide inclusions – tiny impurities in toughened glass that can, in some cases, lead to spontaneous glass breakage.
We say "in some cases" because spontaneous glass breakage is far from common. It is, however, a reality. This is why many architects, designers and builders ask for toughened glass to undergo
heat soak testing – an extra precautionary stage in the manufacturing process to ensure the glass is the best of the best.
In this article, we take a closer look at the heat soak testing process and why it matters. We hope it helps you when sourcing glass products for your building project.
What is heat soak testing?
When toughened glass is manufactured, nickel sulfide impurities can get baked into the finished product. Over time, these can expand. This could happen in days, months or years. Eventually, they can cause panes to fracture spontaneously.
Heat soak testing is a process that seeks to spot these flawed panes before they get anywhere near a building site. After the initial toughening process – where the glass is heated to a high temperature and then rapidly cooled – it's placed in a separate furnace. Here, it's heated at around 290°C (554°F) for four hours.
If a pane of toughened glass contains nickel sulfide inclusions, it will more than likely shatter in the furnace. This allows manufacturers to remove faulty products and replace them before shipment.
What are nickel sulfide inclusions?
Nickel sulfide inclusions are near-invisible particles of nickel sulfide that can expand inside toughened glass and cause it to break spontaneously.
They're an unwanted byproduct of the manufacturing process. While manufacturers use different methods to reduce or eliminate the threat, the most reliable method at the time of writing is heat soak testing.
Why is nickel present in the glass manufacturing process?
Contaminants are an unavoidable part of the glass manufacturing process, especially in large-scale production. In this respect, they're similar to
roller-wave distortion and other imperfections in otherwise high-quality toughened glass products.
Sometimes, the stainless steel machinery used to make and handle toughened glass contaminates the glass with nickel shavings. These get baked into the glass and can eventually lead to spontaneous glass breakage.
Why is heat soak testing important?
The key reason why heat soak testing is important is, of course, safety. Toughened glass tends to be installed in "critical locations" – those parts of a building where people could be injured were the glass to break.
On top of this, heat soak testing can save you money in the long run. Even if no one gets hurt, you don't want to be spending money on repairs and throwing money away during downtime. By increasing the lifespan of toughened glass, you're making a prudent investment.
Heat soak testing is a preventative measure that allows manufacturers, designers and installers peace of mind. While it's not 100% reliable, it significantly minimises the chances of the glass spontaneously shattering.
The trouble with nickel sulfide inclusions, you see, is that you can rarely spot the problem simply by looking at the glass. Once it's shattered, the breakage pattern makes it obvious. But a pane of toughened glass with nickel sulfide inclusions usually looks much the same as one without.
What types of glass products is heat soak testing used for?
Heat soak testing can be used on toughened glass intended for any application. It's especially important, however, when the glass will be used for structural or architectural glazing. This includes skylights, load-bearing glass walls, balustrades, partitions and more.
What are the limitations of heat soak testing?
Heat soak testing is a good idea. It's not, however, a completely reliable method. Some panes of glass with nickel sulfide inclusions can still end up in buildings despite undergoing the process.
Another limitation of heat soak testing is that it's a destructive process. The only way to know if a piece of toughened glass contains nickel sulfide inclusions is by destroying it in a furnace.
Nickel sulfide inclusions can be spotted by other methods including ultrasonic testing and polarised light microscopy. In most cases, however, heat soak testing remains the most affordable and reliable method.
What else can cause spontaneous glass breakage?
Nickel sulfide inclusions aren't the only cause of spontaneous glass breakage. It can also be caused by nicked or chipped edges, thermal stresses and using glass that isn't thick enough to resist wind load.
The case of London City Hall
Between 2002 and 2021, London City Hall in Bermondsey was the headquarters of the Greater London Authority. Designed by renowned architect Norman Foster, it cost around £43 million to complete.
Like many recent additions to the London skyline, London City Hall split opinion. Some marvelled at its raised concentric walkways, while others compared it to Darth Vader's helmet.
Soon after its completion, London City Hall experienced problems. Several floor-to-ceiling glass partitions spontaneously broke – and it was agreed that the cause was nickel sulfide inclusions.
While these problems at London City Hall were unusual, they could happen again. If your project involves installing toughened glass in critical locations, heat soak testing is your best bet.
Here at ToughGlaze, we put the quality of our products first. Whatever kind of toughened or laminated glass you need for your project, we can heat soak test it here at our state-of-the-art facilities. Learn more about our
heat-soaked glass or
get in touch for a quick, competitive quote.
