When Should You Specify Heat-Soaked Glass for a Curtain Wall?
A pane of tempered glass shatters on the nineteenth floor of a finished building.
Nothing hit it. Nobody was near it. The weather was unremarkable, the facade had passed every test it was ever given, and three years later another pane goes on a different elevation for exactly the same reason.
That is nickel sulfide failure. Heat soaking exists because of it, the mechanism behind it is thoroughly understood, the testing that addresses it has been standardised for decades, and yet the decision of whether to specify it still gets made late on projects where it should have been automatic from day one.
This article covers when it is justified and what it actually achieves. PRANCE supplies glass facade and curtain wall systems, and heat soak is one of the glazing questions we work through with project teams early.
Nickel Sulfide Inclusions in Glass
Microscopic particles of nickel sulfide form occasionally in float glass during manufacture, when trace nickel contamination meets sulfur in the melt.
They are tiny. Well under a millimetre, usually.
In annealed glass they do nothing at all. The glass carries no locked-in stress, so a small particle sitting inside it is simply a small particle sitting inside it.
Tempering changes everything.
Why Tempering Turns a Harmless Particle Into a Failure Mode？
Tempered glass carries stress by design: surfaces in compression, core in tension, and that balance is precisely what gives the material its strength.
An inclusion sitting in the tension zone is a different proposition, because the particle itself is unstable. At high temperature it exists in one crystalline form. At ambient temperature it slowly converts to another that takes up slightly more room, commonly quoted as a two to four percent volume increase.
Rapid cooling during tempering freezes the particle in its high-temperature form. Over months and years it converts, it expands, and if it happens to sit in the wrong place with enough tension around it, the pane fails.
No impact. No warning.
The Timing Pattern
Breakage typically shows up between one and seven years after installation, peaking in the first few.
Heat accelerates the conversion, so facades with heavy solar exposure tend to fail earlier than shaded ones. That same relationship is what the test exploits.
Heat-strengthened glass carries far lower core tension, which is why it is much less susceptible. The furnace process behind the inclusion risk is identical, but the stress state that turns an inclusion into a broken pane simply is not there to the same degree.
How Heat Soak Testing Works
Deliberately accelerating the conversion so that susceptible panes break in a factory oven instead of on a building.
EN 14179-1 governs it, and the standard is specific about process.
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Heating phase: panes raised to soak temperature under controlled ramp conditions
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Soak temperature: approximately 290 degrees Celsius, held within a narrow tolerance
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Hold time: a minimum holding period, commonly cited as two hours at temperature
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Cooling phase: controlled cool-down
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Batch placement: glass positioned so every pane actually reaches and holds temperature
The process is destructive on purpose. Panes containing critical inclusions break in the oven, and those are exactly the panes that would otherwise have broken in service years later.
Two things get misunderstood.
It is not a treatment. Heat soaking does nothing to the glass that survives it. No added strength, no modified properties. It removes the panes that were going to fail.
It does not reach zero. The standard reduces probability substantially and some residual risk remains, and no supplier worth dealing with will tell you otherwise.
Heat Soaked Glass for Curtain Walls: When It Is Justified？
The decision turns on consequence rather than probability. On any individual pane the odds are low. What matters is what happens when the low-probability event occurs, and where.
Specify It Where
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The glass is at height, because falling fragments from tempered glass are a public safety matter
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Access for replacement is difficult or expensive, meaning rope access, mast climbers, or crane mobilisation
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The glazing is overhead, above anything occupied or trafficked
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The facade is structurally glazed, where a failure raises questions about the bonded assembly too
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The glass is a balustrade or guarding element, where failure has an immediate safety consequence
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The project is a landmark, where a breakage event carries reputational weight
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Regulation or the specification requires it, which is increasingly common
It Matters Less Where
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The glass is heat-strengthened rather than fully tempered
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The glazing sits at ground level with easy access and nothing overhead
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The glass is laminated, since a failed ply stays retained by the interlayer
That last point deserves weight. Lamination addresses the consequence while heat soaking addresses the cause, and on demanding applications both get specified rather than one being treated as a substitute for the other. How each glass type behaves when it breaks lays out the retention argument properly.
Cost and Programme Impact
Both, and both belong in the programme from the start.
The cost premium over standard tempered glass is meaningful but modest against a whole facade package. Programme is usually the bigger problem, since a soak cycle takes hours per batch and furnace capacity is finite.
Three consequences follow:
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Lead time extends, per batch rather than once
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Replacement panes take longer, which bites when a unit is damaged during construction
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Oven breakage means replacement panes, so an allowance for yield loss belongs in the order quantity
Order spare stock in the same production batch. It protects colour matching and it avoids running a full soak cycle for a single pane eighteen months later.
Specification Wording
Loose requirements get interpreted loosely. Four elements make it stick.
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All fully tempered glass shall be heat soak tested to EN 14179-1
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Testing shall be carried out on the final cut and processed panes, after all fabrication
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Certificates shall be provided identifying batch, furnace, cycle data, and the panes covered
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The soak temperature and hold time shall be recorded and available for inspection
The second clause matters more than it looks. Glass soaked before final processing is not covered for anything done to it afterward, and that gap is the one most commonly exploited.
Ask the glass processor for heat soak records and furnace certification at tender. Not at delivery. Technical documentation for the rest of the facade system belongs in the same submittal package.
What Heat Soaking Does Not Address？
Worth being blunt, because it occasionally gets treated as general insurance.
Thermal stress breakage, no. That comes from temperature differentials across a pane and is handled through glass type and shading detailing.
Edge damage from handling, no. That is fabrication and installation quality.
Impact breakage, no.
And it has no bearing on how well the pane bonds to its frame, since structural bonding on heat-treated panes depends on surface condition and sealant compatibility rather than on anything happening inside the glass.
Whether a third pane changes the calculation is independent of all this. Each tempered pane carries its own inclusion risk, so a triple-glazed unit with three tempered panes carries three times the exposure of a single one.
Frequently Asked Questions
1. Does heat soaking eliminate the risk of spontaneous glass breakage?
No. It cuts the probability substantially by causing susceptible panes to fail in the furnace rather than in service, and a residual risk remains. No supplier should claim complete certainty, and specifications should be written on that basis.
2. Is heat-strengthened glass susceptible to nickel sulfide failure?
Far less so. Heat-strengthened glass carries much lower core tension than fully tempered glass, so an inclusion is considerably less likely to trigger failure, which is one reason heat-strengthened plies turn up so often in laminated curtain wall units.
3. When does nickel sulfide breakage usually occur?
Typically between one and seven years after installation, peaking in the first few. High solar exposure accelerates the conversion, so sun-facing elevations tend to fail earlier than shaded ones.
4. Does laminated glass need heat soaking?
It depends on consequence. Lamination retains a broken ply so the pane does not fall, which handles the main safety concern. On high-rise, overhead, and structurally glazed work many specifications require both, because one addresses the cause and the other the result.
5. Should heat soaking happen before or after final processing?
After, without exception. The glass must be in its finished cut and processed state going into the furnace, since any fabrication carried out afterward sits outside the test. Specifications should say so explicitly.
Making the Decision
Weigh consequence, not probability.
At height, overhead, structurally glazed, or anywhere replacement means mobilising access equipment, heat soaking is cheap against what a single spontaneous failure actually costs once you add disruption, remediation, investigation of every other pane on the building, and the conversation with the client.
At ground level with easy access and laminated construction, the case is much weaker and the money may well be better spent elsewhere.
Send us your facade specification and building height. Our team will review the glazing build-up with you, including whether heat soak testing belongs in the specification and how it should be evidenced. You can talk to our facade team to start.