Heat soak testing is an additional tempering step that reduces the risk of nickel sulphide driven spontaneous breakage in fully tempered glass, but it does not guarantee zero risk. When we specify heat soaked glass, we reference the controlling standards, BS EN 14179-1 and ISO 20657, and treat the process as risk reduction rather than a warranty against failure.
TL;DR:
- Specify heat soak testing for fully tempered glass in overhead glazing, free standing barriers, point fixed structural glazing, and frameless or sloping assemblies.
- Name EN 14179 part 1 or ISO 20657, require a surface temperature near 260°C ±10°C, and keep the cycle below 290°C.
- Require batch calibration certificates, complete temperature logs, loading records, marking, traceability, and the cooling method; schedule testing separately with extra time for failed batches.
- Consider glass strengthened by heat when strength, rather than safety glazing, drives the choice; it avoids nickel sulphide risk, but check coating tolerance.
Table of Contents
- What heat soak testing is and why it matters
- Standards and the key process parameters to specify
- When to specify heat soaked glass and practical alternatives
- Practical process implications for projects and fabricators
- Limitations, residual risk and common failure modes
- Cost, scheduling and specification text to include
- Precision Glasses perspective and technical notes
- A specifier’s decision checklist
- How Precision Glasses supports your specification
- FAQ
- Sources
What heat soak testing is and why it matters
Nickel sulphide inclusions form during float glass manufacture when trace nickel and sulphur compounds crystallise inside the glass mass. In their stable form these inclusions are harmless, but when the surrounding glass is tempered, some inclusions remain in an unstable crystal phase that slowly converts over months or years, expanding and generating enough internal stress to shatter an otherwise sound pane. Heat soak testing forces that conversion to happen early, inside a controlled oven rather than above a building entrance.
The process raises the glass surface temperature to accelerate the phase change in any unstable inclusions, so a pane carrying a dangerous inclusion fails during the test cycle rather than years later in service. The scope matters as much as the mechanism:
- Fully tempered clear, tinted, coated and low-E glass are generally included in heat soak testing scope according to standards.
- Heat-strengthened and annealed glass are typically excluded from heat soak testing, as nickel sulphide risk mainly applies to fully tempered glass.
- Laminated units and insulated glass units need traceability back to each constituent toughened pane rather than testing as an assembly.
Standards and the key process parameters to specify
Two standards govern a correct heat soak test, and both belong in a specification clause rather than a general reference to “heat soaked glass”. BS EN 14179-1:2016 defines heat-soaked thermally toughened soda-lime-silicate safety glass and sets out the process system requirements, tolerances, calibration regime and traceability clauses that a compliant batch must satisfy. ISO 20657 mirrors the same approach for markets working to the international standard.
The glass surface itself must reach approximately 260°C ±10°C during the holding phase, and the standard restricts the surface temperature from exceeding 290°C at any point in the cycle, a limit set to avoid reconverting stable inclusions or damaging the temper, according to the EN 14179-1 standard extract.
A compliant process also requires:
- Regular oven calibration tests, following the annex A procedure, covering heating rates, separation distances and measurement points.
- Defined loading rules, since overloaded ovens produce uneven heating across a batch.
- Marking, documentation and batch traceability so each delivered pane can be linked back to its calibration and temperature records.
We ask suppliers for calibration certificates and full temperature logs as a matter of course, because a heat soak certificate without supporting records tells a specifier very little.
When to specify heat soaked glass and practical alternatives
Not every tempered pane needs heat soak testing, and deciding where to specify it comes down to consequence of failure as much as probability.
- Overhead glazing, including rooflights and canopies, where a spontaneous break could fall onto people below.
- Free-standing balustrades and barriers where the glass itself is the only line of protection.
- Bolt-fixed and point-fixed structural glazing, where a cracked pane is harder to replace safely and quickly.
- Sloping or frameless assemblies where broken fragments cannot be contained by a frame.
Where tempered glass is specified mainly for strength rather than safety glazing requirements, heat-strengthened glass often removes the nickel sulphide risk entirely, since it is not fully tempered and does not carry the same residual hazard. Coated and surface-finished products need checking with the supplier before specification, since not every coating tolerates the heat soak cycle without visual change.
Practical process implications for projects and fabricators
Heat soak testing is almost always carried out off-line, as a discrete batch process after tempering rather than inline with production. Oven capacity is finite, and a batch run that includes a failure extends the whole cycle, so lead times for heat soaked glass typically run longer than for standard tempered glass.
- Adjacent panes in the same oven load can be damaged if one pane breaks during the cycle, so proper spacing and handling precautions are advisable.
- Coated glass needs a check with the coating supplier before the batch runs, since some coatings are sensitive to extended exposure at soak temperatures.
- Oven calibration certificates, temperature logs and loading records should be requested for every batch, not sampled occasionally.
- Schedule float should account for a repeat run if a batch fails or records are incomplete.
Pro Tip: Build heat soak testing into the programme as a separate line item with its own float, rather than folding it into standard glass lead time.
Limitations, residual risk and common failure modes
Heat soak testing reduces but does not eliminate the risk of nickel sulphide related breaks, a point the BS EN 14179-1 standard states directly rather than implying. Peer-reviewed microanalysis research shows that where field breakages do occur, it is often possible to determine that heat soak testing was not performed correctly, or not performed at all, according to analysis published in Glass Structures & Engineering. That finding matters for specifiers: a correctly run heat soak test appears to be considerably safer in practice than the industry’s general perception of residual risk suggests.
The practical causes behind a missed inclusion tend to repeat: an incorrect temperature profile, an overloaded furnace that heats unevenly, or, more rarely, reconversion when a pane overheats beyond the 290°C ceiling. Demanding calibration records is the most direct defence against all three.
Cost, scheduling and specification text to include
A specification clause for heat soak testing should reference EN 14179-1 or ISO 20657 by name, require calibration certificates for the specific batch, call for marking and full traceability documentation, and state the cooling method used. Cost drivers worth flagging to clients include batch breakages, extra handling and stillage, and oven throughput limits that can push testing onto a separate schedule from standard glazing.
- Reference the controlling standard explicitly rather than writing “heat soaked glass” alone.
- Require full-batch calibration certificates and temperature logs as delivery documents.
- Confirm the cooling method and marking requirements in writing before fabrication starts.
| Decision point | Specification action |
|---|---|
| Risk area identified | Confirm fully tempered glass is required before specifying heat soak testing |
| Supplier appointed | Request EN 14179-1 or ISO 20657 calibration certificates per batch |
| Delivery | Confirm marking, traceability and temperature log documentation |
Weighing the additional cost against the consequence of a field breakage is usually straightforward once the risk area is identified: overhead and safety-critical glazing justifies the extra cost and lead time, while glazing chosen mainly for strength may not need it at all.
Precision Glasses perspective and technical notes
Oven calibration schedules and batch traceability are key factors in quality control, with small batches sometimes creating greater calibration challenges, due to shorter runs leaving less margin to spot uneven heating profiles before delivery. Traceability back to specific calibration records helps specifiers respond quickly if questions arise later.
When we review a tender enquiry, we ask suppliers to confirm:
- Which standard, EN 14179-1 or ISO 20657, governs the batch and whether calibration certificates will be supplied.
- What cooling method is used and whether coated products have been tested at soak temperature.
- How traceability is maintained from oven load through to delivered, marked panes.
A specifier’s decision checklist
On a live project, we identify the risk area first, overhead, balustrade or structural glazing, then weigh heat-strengthened glass as an alternative before committing to heat soak testing. Early engagement with the fabricator on calibration records and schedule float avoids surprises later.
— Alexandra
How Precision Glasses supports your specification
Specifying heat soaked glass correctly depends on documentation as much as process, and that is where we focus. We supply toughened glass and related technical fabrication with batch-level traceability and calibration records available to specifiers who need them for contract files, and our quality processes are built around the audit-ready documentation that regulated projects require.
- Calibration and traceability documentation can be provided alongside toughened glass orders.
- Specification queries on coatings, tolerances and process parameters can be supported before fabrication begins.
- It is possible to work from prototype quantities through to volume production while maintaining consistent documentation standards.
If you are preparing a specification or tender enquiry involving heat soaked glass, get in touch through our services page to discuss your project requirements and documentation needs.
FAQ
Is BS EN 14179-1 still valid?
Yes, BS EN 14179-1:2016 remains the current, applicable standard for heat soaked thermally toughened soda-lime-silicate safety glass. Specifiers should obtain the formal standard text from BSI for contract use rather than relying on summaries.
What is the difference between tempered glass and heat-soaked glass?
Tempered glass is toughened through rapid heating and cooling, which gives it strength but leaves a small residual risk of spontaneous breakage from nickel sulphide inclusions. Heat-soaked glass is tempered glass that has gone through an additional oven cycle, following EN 14179-1, designed to trigger that failure in the factory rather than on site.
What is glass heat soaking?
Glass heat soaking is a controlled oven process applied to fully tempered glass after manufacture, where panes are held at an elevated surface temperature for a defined period to accelerate any unstable nickel sulphide inclusions into failure before installation. It is a batch process governed by EN 14179-1 and ISO 20657, not an inline step of standard tempering.
What is the purpose of soak testing?
The purpose of heat soak testing is to reduce the likelihood of spontaneous, unexplained breakage in fully tempered glass once it is installed, by forcing unstable inclusions to fail during a controlled test cycle instead. Trade guidance from Glass Magazine confirms it is a risk reduction measure, applied as an off-line batch process, rather than a guarantee of zero future breakage.
Sources
- BS EN 14179-1:2016 – Glass in building: Heat soaked thermally toughened soda lime silicate safety glass (BSI Knowledge)
- SIST EN 14179-1:2016 – Heat soaked thermally toughened safety glass (Standards extract)
- Heat Soak Testing of Tempered Glass | Glass Magazine
- Spontaneous breaking of thermally toughened safety glass in facades: can we state if the HST was done according to EN14179-1? | Glass Structures & Engineering
