Heat Soaked Glass

The Problem Heat Soaking Exists to Solve

Tempered glass can break with no external cause. Not from impact, not from thermal stress, not from anything anyone did — the pane simply fails and falls. The trade calls it spontaneous breakage, and the usual cause is a microscopic nickel sulphide inclusion trapped inside the glass when it was made.

Nickel finds its way into the batch from stainless steel contamination and reacts with sulphur to form a particle a fraction of a millimetre across. Invisible, undetectable by inspection, and harmless in ordinary annealed glass. In tempered glass it is a different matter, because tempering leaves the core of the pane in high tension. Over years, the inclusion changes crystal phase from its high-temperature alpha form to the expanded beta form. It grows slightly. And if it happens to sit in that tensile core, the expansion is enough to start a crack that runs through the whole pane at once.

The timing is what makes it awkward. Most nickel sulphide failures occur within five to seven years of installation, with the peak in years two to four — long after the contractor has gone and the warranty conversation has become difficult. Failures more than a decade out are rare but documented.

What the Heat Soak Test Actually Is

It is worth being precise, because the name misleads people. Heat soaking is not a treatment and it does not make the glass stronger. It is a destructive screening test: it makes the suspect panes break in the factory instead of on the building.

Under the European standard EN 14179-1, already-tempered glass is loaded into an oven, brought to 290°C ± 10°C, then held through a soak phase at 260°C ± 10°C for a minimum of two hours before controlled cooling. That temperature accelerates the phase change that would otherwise take years. A pane carrying a dangerous inclusion breaks in the chamber. What comes out unbroken has been screened.

So some of your order is expected to fail, and that is the test working. A fabricator quoting heat soaked glass has priced that loss in.

How Much Risk It Actually Removes

Roughly 95 percent, not 100. That distinction matters and any supplier who claims otherwise is overselling.

The accepted figure is that after a properly conducted test to EN 14179-1, the residual risk is no more than one breakage per 400 tonnes of heat soaked toughened glass. Converted to something usable, that works out at roughly one in seven thousand sealed units, or about one in fourteen thousand panes. Some inclusions are stable enough to survive the soak but still transition across a much longer service life.

One caveat worth passing on: not every unexplained breakage is nickel sulphide. It gets blamed for a great deal that is actually edge damage from handling, impact, or poor installation putting the pane under load it was not meant to carry. A genuine nickel sulphide failure leaves a characteristic butterfly-shaped fracture origin, and that is what to look for before assuming.

Heat Soaked vs Heat Treated — Not the Same Thing

These two terms get used interchangeably on quotations and they mean quite different things. The confusion is worth clearing up before you order, because it changes what you actually receive.

Heat treated is a category. ASTM C1048, the governing specification, covers heat-treated flat glass in two kinds: Kind HS, heat-strengthened, and Kind FT, fully tempered. Glass that has not been heat treated at all is annealed, and in a specification it is designated AN. So "heat treated" tells you the glass has been through a heating and controlled-cooling process, and nothing more precise than that.

Heat soaked is not a kind of glass. It is a test performed on glass that is already fully tempered. Heat soaked glass is Kind FT that has additionally been screened. It is not a third category alongside HS and FT, and asking for "heat soaked instead of tempered" does not make sense — the tempering is a prerequisite.

The Three Kinds Side by Side

Annealed (AN). Not heat treated. Breaks into large shards. Can be cut and drilled after manufacture. Baseline strength.

Heat-strengthened (Kind HS). Roughly twice the strength of annealed glass of the same thickness and type. Surface compression between 3,500 and 7,500 psi — 24 to 52 MPa — for glass of 6mm and under. Breaks into large shards much like annealed, which is why it does not qualify as safety glazing on its own and is normally used laminated. Better resistance to thermal stress and wind load than annealed, and less prone to nickel sulphide breakage because the core tension is lower.

Fully tempered (Kind FT). Substantially higher surface compression again, and the only one of the three that qualifies as a safety glazing material under C1048, because it fractures into small blunt granules. It is also the one carrying the nickel sulphide risk, precisely because that high core tension is what makes an inclusion dangerous.

Heat soaked. Kind FT, plus the EN 14179-1 screening described above. Same specification, same strength, same break pattern, same code compliance — with the suspect panes already removed.

The Order of Operations, and Why It Matters

Glass goes annealed, then fabricated, then heat treated, then heat soaked if it is being screened. That sequence is fixed, and getting it wrong is expensive.

All cutting, drilling, notching, grinding and edgework must be completed before heat treatment. Once glass is heat-strengthened or tempered it cannot be worked — any attempt to cut or drill it releases the stress and the pane fails. This is why a tempered panel that turns out to be a quarter-inch oversize is scrap rather than a trim job, and why we take dimensions carefully at survey rather than ordering from a drawing that has not been checked on site.

One other consequence worth knowing: heat treatment slightly modifies the flatness of the glass. Roller wave, bow and edge kink are inherent to the process, and a faint strain pattern can become visible under certain light, particularly through polarised sunglasses. This is normal and not a defect. On large or highly reflective facades it is worth discussing before installation rather than after.

Compared With the Alternatives

Against Ordinary Tempered

Heat soaked glass is tempered glass. Identical strength, identical break pattern into blunt granules, identical thickness and appearance, and it meets the same safety glazing requirements. The only difference is that it has been screened. Nothing about its performance in the building changes — you are buying a lower probability of an unpleasant surprise, not a better product.

Against Heat-Strengthened

Heat-strengthened glass is heated and cooled more gently, so it ends up with lower surface compression and, crucially, lower tension in the core. Because crack initiation depends on the size of the inclusion in combination with the stress around it, a larger inclusion is needed to start a failure — so heat-strengthened glass is substantially less prone to this, though not immune.

The trade-off is that heat-strengthened glass is not a safety glazing product on its own. It breaks into large shards rather than granules, which is why it is almost always used laminated. Where the code requires safety glazing, heat-strengthened alone will not satisfy it.

Against Laminated

This is the option people forget, and often it is the better answer. Laminating does not prevent nickel sulphide breakage — but it changes the consequence completely. When a laminated pane fails, the interlayer holds the fragments and the glass stays in the opening rather than coming down.

For overhead glazing and anything above a walkway, laminated construction addresses the actual hazard more directly than screening does. Heat soaking reduces the chance of failure; lamination handles what happens if it fails anyway. On genuinely critical applications the answer is usually both.

Cost, and When It Is Worth It

Heat soaking adds cost in three ways, and it is worth understanding all three rather than looking only at the unit price.

Oven time. A soak cycle takes hours per batch and the oven capacity is finite. That is a real production cost.

Breakage loss. Panes that fail in the chamber are scrapped, and the price carries that. On a small order this hurts proportionally more, because the loss cannot be spread.

Lead time. The soak cycle sits between tempering and delivery. On a tight programme this is often the more painful cost.

Against that, weigh what a failure actually costs. Replacing one pane in an installed facade means access equipment, possibly a sidewalk shed, permits for work over the public way, and a building that has had glass fall from it. The unit price of heat soaking is small next to any of that.

Where we would specify it: anything overhead or above a walkway, glass balustrades and terrace railings, point-supported and structurally glazed facades, canopies, large single panes where replacement means specialist access, and any application where the glass cannot practically be reached again. For structurally glazed point-supported work, the specification worth asking for is 100 percent heat soaking to EN 14179-1 — not statistical sampling of one lite in several hundred, which tells you very little about the ones you are actually installing.

Where we would not: ordinary vision glazing at low level in a frame, in a room, at a size that could be replaced from a ladder. The risk is low and the consequence is manageable.

Where This Matters Across the City

The pattern is consistent: heat soaking earns its cost wherever falling glass would be dangerous or replacement would be expensive. These describe the kind of work each area brings rather than any specific client project.

Balcony and Terrace Balustrades — Long Island City and Downtown Brooklyn

The residential towers through Long Island City and Downtown Brooklyn are full of glass balustrades at height. This is the single clearest case for specification care in the whole city: a pane failing at the fortieth floor is not a maintenance issue, it is a public safety incident on the pavement below. Heat soaked and laminated is the combination we would put on a balustrade, and we would decline to supply a monolithic tempered balustrade at height without saying plainly why.

Canopies and Overhead Glazing — Manhattan Retail Frontage

Entrance canopies over a sidewalk are directly over people, continuously, on a public way. Overhead glazing is the application where laminated construction is not negotiable and heat soaking is a sensible addition. It is also where a failure attracts the most attention.

Structural and Point-Supported Facades — Midtown and Hudson Yards

Point-supported glazing puts concentrated load into the glass at the fittings and typically uses large panes that are expensive and awkward to replace. This is exactly the application where 100 percent heat soaking to EN 14179-1 is worth specifying, and where the fabricator should be able to produce the documentation to prove it was done.

Rooftop Amenity Decks — Williamsburg and the Waterfront

Roof terraces with glass guarding combine height, weather exposure and public access. Add the thermal cycling a fully exposed south-facing balustrade goes through, and the case for screening the glass before it goes up is straightforward.

Getting It Specified

Tell us where the glass is going, what is underneath it, and how it would be replaced. If the answer to the last question involves a lift, a shed or a permit, heat soaking is almost certainly worth it. We supply and install across Manhattan, Brooklyn, Queens, the Bronx and Staten Island, and throughout Northern New Jersey, the Jersey Shore, Philadelphia and the Lehigh Valley. See also our pages on tempered glass and laminated glass.