Heat strengthened glass
Heat strengthened glass is the quiet middle child: about twice as strong as annealed, but, and this trips people up constantly, it is not a safety glazing material. Knowing exactly where it sits between plain annealed and fully tempered is the difference between a clean spec and a rejected submittal.
What is heat strengthened glass?
Heat strengthened glass (HS) is annealed float that has been reheated and then cooled faster than annealed but slower than fully tempered. That gentler quench builds up a modest surface compression, enough to make it roughly twice as strong as annealed at resisting bending.
It sits right in the middle of the strength ladder: tougher than annealed, softer than tempered. ASTM C1048, Kind HS pins it down with a surface-compression window of 3,500 to 10,000 psi. Push past 10,000 and you have crossed into tempered territory (Kind FT).
How it differs from tempered
Tempering and heat strengthening run almost the same playbook: reheat the annealed glass, then cool it fast. The whole difference is how fast you quench it. Tempered gets hit hard with high-pressure air, driving surface compression above 10,000 psi. Heat strengthened gets a gentler blast and lands between 3,500 and 10,000 psi.
This difference in quench rate produces two important downstream effects:
| Property | Annealed | Heat strengthened | Tempered |
|---|---|---|---|
| Surface compression | Near zero | 3,500–10,000 psi | ≥ 10,000 psi |
| Strength vs annealed | 1× | ~2× | 4–5× |
| Safety glazing? | No | No | Yes |
| Fracture pattern | Large sharp shards | Large irregular pieces | Small cubes (dice) |
| Roller wave distortion | None | Minimal | More visible |
| Nickel sulfide risk | Low | Very low | Higher |
| ASTM standard | C1036 | C1048 Kind HS | C1048 Kind FT |
Fracture pattern and why it matters
When heat strengthened glass breaks, it comes apart in larger, irregular pieces, about annealed-sized, though a bit less vicious. And that is the catch: that pattern means it does not clear safety glazing under ANSI Z97.1 or CPSC 16 CFR 1201.
Here is the twist, though: that very fracture pattern is why HS is so often the inner lite of a laminate. Bonded to a PVB or SentryGlas interlayer, fragment size stops mattering, the interlayer holds everything in place. So for overhead glazing, canopies, and sloped skylights where a falling shard could hurt someone below, laminated heat strengthened glass is frequently the pick: the interlayer does the safety job, and you sidestep the spontaneous-breakage risk that haunts tempered.
Where heat strengthened glass is preferred
Spandrel panels and curtainwall
Heat strengthened glass is the workhorse for spandrel panels, the opaque bands between floors in a curtainwall. Spandrel glass cooks: it soaks up solar heat and, with insulation packed behind it, can run far hotter than the frame around it. That thermal stress will crack annealed glass. HS shrugs it off far better, and it shows less roller-wave distortion than tempered, which matters when a whole elevation of it is staring back at the street.
Laminated safety glass assemblies
Plenty of architects spec laminated heat strengthened glass for overhead work, canopies, sloped roofs, glass floors. The logic: tempered glass can let go on its own from a nickel sulfide inclusion (see the tempered glass article), and when it does it rains down small pieces that can still hurt. HS breaks into bigger chunks that stay stuck to the interlayer, a much gentler way to fail over people's heads.
Large-format glazing where distortion matters
On big curtainwall panels viewed from across the street, roller-wave distortion in tempered glass turns reflections wavy, one of the most common client gripes. Heat strengthened glass picks up less roller wave, so it is the go-to for large, high-visibility facade panels where flatness is the whole game.
Painted and coated back surfaces
Ceramic frit and silicone-based spandrel coatings on the back of the glass can fight with the tempering process, they absorb heat differently and can throw off the quench or pile up stress in spots. Heat strengthening is more forgiving with some of those coatings.
Thermal stress resistance
One of the most useful things heat strengthening buys you is resistance to thermal-stress fracture. Glass cracks when one part of a pane is hot while another stays cool: the hot part expands, and if that stress beats the glass's strength, a crack starts at the edge.
Annealed glass tops out at only about a 35°F to 50°F (20–28°C) difference across the pane before it is at risk. Heat strengthened takes up to 100°F (56°C), and tempered more still. So on glass with heavy sun, a dark coating, or a frame that shades the edges, HS often gets specified purely to stop thermal cracking, even when no safety product is required.
What you cannot do with heat strengthened glass
Just like tempered, heat strengthened glass has to be cut, drilled, notched, and edge-finished before it goes in the furnace. After heat treatment it is locked, try to work it further and it breaks. Every bit of cutting and drilling happens on the original annealed lite.
And to say it once more, because it matters: heat strengthened glass is not a stand-in for tempered in code-required safety spots. Doors, shower enclosures, sidelites, balustrades, and the rest of the hazard zones need tempered or laminated.