How is tempered glass made, and why does it break into pieces?
It’s ordinary glass put through a heat treatment. AGC, which makes car glass, says its tempered glass is heated “to near its bending point (approx. 650℃)” and then the surface is cooled “rapidly” and “uniformly with air.”
That quick cooling locks compressive stress into the surface, which AGC says leaves the glass three to five times stronger than ordinary glass of the same thickness. When it does fail, it goes all at once, fracturing into small “nuggets” rather than long shards. Hyundai’s glass bulletin puts it the same way: broken tempered glass scatters into small fragments without sharp edges.
Where is tempered glass used on a car?
Mostly in the side windows and the back glass, with some roofs too, per AGC. It isn’t fixed by model, though; trims can differ. Hyundai’s bulletin lays out the 2023 Palisade:
| Seating row | SE | SEL, XRT | Upper trims (SEL Premium, Limited, Calligraphy) |
|---|---|---|---|
| Row 1, front doors | Tempered | Acoustic laminated | Acoustic laminated |
| Row 2 doors | Tempered privacy | Tempered privacy | Acoustic laminated privacy |
| Row 3 | Tempered privacy | Tempered privacy | Tempered privacy |
Windshields sit outside the table, since AGC says road regulations require laminated glass there.
What does tempered glass mean for UV and tint?
It lets more UVA through than a windshield, which is where film earns its keep. The Skin Cancer Foundation groups side, back and sunroof windows as tempered glass and says it lets UVA through more readily than a windshield. When a 2016 study measured 29 cars, side windows stopped 71% of UVA on average, and just 4 of them cleared 90%. LLumar’s spec sheet rates each of its films above 99% UV blocking between 300 and 380 nm; the wider picture is on whether tint blocks UV.
In BC, MVAR s. 7.05(8) lets tempered glass behind the driver take film of any darkness, and the back window too, provided both outside mirrors are in place. The front doors, by contrast, can take none at all, clear film included. Front film goes on only for customers who still want it after hearing the downside: it fails the road rules, a ticket usually runs $109, and an officer can issue a Notice & Order; see tinting front windows in BC. What film does when tempered glass is smashed is answered on tint in a break-in, and rear packages from $220 are listed on back window tint.