Tempered Glass

What Tempering Actually Does

Tempered glass starts as ordinary annealed glass. It is heated to around 620 °C and then cooled rapidly with jets of air. The surfaces cool and set first while the core is still hot; as the core then cools and contracts, it pulls the already-rigid surfaces into compression and leaves itself in tension.

That locked-in stress is the whole product. Glass fails when a surface flaw opens under tension, so a surface held permanently in compression has to be pushed much harder before anything can start. The result is roughly four times the strength of annealed glass of the same thickness, and far better resistance to thermal stress.

The governing specification is ASTM C1048, Kind FT — fully tempered — which is the only one of the heat-treated categories that qualifies as a safety glazing material. Kind HS, heat-strengthened, is about twice annealed strength and breaks into large shards, so it does not qualify on its own.

Why It Breaks the Way It Does

All that stored energy has to go somewhere. When a tempered pane finally fails, the energy releases through the whole panel at once and it disintegrates into small, relatively blunt granules rather than long shards. That behaviour is what makes it a safety product, and it is tested under CPSC 16 CFR 1201 and ANSI Z97.1. Category I covers smaller glazing at a lower impact energy; Category II is the higher requirement used for larger panes and for guards.

The trade-off is that there is no intermediate state. Tempered glass does not crack and hold. It is intact, and then it is granules on the floor and an open hole where the glazing was. Where that matters — overhead, at height, in a guard, in a door that must stay closed — laminated construction is the answer, because the interlayer keeps the fragments in the frame.

The Constraints Nobody Mentions Until It Is Too Late

It cannot be worked after tempering. No cutting, no drilling, no notching, no grinding. Any attempt releases the stress and the panel disintegrates. Every hole, cut-out, notch and edge finish has to be completed before the glass goes into the furnace.

The practical consequence is that a tempered panel a quarter of an inch oversize is scrap, not a trim job. This is the single biggest argument for surveying an opening properly rather than ordering from a drawing nobody has checked on site.

It is not perfectly flat. Passing through a roller hearth furnace leaves roller wave — a subtle ripple running across the pane — along with slight bow and edge kink. These are inherent to the process, not defects, and they become visible as distortion in reflection on large or highly reflective facades. Worth discussing before installation rather than after.

The strain pattern is visible in polarised light. Faint dark blotches or stripes appear through polarised sunglasses or in certain daylight. Normal, and evidence the glass really is tempered.

Edges are vulnerable. The compression layer is thinner at the edge, and most unexplained tempered breakage is actually edge damage from handling or from being forced into a frame too tight for it.

Spontaneous breakage is real. Microscopic nickel sulphide inclusions sitting in the tensile core can expand over years and trigger failure with no external cause, typically two to seven years after installation. Heat soaking screens most of them out and is worth specifying anywhere replacement means access equipment.

Where the Code Requires It

Safety glazing is not an upgrade you choose. In defined hazardous locations it is required, and the list is broader than most people expect.

Doors. Glazing in any fixed or operable panel of swinging, sliding and bifold doors.

Beside doors. Panels within a defined distance of the door edge and below a defined height — the classic sidelite.

Large low panes. Glazing over a threshold area with its bottom edge close to the floor and its top edge above a set height, where the walking surface is within reach.

Wet areas. Glazing in and around bathtubs, showers, saunas, steam rooms, whirlpools and indoor pools, below a defined height above the standing surface. Shower enclosures are tempered without exception.

Stairs, landings and ramps. Glazing adjacent to the walking surface within defined distances.

Guards and railings. All glazing in guards and railings, structural or in-fill, regardless of area or height — and in NYC these must additionally satisfy Category II and the structural provisions on our railings page.

Where the code requires safety glazing, tempered and laminated both qualify. Which one is correct depends on what should happen when it breaks, not on which is cheaper.

Tempered Glass Work Across the Region

These describe the kind of work each area brings rather than any specific client project.

Shower and bath enclosures — citywide. The highest volume of tempered work we do. Panels are 3/8″ or 1/2″, every hinge and handle position is drilled before tempering, and the enclosure is templated on site after tiling because tiled openings are almost never square. Detail on our shower doors page.

Storefront and entrance glazing — Flatbush, Coney Island Avenue, Third Avenue Sunset Park, Astoria. Doors and the panels beside them are required safety glazing. On a busy retail entrance the practical question is edge protection and correct glazing clearance, since most breakage on frontage is edge damage rather than impact.

Balustrades and terraces — Long Island City, Downtown Brooklyn, the waterfront. Tempered alone is not appropriate here. Laminated, and heat soaked.

Partitions and interior glazing — Tribeca, Boerum Hill, Midtown. Large panes where roller wave becomes visible in reflection, so orientation through the furnace is worth specifying on a long run of panels that will be seen together.

Coastal work — the Rockaways, Manhattan Beach, Staten Island's South Shore, the Shore. The glass is unaffected by salt; the hardware and fixings are not. Stainless rather than plated.

Tell us the size, the thickness, and every hole and notch you need, because all of it has to be done before the glass is tempered. We supply and fabricate across Manhattan, Brooklyn, Queens, the Bronx and Staten Island, and throughout Northern New Jersey, the Jersey Shore, Philadelphia and the Lehigh Valley, with collection from Staten Island by appointment.