Tempered glass, also called toughened glass, is made by heating a pane and cooling it rapidly to increase its resistance to breakage. When fully tempered glass breaks, it typically shatters into small fragments, which helps reduce the risk of serious cuts.
How to Tell if Glass Is Tempered
Look for a small stamp near a corner of the pane. The mark may be etched or printed on the glass. Check each pane separately in a shower enclosure or glass entrance, since panes in the same assembly can have different specifications.
What to Look for on the Glass Stamp
Look for “Tempered” or a code that identifies the glass type. The stamp may also name the fabricator and list a safety glazing standard, such as ANSI Z97.1 or CPSC 16 CFR Part 1201. If it includes an SGCC number, look up the product in the SGCC Certified Products Directory.
What to Do if You Can’t Find a Stamp
Ask the installer or fabricator for a record that identifies the specific pane. A glazing schedule, product submittal, or order record can tell you what was supplied. Dark bands or spots seen through polarized lenses can indicate heat treatment. They can appear in both fully tempered and heat-strengthened glass, as the NGA explains in its technical paper on iridescence.
How Is Tempered Glass Made?
Fabricators start with an annealed glass pane that has already been cut and prepared for the job. The furnace heats the pane evenly to near its softening point. As the glass leaves the furnace, air jets cool both faces quickly and evenly. This step is called quenching.
The surfaces cool and stiffen while the center is still hot. As the center cools and contracts, it puts the surfaces in compression and leaves the interior in tension. The surface compression makes it harder for a crack to spread, giving the pane greater resistance to breakage. The National Glass Association describes this process in its production guidelines for heat-treated flat glass.
How Strong Is Tempered Glass?
Fully tempered glass has about four times the breakage strength of annealed glass of the same thickness under comparable conditions. The National Glass Association gives this figure as a rule of thumb for heat-treated glass.
Tempering leaves the glass’s stiffness unchanged. Take two panes of the same size and thickness, one annealed and one fully tempered. With the same supports and the same evenly distributed load, both panes bend by the same amount. This distinction matters when the design sets a limit on deflection, as described in the GANA Glazing Manual.
Does Tempered Glass Shatter?
Yes. When a single pane of fully tempered glass shatters, the fragments can fall from the opening. A hard impact can break the pane, and a chipped edge can lead to breakage later. A nickel sulfide inclusion is a less common cause of spontaneous breakage.
An interlayer helps hold fragments together after breakage in a laminated assembly, which can also incorporate tempered panes. That behavior is one factor in choosing laminated or monolithic tempered glass for an opening.
What Is Tempered Glass Used For?
In buildings, tempered glass is used where the design calls for greater breakage resistance or safety glazing. Certain doors and shower enclosures are covered by the U.S. safety standard for architectural glazing. The locally adopted building code can also set requirements for a particular location or assembly.
Application | What to Check |
Shower doors and enclosures | Check the safety glazing mark on each panel, including fixed panels. |
Glass entrance doors and sidelites | Check the requirements for the door and each adjacent panel. Confirm which glass the framing and hardware can accept. |
Windows and interior partitions | Check whether the pane’s size, height above the floor, or distance from a door affects the glazing requirements. |
Glass guards and overhead glazing | Check the required glass composition and any provisions for retaining or catching fragments after breakage. |
Can Tempered Glass Be Cut or Drilled?
A finished tempered pane can’t be cut or drilled to fit. Cutting, drilling, notching, and edge finishing must be completed before tempering. If the finished size or hole pattern changes, the fabricator needs to make a new pane. This sequence is part of the fabrication requirements in ASTM C1048.
When replacing the glass in a frameless shower door and keeping the existing hinges and handle, the new pane has to match both the opening and the hardware. A pane with the right width and height may still fail to fit if a hinge cutout is out of position. Use a hardware template or dimensioned drawing to specify the hole and cutout locations.
For a custom tempered glass panel, send the application, finished dimensions, and any hardware drawings to PRL Glass & Aluminum. Include the glass thickness and edge finish if you know them.
