Non-tempered glass, commonly called annealed glass, is standard flat glass that has been slowly cooled during manufacturing but not subjected to any additional strengthening treatment. It’s the most basic and widely used type of commercial glass, found in everything from picture frames to storefront windows. If no one specifies a glass type, annealed glass is almost always what they mean.
How Non-Tempered Glass Is Made
All glass starts the same way: raw materials are melted at extremely high temperatures and formed into flat sheets. What separates non-tempered glass from tempered glass is what happens next.
Non-tempered glass goes through a process called annealing, where the molten glass is cooled very slowly in a controlled environment. This gradual cooling allows heat to distribute evenly throughout the sheet, giving molecules enough time to settle into stable positions. Once the glass reaches a point where its internal structure is essentially locked in place, it can be cooled more quickly to room temperature. The result is a stable, workable sheet of glass with minimal internal stress.
Tempered glass, by contrast, is reheated after annealing and then rapidly cooled with blasts of air. That rapid cooling creates strong compressive forces on the surface that make tempered glass roughly five times stronger. Non-tempered glass skips this extra step entirely.
Strength and Breakage
Non-tempered glass has a bending strength of about 40 MPa, which is significantly lower than tempered glass. Its compressive strength is excellent at around 1,000 MPa, meaning it handles direct, even pressure well. But it’s far more vulnerable to impacts, sudden force, and uneven stress.
The biggest practical difference shows up when the glass breaks. Non-tempered glass fractures into sharp, jagged shards, often long and triangular, that can cause serious cuts. A rock or ball hitting an annealed window creates a circular puncture with cracks radiating outward. Tempered glass, on the other hand, crumbles into small, cube-like pieces sometimes called “dice” that are much less likely to cause deep lacerations. This is why tempered glass is often referred to as safety glass.
Thermal Stress Limits
Non-tempered glass is also more sensitive to temperature differences across its surface. When one area of a glass pane heats up while an adjacent area stays cool (like the exposed center of a window versus the portion hidden inside the frame), the uneven expansion creates stress. Research on glass in fire conditions has found that annealed glass can crack when the temperature difference between the exposed area and the shaded edge reaches roughly 90°C (about 160°F). This is why non-tempered glass near heat sources, fireplaces, or in direct sunlight with partial shading can sometimes crack spontaneously.
Why It’s Easier to Work With
One major advantage of non-tempered glass is that it can be cut, drilled, and shaped after manufacturing. Fabricators routinely cut annealed glass to custom sizes, grind edges, and drill holes for hardware. All of this work has to happen before tempering if tempered glass is needed, because drilling or cutting tempered glass causes it to shatter instantly. That flexibility makes annealed glass the go-to choice for custom projects where precise sizing or modifications happen on site or after purchase.
Where Non-Tempered Glass Is Used
Non-tempered glass shows up in a wide range of everyday products and settings:
- Windows: Standard residential windows in locations not classified as hazardous by building codes.
- Storefronts and display cases: Shopping mall storefronts, restaurant dividers, and retail display counters where floor-to-ceiling visibility matters but high-impact resistance isn’t required.
- Picture frames and shadow boxes: Framing artwork, photographs, and family heirlooms.
- Furniture protection: Custom-cut glass tops for desks, dressers, coffee tables, and antique wood furniture.
- Mirrors: Most standard mirrors use annealed glass as the base layer.
- Room dividers: Clear or frosted glass partitions in offices, lobbies, and open-concept homes.
Where Building Codes Restrict It
Because of its sharp breakage pattern, building codes limit where non-tempered glass can be installed. The International Building Code requires safety glazing (tempered or laminated glass) in what it calls “hazardous locations,” areas where people are likely to walk into or fall against glass. These include glass doors, shower enclosures, glass panels near bathtubs, low windows near floors, and glass adjacent to stairways or ramps.
For sloped overhead glazing like skylights and glass roofs, annealed glass is generally not permitted as the primary glazing material. Codes typically require laminated, heat-strengthened, or tempered glass in these applications because falling shards from broken overhead glass pose an obvious danger. Annealed glass is allowed only under specific exceptions outlined in the code.
Cost Difference
Non-tempered glass is the least expensive option. Tempered glass typically costs 20% to 50% more than standard annealed glass because of the additional heating and rapid-cooling process. For projects where safety glazing isn’t required by code, and where the glass won’t be subject to heavy impacts or extreme temperature swings, annealed glass keeps costs down without sacrificing basic functionality.
How to Tell If Glass Is Non-Tempered
The easiest check is to look for a small etched stamp in one corner of the glass. Most manufacturers mark tempered glass with a label that reads “tempered” or “safety glass.” If there’s no stamp, the glass is likely non-tempered, though the absence of a label isn’t a guarantee since stamps can sometimes be hidden by frames or wear away over time.
You can also look at the glass at an angle in bright light. Non-tempered glass tends to appear colorless with a smooth, flawless surface. Tempered glass sometimes shows faint wavy distortions or a slight haze visible at certain angles, a byproduct of the rapid cooling process. If you’re still unsure and it matters for safety or a renovation project, a glass professional can test the pane with polarized light, which reveals the stress patterns unique to tempered glass.