Identifying Safety Glass: The 'Tempered' Logo Etching

Every week, someone walks into a glass shop in Toronto holding a pane they pulled from a shower door or a patio sidelight, asking us to cut it down to size. We look at the corner, find the bug, and deliver the bad news: this glass is tempered, and it cannot be modified. Not trimmed. Not drilled. Not notched. The conversation that follows is the reason this article exists.

Tempered glass identification is not complicated. But most homeowners, property managers, and even some general contractors have never been told what to look for. This is the full explanation.

What Is the "Bug" on Tempered Glass?

The bug is a small permanent marking — typically 15 mm to 25 mm in size — etched or sandblasted into one corner of a tempered glass pane. It is not a sticker. It is not paint. It is physically ground into the glass surface and cannot be removed without scratching the glass.

The term "bug" comes from the auto glass industry, where the manufacturer's tiny logo on a windshield looked like a small insect. The name stuck, and the glazing trade adopted it for all safety glass markings.

What the bug contains:

• The word "Tempered" or "Temp" — confirming the glass type
• Manufacturer name or logo — the company that fabricated the glass (e.g., Cardinal, Vitro, Guardian)
• Safety standard reference — in Canada, this is CAN/CGSB-12.1; in the US, ANSI Z97.1 and/or CPSC 16 CFR 1201
• Glass thickness — typically stated in millimetres (4 mm, 5 mm, 6 mm, etc.)
• SGCC number — a certification number from the Safety Glazing Certification Council, a voluntary but widely used industry program
• Impact classification — Class A (passes the higher 1219 mm drop height test) or Class B/C (lower drop heights)

The bug is not optional. Under both Canadian and American safety glazing standards, every piece of tempered glass installed in a hazardous location must carry this permanent identification. If the glass has no bug, it is either not tempered or the mark was improperly removed — both of which are problems.

Why Does This Mark Exist?

The bug exists for one reason: liability and code enforcement.

When a building inspector checks a new construction or renovation in Toronto, they need to verify that glass in hazardous locations meets CAN/CGSB-12.1. They cannot test the glass without breaking it. So they look for the bug. If it is there, the glass passes visual inspection. If it is not, the inspector can flag it and require proof of compliance.

The bug is the glass equivalent of the CSA label on an electrical panel. It is the visible proof that the product was manufactured, tested, and certified to a specific safety standard.

The Canadian General Standards Board (CGSB) published CAN/CGSB-12.1-2017, the current edition of the national safety glazing standard. This standard defines the impact test requirements (a steel ball dropped from specified heights onto the glass), the fragmentation pattern requirements (tempered glass must break into small, relatively blunt pieces rather than sharp daggers), and the permanent marking requirements.

If you are a Toronto homeowner or property manager, the bug is your quick visual audit. Walk up to any glass door, shower enclosure, or low window in your building. Look at the corners. If there is a bug, you know the glass was manufactured as safety glass. If there is no bug in a location where safety glass is required, you may have a code compliance issue worth investigating.

Where Is Tempered Glass Required in Ontario?

The Ontario Building Code (OBC), referencing CAN/CGSB-12.1, mandates tempered or laminated safety glass in what it calls "hazardous locations." These are spots where a person is likely to fall into, walk into, or otherwise contact glass with enough force to break it.

Hazardous locations requiring safety glass:

Location	Why It Is Hazardous
Glass in doors (all types)	People walk into doors, push them, and fall against them
Sidelights within 600 mm of a door	Easily mistaken for a door opening
Shower and bathtub enclosures	Wet, slippery surfaces increase fall risk
Glass panels with bottom edge less than 600 mm from floor	A person can trip and fall into the glass
Glass adjacent to stairways and ramps	Fall zones with high impact potential
Sliding patio doors	Full-height glass that people walk toward at speed
Storm doors	High-traffic entry points

In Toronto's housing stock — from Victorian row houses in Cabbagetown to post-war bungalows in Scarborough to highrise condos in CityPlace — these locations appear in virtually every dwelling. If your home has glass in any of these locations without a tempered glass bug, it is worth a closer look.

For residential window replacement projects, we verify that all glass in hazardous locations meets current code requirements during the quoting process. Old homes frequently have annealed glass in locations that now require safety glass under the current OBC.

The Physics: Why Tempered Glass Cannot Be Cut

This is the part that surprises people. Tempered glass looks identical to regular annealed glass. Same clarity. Same thickness. Same weight. But internally, it is a completely different material.

How tempering works:

1. A sheet of annealed (regular) glass is cut to its final size and shape
2. All edges are ground and polished — no modifications can happen after this point
3. The glass enters a tempering furnace at approximately 620°C (1,148°F)
4. The glass is rapidly cooled ("quenched") with high-pressure air jets

This rapid cooling creates a specific internal stress profile. The outer surfaces of the glass are in compression (squeezed together). The interior core is in tension (pulled apart). These opposing forces are locked in permanent balance.

This compression-tension balance is what makes tempered glass strong. It is roughly four to five times stronger than annealed glass of the same thickness. It resists impact, wind load, and thermal stress far better than regular glass.

But the same stress balance is why you cannot cut it.

The moment you score, cut, or drill tempered glass, you breach the compressed outer surface. The stored energy in the tension zone releases instantaneously and catastrophically. The entire pane does not just crack — it explodes into thousands of small, roughly cube-shaped fragments. This is not a gradual failure. It happens in a fraction of a second, and the glass is destroyed.

Think of tempered glass like a tightly inflated balloon. You cannot cut a small hole in a balloon. The moment you pierce it, the whole thing goes.

This is not a theoretical risk or a "sometimes" outcome. It happens every single time. There is no technique, tool, laser, or method that allows you to cut tempered glass after it has been quenched. The physics simply do not permit it.

How to Identify Tempered Glass: Five Methods

Method 1: Look for the Bug (The Fast Way)

Check all four corners of the glass pane. The bug is almost always in a bottom corner but can appear in any corner. It is a small etched or sandblasted rectangle containing text. You may need to clean the glass and look at an angle to spot it — the etching is subtle and sometimes obscured by the frame.

If you see the bug: the glass is tempered. Full stop.

Method 2: Examine the Edges

If the glass is exposed (not framed), look at the edges.

• Tempered glass: Edges are smooth and slightly rounded from the grinding process that occurs before tempering. They feel finished.
• Annealed glass: Edges are sharper, sometimes with visible "shell" marks from the cutting process. They feel raw.

This method is not foolproof — some annealed glass has polished edges — but rough, sharp edges almost certainly mean annealed glass.

Method 3: The Polarized Lens Test

Put on a pair of polarized sunglasses and look at the glass with a bright light source behind it (daylight works). Tempered glass will show faint rainbow-coloured stress patterns — typically appearing as diagonal lines or a grid pattern. These patterns are caused by the internal compression-tension stress field and are invisible to the naked eye.

Annealed glass shows no such patterns through polarized lenses. This is one of the most reliable field tests when the bug is missing or hidden by framing.

Method 4: Check for Roller Wave Distortion

During tempering, the glass rides through the furnace on ceramic rollers. These rollers leave very faint surface distortions called "roller wave" or "roller distortion." Look at a reflection in the glass (a straight roofline or a window frame across the street). If the reflected line appears slightly wavy, the glass has been through a tempering furnace.

This distortion is extremely subtle on modern high-quality tempered glass but is more noticeable on older or lower-quality panels.

Method 5: The Sound Test

Tap the glass with a knuckle or a coin.

• Tempered glass: Produces a higher-pitched, clear ring
• Annealed glass: Produces a lower, duller thud

This test requires some experience and is best used as a supporting indicator alongside other methods, not as the sole identifier.

The Consequences of Getting It Wrong

Scenario 1: Cutting Tempered Glass

A homeowner in Etobicoke brings in a glass panel from a patio door sidelight. They want us to trim it by 2 inches to fit a slightly different frame. We find the bug. The glass is tempered. If we attempted to score it with a glass cutter, the entire panel would shatter on the cutting table. No partial cut. No clean break. Total destruction and a pile of small fragments on the floor.

The fix: Order a new pane of annealed glass cut to the correct dimensions, then send it for tempering. Turnaround: typically 5-10 business days for custom tempered glass in the GTA.

Scenario 2: Missing Safety Glass

A property manager in North York discovers that a ground-floor apartment has regular annealed glass in a patio sliding door — no bug, sharp edges, confirmed annealed. This glass is in a hazardous location and does not comply with CAN/CGSB-12.1. If a tenant falls through it, the glass breaks into large, razor-sharp shards instead of the blunt fragments tempered glass produces. The liability exposure is significant.

The fix: Replace the glass with properly tempered units. For commercial and multi-residential properties, our commercial glass repair service includes compliance audits to identify non-conforming glass.

Scenario 3: The Renovation Surprise

A contractor renovating a bathroom in a 1970s Toronto home removes a glass shower enclosure. The glass has no bug. It is annealed glass — installed decades before the current safety glazing requirements were widely enforced. The new shower enclosure must use tempered glass conforming to CAN/CGSB-12.1. No exceptions.

Tempered vs. Laminated: Both Are Safety Glass

Not all safety glass is tempered. Laminated glass — two sheets of glass bonded with a polyvinyl butyral (PVB) interlayer — also qualifies as safety glass under CAN/CGSB-12.1. The bug on laminated glass will say "Laminated" instead of "Tempered."

Key differences:

Property	Tempered	Laminated
Break pattern	Small blunt fragments	Cracks but holds together (spiderweb)
Can be cut after manufacturing?	No — shatters	No — the interlayer complicates cutting and the tempered layers (if present) will shatter
Sound transmission	Standard	Better (the PVB interlayer dampens sound)
UV blocking	Minimal	PVB blocks ~99% of UV
Cost (6 mm, per sq ft)	$8–$15	$12–$22
Common uses	Shower doors, patio doors, storefronts	Storefronts, skylights, security glazing

For Toronto storefronts deciding between the two, our comparison of tempered vs. laminated glass for Etobicoke storefronts breaks down the security implications in detail.

If you are planning a bathroom renovation with custom shower glass, our guide on custom shower glass thickness for Oakville bathroom renos covers the 8 mm vs. 10 mm vs. 12 mm decision.

What to Do When You Need a Different Size

Because tempered glass cannot be cut after tempering, the sizing process works in reverse compared to regular glass:

1. Measure the opening precisely — to 1/16 inch tolerance
2. Order annealed glass cut to exact dimensions — including any cutouts for handles, hinges, or notches
3. All edge work is completed — grinding, polishing, drilling any holes
4. The glass enters the tempering furnace — 620°C heat treatment followed by rapid air quenching
5. The finished tempered pane is delivered — it is now locked at that exact size forever

This means you cannot order tempered glass and "trim it to fit" on site. If the measurements are wrong, the glass must be reordered and re-tempered. This is why accurate field measurement is critical for every tempered glass installation.

For residential projects in Toronto, we send a technician to template the opening after all framing, tile, or finishing work is complete. This prevents the costly cycle of ordering, discovering it does not fit, and reordering.

The Bug as a Decision Tool

Here is the practical takeaway for anyone working with glass in Toronto — whether you are a homeowner doing a renovation, a contractor on a job site, or a property manager assessing a building:

Before you do anything to a piece of glass, check the corners for the bug.

• Bug present, says "Tempered": Do not cut it. Do not drill it. Do not try to modify it. If you need a different size, order new glass.
• Bug present, says "Laminated": Do not cut it. Same rule applies — though for different reasons (the interlayer makes clean cutting nearly impossible without delamination).
• No bug, glass is in a hazardous location: Investigate. The glass may be non-compliant annealed glass that should be replaced, or the bug may be hidden under the frame gasket.
• No bug, glass is not in a hazardous location: The glass is likely annealed and can be cut, drilled, and modified using standard glass-working techniques.

The bug is the single most useful piece of information on any pane of glass. It takes two seconds to check and can save you hundreds of dollars in destroyed glass — or prevent a serious safety hazard.

A Note on Heat-Strengthened Glass

There is a third category worth mentioning: heat-strengthened glass. It goes through a similar heating process as tempered glass but is cooled more slowly, creating lower internal stress. Heat-strengthened glass is roughly twice as strong as annealed glass (compared to four to five times for fully tempered).

The key difference: Heat-strengthened glass breaks into larger fragments similar to annealed glass — not the small blunt cubes of tempered glass. For this reason, heat-strengthened glass does not qualify as safety glass under CAN/CGSB-12.1 and cannot be used in hazardous locations.

Heat-strengthened glass may or may not carry a bug. If it does, it will say "Heat Strengthened" or "HS" rather than "Tempered." Like tempered glass, it cannot be cut after processing.

Frequently Asked Questions

How do I identify tempered glass?

Look for a small etched stamp — the "bug" — in one corner of the glass. It will include the word "Tempered" or "Temp," the manufacturer name, a safety standard reference (CAN/CGSB-12.1 or ANSI Z97.1), and the glass thickness. If no bug is present and the glass is in a hazardous location, it may not be code-compliant.

Can you cut tempered glass?

No. Tempered glass has extreme internal stress from the heat-treatment process. Any attempt to score, cut, or drill it releases that stored energy instantly, and the entire pane shatters into small fragments. All cutting must be done before tempering.

What does the bug stamp on tempered glass mean?

The bug is a permanent etched or sandblasted label certifying the glass has passed safety impact tests. It identifies the manufacturer, the applicable safety standard (CAN/CGSB-12.1, ANSI Z97.1, or CPSC 16 CFR 1201), the glass thickness, and whether the glass meets Class A or Class B impact ratings.

Where is tempered glass required by the Ontario Building Code?

Tempered or laminated safety glass is required in all glass doors, sidelights adjacent to doors, shower and bathtub enclosures, glass panels within 600 mm of the floor in hazardous locations, glass near stairways, and sliding patio doors. The glass must conform to CAN/CGSB-12.1.

What happens if tempered glass has no bug stamp?

Unstamped glass in a code-required safety location is a compliance problem. It may be annealed (non-safety) glass that was installed incorrectly, or the bug may have been removed during framing. A glazing professional can verify the glass type using polarized light inspection or edge examination.

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