Blog · Glass and quality · 8 October 2026
Why new glass isn't always perfect, and how a defect is judged
Every pane of glass picks up small marks between the float line and your site. Australian standards set a fair way to decide which ones are defects: stand 3 m away, square on, in daylight, and look for no more than 60 seconds. If you can't see it that way, it isn't one.
One test for every pane, small or large
Most customers expect new glass to be flawless, and we understand why. You've paid for new windows, you clean them for the first time, the light catches the glass and there's a tiny scratch or bubble. It's frustrating.
On a small window, swapping the unit is straightforward. Large glass is different. Getting a big unit in can mean closing the street for a day and craning it up three storeys, so replacing it means doing all of that again and weeks of waiting. When a customer finds a mark they can only see from 10 cm away, a good working relationship can fall apart over something nobody could see from the room.
That's why the industry uses one agreed test, and it's the same for every pane, whatever its size. If glass fails the test, it's a defect and it gets fixed. If it passes, it meets the standard. The standards protect the customer from genuinely faulty glass, and they protect the builder and the supplier from removing good glass over marks the test accepts.
Why glass picks up marks at all
Building glass in Australia is almost all float glass, which comes out of manufacture very flat with few imperfections. From there it's cut, drilled, toughened, laminated and assembled into double or triple glazed units. Each step involves handling, and the glass is handled again in transport, when it's glazed into the frame, on delivery and during installation. Small marks can happen at any of those points.
We buy our glass units from several Australian glass processors, and Australian Glass Group (AGG) is one of the largest of them. Before dispatch, units are inspected under strong lights set up to show up scratches and bubbles; at AGG, three people check each unit. The glass is checked again on delivery. Even so, a mark can be missed, or appear later in transport or on site. Inspection can't make glass perfect, and the standard doesn't expect it to.
The method
How to inspect glass after installation
This is how AGG, one of our glass suppliers, sets it out in Technical Bulletin 1008. The method comes straight from the Australian standards, so it applies whichever processor made the glass. The relevant standards are AS 4666 (insulating glass units) and AS/NZS 4667 (cut-to-size and processed glass):
- Inspect early, as soon as practical after supply or installation.
- Clean the glass the way the manufacturer recommends, and make sure it's dry.
- Use natural daylight, but not direct sunlight.
- Stand 3 m inside the room, square on to the glass.
- Look for no more than 60 seconds.
- Ignore the glass inside the frame rebate; it's hidden by the frame.
Any scratch or bubble you can see this way in the primary vision area is a defect. The primary vision area is the centre of the visible glass. The outer strip, 10 to 15% of the visible glass size (AS 4666 tables 5.5 and 5.6), is the secondary vision area, where the allowances are wider.
Torches held against the glass, low-angle light and inspecting from a few centimetres away aren't part of the test.
What isn't a defect
Some effects come from the physics of glass and light, not from faulty manufacture. The Australian standards and AGG don't treat these as defects:
- Anisotropy: light and dark patches on toughened glass, most noticeable through polarised sunglasses. It's an inherent property of toughened glass and can be stronger in a unit with several toughened panes.
- Haze: a faint milky look in some light, more visible on low-E coated glass and laminates, and sometimes slightly blue. It can come and go with the time of day.
- Newton's rings: rainbow circles that appear when the two panes of a double glazed unit flex towards each other, from air pressure or wind. Thicker glass reduces the chance.
- Brewster's fringes: random rainbow lines caused by light reaching the glass from several directions.
- Preferential wetting: patterns that show only when the glass is wet, left by the suction lifters used to handle it. They fade over time and don't affect performance.
- Multiple reflections: a double glazed unit shows two reflections, which you notice most looking at city lights at night.
- Vertical roller wave: toughened glass rests on rollers in the furnace and keeps a very slight wave. Vertical roller wave isn't a defect. Horizontal roller wave is allowed up to 0.15 mm between peak and trough, and edge kink at the edge of toughened glass up to 0.2 mm. Both are measured with a steel rule and feeler gauges.
What builders can do
- Tell your client how glass is judged before handover, not after. A one-line note in the contract or handover pack, pointing to AS 4666 and AS/NZS 4667, saves the argument later.
- Inspect at delivery and again straight after installation, before other trades work near the windows. A scratch from render, a scraper or a ladder isn't a manufacturing defect, and it's hard to tell the difference later.
- For large or hard-to-reach units, agree who inspects, when and how before the crane day. That's when a replacement is cheapest.
If a mark fails the test
Send us a photo, the window reference from the schedule and where the mark sits on the glass, and tell us it was seen from 3 m in daylight. We'll check it against the standard with you. If it fails, it's a defect and we sort it out.
The full method, with tolerance tables for flatness, squareness, size and sealant depth, is in AGG's bulletin: AGG Technical Bulletin 1008: Inspection of Glass Defects (August 2024, PDF).
More on the glass we use: glass options for Schueco uPVC windows.
Questions about glass on a job?
FAQ
Questions from builders and architects
How do you check new glass for defects?
Clean and dry the glass, then stand 3 m inside the room, square on to the pane, in natural daylight but not direct sun. Look for no more than 60 seconds. A scratch or bubble you can see that way in the central (primary) vision area is a defect. This method comes from AS 4666 and AS/NZS 4667 and is set out in AGG Technical Bulletin 1008.
Is a mark I can only see up close a defect?
Usually not. The standard is judged from 3 m. A mark you can only find from 10 cm away, or with a torch held against the glass, does not fail the test. Marks near the edge, in the outer 10 to 15% of the visible glass, are judged more leniently than the centre.
What are the rainbow patterns or patches in my glass?
Most likely Newton's rings, Brewster's fringes or anisotropy (light and dark patches seen through polarised sunglasses). These are optical effects of how light behaves in glass, especially toughened glass and double glazing. AGG and the Australian standards don't treat them as defects.
Why does the inspection method matter so much on large glass?
Access. Replacing a large unit can mean closing the street and craning it up several storeys again. Agreeing the inspection method before the job, and inspecting early, means everyone judges the glass the same way. The test is the same for small and large panes.
When should glass be inspected?
As early as practical after delivery or installation. Inspect before other trades work around the windows, so a mark from site work isn't confused with a manufacturing one.