Glass Quality Standards and Quality of Vision

The quality of vision check

This is the recognised method for deciding whether a blemish in glass is acceptable. It is a defined test rather than an opinion, and it is carried out as follows.

• Look through the glass, not at it. This is the whole principle. A blemish is judged by whether it interferes with the view through the pane, not by whether it can be found when you go looking for it with your nose against the surface.
• Stand at least 2 metres back for ordinary transparent glass, and at least 3 metres back for toughened, laminated or coated glass. Almost everything we supply is toughened, laminated, or both.
• View at 90 degrees to the pane, straight on rather than at a glancing angle.
• In natural daylight, not looking towards the sun, and not with a lamp or a torch held against the glass.
• With the glass dry, with no visible moisture, condensation or dirt on either face.
• Excluding a 50mm band around the edge of each pane. The perimeter is where the cutting, grinding and handling happens, and it is excluded from the assessment by the standard.

This is the standard we supply to

The GGF visual quality standard is what the UK glass industry works to, and it is what we supply to and assess against. That is deliberate: it is a defined, published test rather than a matter of opinion, so it gives you and us the same yardstick if a question ever arises.

Our full acceptability notes, including tolerances, bow limits and the detail on edges and finishes, are in the toughened glass section of our material finishes and quality standards page.

Spontaneous breakage: also not a defect

Roller wave and anisotropy are inherent characteristics you can see and live with. There is one more inherent property of toughened glass that belongs on this page, because it is the other thing the industry does not treat as a manufacturing fault, and it is why glass is excluded from our 10 year warranty.

Very occasionally a toughened pane breaks with nothing having touched it, sometimes years after it was fitted. The usual cause is a microscopic nickel sulphide inclusion formed in the original float glass, which slowly changes volume and releases the stress locked into the pane. The Glass and Glazing Federation describes inclusions of this kind as "a natural part of glass manufacture" and "so small that it is currently impossible to detect them", and states that "there is currently no method to calculate the likelihood". Its guidance to consumers is that where standard toughened glass has been accepted and a spontaneous breakage happens later, "the cost of replacements will be borne by you".

The likelihood can be reduced by heat soak testing to BS EN 14179-1, which the same GGF guidance is clear "will still not offer a 100% guarantee". Lamination does something different and is often confused with it: it does not make a failure any less likely, it keeps the broken pane in place.

The exclusion is worded in our product warranty as "any glass, as a materially fragile product, would not be covered as part of the warranty". The full explanation, including the mechanism, the figures that can and cannot be sourced, and how spontaneous breakage differs from thermal stress and edge damage failures, is in the spontaneous breakage section of our toughened and laminated glass guide.

Walk-on and floor glass

Walk-on glass is thicker, heavier and more highly processed than anything in a window, usually with a painted or printed border and an anti-slip treatment on the top face. The anti-slip treatment stays clear. It alters the surface at a near-microscopic level rather than adding a texture, a pattern or a film, so there is no clouding, no haze and no interruption to the view down through the panel. From standing height, looking down at the floor, which is how anyone actually sees it, there is nothing to notice. To be straight about the limit: very close up, or in raking light striking the surface at a particular angle, you may be able to tell the treatment is there. Our anti-slip glass treatment page sets out the pendulum test figures behind it, our walk-on glass care guide covers how to keep it looking right, and the glass floors and well covers range shows what the finished units look like.

What is not acceptable

The standard is not a shield for poor workmanship, and we would not use it as one. The following are not acceptable and we will put them right:

• Chips, cracks or damaged edges that affect the structural integrity of the pane
• Scratches that are obtrusive when viewed as described above, or longer than the standard allows
• Blemishes bunched together in the vision area
• Smears, fingerprints, labels, dirt or debris, which are a cleaning matter rather than a glass matter but should not be there when we hand a job over
• Glass that is the wrong size, wrong specification, wrong tint or wrong thickness for the order

Checking your glass when it arrives

Glass is the one thing on a delivery worth looking at before the driver leaves. Check the faces and the edges for chips and cracks, and note anything you find on the delivery paperwork. Our delivery information page sets out how that works, and returns information covers what happens next.

If something does not look right, send us photographs taken in daylight from a normal viewing distance, and a close-up for reference. We would far rather look at a photograph early than argue about it later. Call 0114 243 9009 or email sales@diomet.co.uk.

See it in use

• Frameless infinity glass balustrade, Ranmoor - 25 metres of laminated glass with no handrail, which is where roller wave and anisotropy show most
• Laminated glass balustrade, Cardoness, Scotland - toughened against laminated on a coastal site
• Walk-on floor glass in a kitchen, Newcastle - painted borders and a glass floor in daily domestic use
• Electric hinged walk-on glass floor, Swindon - three 25.5mm panels with black borders
• Toughened glass supplied to Devon and Cornwall - cut to size panels shipped nationwide
• All our projects