In a word, yes. The optical quality of transparent glass is directly affected by its manufacturing method. 

Float glass generally produces the most even glass surface and the fewest imperfections. Thermal treatment during the toughening process inevitably results in a product of (usually marginally) lower quality than the float glass it is produced from.

Some imperfections, covered in more detail below, are considered normal. Visual quality of laminated glass is influenced by the thickness of the glass panes and interlayers. 

Inherent qualities of toughened glass

Thermal treatment during the production of toughened glass can result in minor imperfections such as bows, roller wave distortion (caused by a reduction in surface flatness, which may cause images reflected in the glass appear to be rippled), a degree of haze, and an effect called anisotropy. 

Says the GGF: ‘Anisotropy is the result of stress patterns in the cross section of the glass becoming visible. These areas of stress produce… coloured zones visible in polarised light, sometimes referred to as ‘leopard spots’.

These effects are more noticeable at glancing angles. These are not considered as faults.’

Laminated Glass

Visual quality of laminated glass is influenced by the thickness of the glass panes and interlayers, number of panes, and the lamination process used.

Laminated glass used in Norrsken windows is made by a process called folio lamination, which has a negligible effect on the optical quality of the final glass. 

Below are seven simple guidelines for inspecting glass. 

• Insulated Glass Units (IGUs; the glass in your windows) should always be viewed from the inside at or near right angles to the glass. Inspecting windows from an oblique angle is inherently more likely to display ‘normal’ characteristics of the glass, such as haze, and confuse the observation.

• Inspect from at least 3 metres away - imperfections must be visible from this distance.

• View the glass in natural daylight and not direct sunlight

• There should be no visible moisture on either the inner or outer glass panes

• Strong lamps and / or magnifying glasses should not be used

• A time limit of 1 minute per m² is allocated for viewing

• Focus on the central 80% of the glass (Zone M as discussed below).

Defects must be visible from the given distance (finding marks at close range and marking them to be visible from further away is not permitted). Obtrusiveness of faults is judged by looking through the glass, not at it. 

Standards for acceptable glass are slightly different for float, toughened and laminated glass - so it’s important to check your unit’s glass type(s) as specified in your Norrsken window schedule. 

IGUs with optical defects (e.g. smears or fingerprints) on the cavity faces - e.g. the inside - of the glass are unacceptable if they are beyond the limits set for stains & residue in the table above.

This also applies to any defects within the 50mm edge zone if they are visibly disturbing.

Toughened glass is assessed according to the same criteria as basic float glass - so use the table above to assess the perceived fault.

However, it’s worth remembering that the toughening process can inherently introduce phenomena like bows, roller wave distortion, haze, and anisotropy. So, when evaluating your toughened glass, remember that multiple images, haze, and ‘leopard spots’ are normal and not faults in the glass. 

Laminated glass panes are evaluated according to float glass criteria, with 25% extra tolerance for the number of faults per laminated pane of glass.

This is because the visual quality of the laminated glass may be worse than that of any component pane - so the greater number of laminated glass panes within an IGU, the greater the likely number of defects and the greater the allowance made for imperfections. 

Here'a a simple checklist if you think there may be a fault in your glass.

• Check the glass type from your Norrsken window schedule. 

• Review the guidelines for assessing glass.

• Inspect the glass, noting the size, frequency and type of each imperfection visible from the given distance. Take images if necessary.

• Compare your findings with the GGF table of critical measures for faults, taking into account any tolerance accorded your IGU type. 

• If you believe the fault is not acceptable, get in touch with our Customer Service & Aftercare team on 01202 632777 or email support@norrsken.co.uk, who’ll be able to provide more help, discuss the query and your options. 

You can read more on the GGF website, or read helpful GGF datasheets on glass quality here.