What to expect from your glass

Although we hope you do not have any issues with your glass we would like to take the opportunity to outline what to expect from your glass and Prenton Glass’s criteria for visual defects in glass and their acceptability. The page below will cover the following; double glazed units consisting of toughened, laminate or standard glass. Including any coatings or tints on or within the glass.

With all glazing supplied a certain amount of visual distortion is acceptable in the manufacturing process. There is also an acceptable level of defects that can be evident but occur naturally in this process.

We always class damaged glass as unacceptable, it may be installed temporarily as a necessity to keep your home weather proofed but it will always be replaced when a new glass unit has been ordered and delivered.

Our objective at Prenton Glass if to supply and install glass as free as possible of defects caused in manufacture. However you should know what to expect from your glass and accept minor imperfections/ manufacturing visual distortions provided they fall within the scope of he following definitions and acceptance criteria.

There will however be a distinction between large pane glass units and what would be considered ‘standard glass sizes’. A large pane glass unit would be any pane having square meterage of 2.5 and over. In the case of large glass panels the end user/client must be aware that the defect threshold will increase as the glass area percentage becomes higher.

Visual Distortions

Roller wave
Roller wave is a common type of visual distortion that may be visible in toughened glass. It is caused during the toughening process when the heated and slightly molten glass is passed over large rollers while it is cooled. As a result the glass may have a slightly waved finish to the surface.

This does not affect the strength or durability of the toughened glass and is only visually detectable under certain circumstances.

As a rule, the visual appearance of roller wave in glazing is not classed as a glass defect.

Stain Pattern
Anisotropy is a specific visual effect that can be seen in some toughened glass panels under polarised light. It is caused by the differing layers of ‘stress’ or tension within the thickness of a toughened glass unit. When polarised light passes through these areas of the glass it can appear as an iridescent or coloured pattern on the surface of the glass.

As polarised light is present within normal daylight Anisotropy may be visually apparent at certain times of day or under certain lighting conditions. This appearance in glass is often more apparent when the glass is viewed obliquely or when glass is installed at right angles to each other.

This visual phenomenon can also be known as stain pattern, quench marks or leopard spots. Anisophy is an accepted characteristic of the glass toughening process and is no classed as a glass defect.

Brewsters Fringe
These are visual effects seen as a rainbow within a unit. They are not a deterioration o the unit or the glass, but an effect created when light passes through two panes of glass that are parallel and of the same thickness. The resulting light refraction becomes visible as a rainbow effect.

Brewsters fringes can be confirmed by pressing one surface of the unit. The rainbow effect will move and colours change as the surface is depressed and release. This is not recognised as a glass defect.

External Condensation
Condensation may occur with glass units having low U-Values that significantly reduce heat loss. Under localised climatic conditions this can lead to moisture condensing on to the outside surface of the unit. This is not a fault, but rather demonstrates that the product is thermally efficient.

Other distortions in standard sized units
If there are other visual distortions in glazing, they should be inspected to determine what they are and if they are classed as acceptable within the guidelines from the GGF.

Upon inspection we will use the guidelines as detailed below whilst looking through the glass in natural light. The glass will be classed as ‘acceptable’ if the following are neither obtrusive nor bunched together:

  • Bubbles or blisters no longer than 25mm. Small holes partially or wholly enclosed by glass which normally contain air. These may be spherical or non spherical dependant on the mode or formation.
  • Fine scratches, no longer tan 25mm. A long narrow surface flaw produced by a hard object which produces a perceptible depression.
  • Minute particles no longer than 1mm. Loose manufacturing detritus that may remain within the unit after manufacture.
  • Sleek no longer than 25mm. A fine scratch with no perceptible.
  • Scare no longer than 25mm. A scratch which is visibly white.
  • Inclusions. Insoluble matter retained within or on the surface of the glass during manufacturer.
  • Visual effects. Toughened and Toughened/ laminated glass as used in Prenton Glass installations is a processed glass. It is acceptable that a small amount of distortion may be visible through such glass. Using sealed glass units increases the number of reflections which may accentuate any visible visual distortions. In addition, metal oxide coatings such as low E coatings may produce momentary visual effects in the surface of glass.

Inspection Criteria
The glass area to be viewed is the entire vision area with the exception of a 50mm wide band around the whole perimeter o each of the glass panes. Any defects falling within this area are acceptable.

The IGU’s shall be viewed at near normal incidence. ie at right angles to the glass surface from the room inside, standing at a distance of not less then 2 metres away from the inner glass surface for annealed float glass and 3 metres away for all other glass types e.g. laminated or toughened glass.

The assessment of visual quality of the panes of glass should be carried out in natural daylight but not in direct sunlight and with no visible moisture on the surface of the inner or outer glass panes.

The use of strong lamps and / or magnifying devices is not allowed. It is not permissible to find defects at close range and then mark them so as to be visible from the given viewing distance. Obtrusiveness of faults shall be judged by looking through the glass, not at it.

Prenton Glass note that glass issues are generally subjective.

Please note this document has been produced to quantify acceptable tolerance of glass quality for manufacturer and client. Its purpose i to set a reasonable baseline on what is permissible within specific parameters.

Prenton Glass will however accept independent inspection evaluation from the industry ombudsman the GGF.

Prenton Glass will not be liable for any fees in conjunction with an independent inspection.