Visual Distortions in Glass

A certain element of visual distortion is accepted in glass manufacturing. The glasses that are ‘processed’, such as toughened glass, go through additional manufacturing processes which can change the visual appearance of the glazing.

Damaged glass is always classed as unacceptable and will not be installed on site, however, there are a few types of visual distortion in glass that are well documented and accepted as a natural element of processed glass.

anisotropy pattern in glass

Anisotropy (or Strain 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 a Strain Pattern, Quench Marks or Leopard Spots. Anisotropy is an accepted characteristic of the glass toughening process and is not classed as a glass defect.

Roller Wave

Roller Wave is a common type of visual distortion that may be visible in toughened glass units. It is caused during the glass toughening process when the heated and slightly molten glass is passed over large rollers whilst 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. However, at IQ Glass we follow the GGF Guidelines (Glass and Glazing Federation) to determine what is an acceptable level of roller wave and what is not.

Read More: What is Roller Wave?

roller wave distortion in glass balustrade
structural glass facade with concealed vents

Newtons Rings (Interference Fringes)

Newton Rings are the most common example of Interference Fringes in insulating glass as they are often the easiest to see. They are caused when two panes of an insulated glass unit touch each other.

Where the glass units connect you will see a series of concentric fringes that surround the point of contact.

The visual distortion is caused by the delay in light reflection between these two surfaces of glass causing two light waves to combine.

The presence of a Newton Ring in an insulated glass unit may be a result of a narrow spacer bar being used within the insulated glass unit. If the spacer bar used in your IGU is too narrow the natural deflection in the glass units may cause the two panes to touch.

In some instances, changes in air pressure (ie if the glass is being installed at high altitude) between where the glass unit was sealed and where it is installed may cause glass to touch at the centre.

It is always highly important for you let your architectural glazier know exactly how your glass installations will be used and where they are located. This will enable them to properly specify the best glass make up.

Brewster's Fringes (Interference Fringes)

Brewster’s Fringes are another form of Interference Fringe found in architectural glazing installations. These are less common as they are rarely noticeable under normal lighting conditions.

These interferences fringes are actually caused by the use of high-quality glass with optically flat surfaces that are used parallel to each other (ie in an insulated glass unit). Where both panes of high-quality glass within the unit are the same thickness, light reflected in one glass can combine with that from the other. As the light path differences are so small (due to the high quality and precision manufacturing of the glass panes) these can cause interference.

The visual effect is a series of faint bands or irregular shapes (normally grey or very faintly coloured) which could be seen in any area of the glass unit. However, it is very difficult to see a Brewster’s Fringe under normal conditions. In most instances you will only be able to notice these interference fringes under special lighting or viewing conditions (such as viewing the glass at an angle with a shaded area beyond the glass).

You can check if your visual glass effect is a Brewster’s Fringe by lightly tapping or pressing the glass. If it is a Brewster’s Fringe the fringes will move in response to the small changes in the geometry of the parallel glass panes.

This visual effect is not manufacturing fault or defect and glass displaying this fringe pattern does not need to be replaced.

minimally framed sliding doors
structural glass facade

Inspecting Glass Distortions

All glass is visually checked when leaving our factory and then again when installed on site. If any glass unit does not adhere to our strict quality guidelines it will be rejected and replaced.

We inspect glass using the guidelines from the Glass and Glazed Federation (GGF) which is the government recognised trade body for the glazing industry. All glass units to be inspected must be done under the strict guidelines set out to determine whether a glass unit is ‘acceptable’ or ‘unacceptable’. These guidelines include rules for how we have to inspect the glass:

  • You should stand no closer than 3m away from the glass face.
  • You should face the glass head-on at a 90-degree angle.
  • Glass inspections should be done in natural daylight but not directly towards the sun and with no visible moisture on the glass.
  • You should look through the glass rather than at the glass.
  • You should exclude a 50mm wide band around the edge of the glass from your inspection.

You can download and view the GFF Guidelines for Glass Quality here.

Other Distortions in Glazing

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.

To inspect the glass we will use the guidelines as detailed above 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
  • Fine scratches, no longer than 25mm
  • Minute particles

Toughened and toughened laminated glass as used in IQ installations is a processed glass. It is accepted 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.

structural glass
Toughened Glass

Distortion in Curved Glasses

The guidelines from the GGF only apply to flat glass. There are currently no guidelines for quality for curved glazing. It is accepted that there will be a certain element of visual distortion through curved glass units, especially if they are used in a double or triple glazed unit. Any roller wave on the surface of the glass will be extenuated through the curving process and the way in which glass is curved may result in varying glass thicknesses across the curve.

If you have any questions about visual distortion in modern glass units please do contact the team at IQ on 01494 722 880.

Suction Marks on Glass

When glass is being manufactured and installed rubber suction cups are used to move glass without damaging it. Sometimes, these suction cups can leave invisible residue on the glass surface that dissipates over time.

The presence of this material in the surface of the glass can sometimes be visible when moisture is settled on the surface (such as when condensation forms on the glass).

This is nothing to worry about and is not a defect in the glass. The rubber particles will wear away over time under normal exposure to the elements. They can sometimes be minimised or removed with the use of a glass cleaning materials.

You can read more about visible suction marks on glazing at our technical article: Condensation and Glass Designs

glass lifting machine using rubber suction cups
Rebecca Clayton
Technical Sales
Rebecca is Head of Marketing at the IQ Group and has worked in glazing specification for many years. She has a broad range of technical knowledge about all our glazing products and offers technical advice and guidance to architects for specification. Her easy to digest technical advice is often quoted in magazines and publications. You might also recognise her as one of the IQ Glass CPD presenters.
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