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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 HBD’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. 

Potential Distortions

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 proof. It will always be replaced when new glass has bee ordered and delivered.

Our objective at HBD is 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 imperfection/manufacturing visual distortions provided they fall within the scope of the following definitions and acceptance criteria. 

There will however be distinction between large pane glass units and what would be considered ‘standard size glass’. A large pane glass unit would be any pane over 2.5 sq meters or over. In the vase of large glass the end user/ client must be aware that the defect threshold will increase as the glass area percentage becomes higher.


Condensation forms on an object when the object’s surface temperature goes below the dew point . The dew point is defined as the temperature when the air is 100% saturated with moisture , or where the air is at 100% relative humidity. This is not considered a defect. The window is performing exactly as designed . It is blocking heat on one side of the IGU from reaching the other side . Exterior condensation on energy efficient windows is quite common and is perfectly normal.

Whether or not a window develops exterior condensation is actually a rather complex issue involving environmental and performance parameters. For example condensation is much less likely to form on a cloudy night. Trees or other obstructions close to windows, bushes under windows and even length, angle and projection of soffits or other overhangs can affect the formation of condensation.

A small change in temperature or humidity
from one room to the next could raise or lower the dew point considerably.


The marks visible on the glass surface are in this case referred to as suction cup marks. They may occur on glass surfaces as a result of manufacturing or installation processes. During this process, suction cups are used to manipulate the glass, move it and hold it safely while cutting takes place. If you were to examine these marks under a microscope you would see the surface has peaks and valleys in, it is
possible for minute particles of the suction cup to be deposited on the glass surface and settle into the valleys. These particles are not visible to the unaided eye, however the glass surface composition changes enough that it affects the way water droplets adhere to the glass. Over time with normal exposure to the elements these marks will diminish and disappear. As with any other visual imperfection we can confirm the glass is not defective and is fit for purpose functioning properly.

Roller wave

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

Brewsters Fringe can be seen as a rainbow within a glass unit. These stains are not a deterioration of the unit or the glass, but an effect created when light passes through two panes of glass which are parallell and of the same thickness. This can be confirmed by pressing one the surface of the unit, the rainbow effect will then move and colours may change as the surface is depressed and released.

This visual phenomenon can also be known as stain pattern, quench marks, leopard spots or anisotropy is an accepted characteristic of the glass toughening process and is not classes as a glass defect.

Other visual distortions in standard size units 

If there are any 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 GGF.

Upon inspection we will use the guidelines as detailed below whilst looking through the glass in natural light. The glass will be classed ‘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 dependent on the mode or formation.
  • Fine scratched, no longer than 25mm. A long narrow surface flaw produced by a hard object which produces a perceptible depression.
  •  Sleek, no longer than 25mm. A fine scratch with no perceptible depression.

  • Scare, no longer than 25mm. A scratch which is visibly white.

  • Minute particles, no longer than 1mm. Loose manufacturing detritus that may remain within the unit after manufacture.

  • Inclusions. Insoluble matter retained within or on the surface of the glass during manufacture.

  • Visual effects. Toughened and toughened/ laminated glass as used in HBD 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 umber of reflections which may accentuate any visible visual distortion. In addition, metal oxide coatings such as Low-E coatings may produce momentary visual effects in the surface of glass.
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