Glass pack phenomena

Physical phenomena may emerge in glass packs. All these phenomena are typical of glass packs and are not considered as defects. They are:
  • Interference effect;
  • Glass pack (double-glass) effect;
  • Anisotropy;
  • Condensate formation;
  • Wet glass surface.

Interference effect

The interference effect can be observed in glass packs with polished (float) glass units. This effect is manifested in coloured zones of greater or smaller intensity which change when touching the glass. Optical interference appears due to the clash of light waves in one spot. The effect increases in case of parallel or almost parallel glass surfaces. The interference effect can be seen on the whole glass area or part of the glass area.
The interference phenomenon emerges randomly and cannot be influenced.

Glass pack (double-glass) effect

A glass pack has a closed hermetic amount of air or other gas. The amount of gas in a glass pack is established at the place of manufacturing in terms of altitude, atmosphere pressure and temperature at the time of manufacturing. If a glass pack is mounted at the different altitude, then the air pressure or temperature may change during operation, causing deformation of glass units in the glass pack, i. e. glass can camber or deflect causing distorted view.
View reflections of different size and intensity may be also visible on glass packs. Such reflections can be especially clear if the background is dark or coated glass is used.


Anizotropija - Anisotropy - phenomenon typical of tempered glass due to internal tensions which appear during the tempering process. Due to anisotropy dark circles or strips can be visible which change according to the angle of view, if glass is in polarised light or the view is seen through polarised glass.
Polarised light exists in the normal day light as well. The level of polarisation depends on the air conditions and position of the sun. The effect is clearer when looking at the glass at oblique angle or in glass packs mounted in corner facades at vertical angle.

Condensate formation

Condensate can form on the external side of glass, if the temperature of such glass is lower than the temperature of the air. Formation of condensate on the external surface of the glass pack depends on the coefficient of light transmittance of the glass pack, relative air humidity, air movement near the glass surface as well as outside and inside temperatures.
In certain cases, condensate can form on the external surface of the outside glass in glass packs with good heat insulation, when relative outside air humidity is high and outside air temperature is higher than the glass surface.

Wet glass surface

The degree of water and glass surface coherence may be different due to the stamps left by rollers, fingers, labels, paper texture, vacuum pick-up tools, remains of sealing materials, signs left by glazing or other means. Zones of different humidity can be clearly visible when glass becomes wet due to weeping (condensate), rain or use of water for washing.

Change of glass colour

All the materials used for manufacturing of glazing products have their own typical colour which does not depend on raw materials and becomes more visible when thickness of glass increases. Glass units with special coating are used for ensuring the requirements of energy savings. Coated glass also has a special colour which may be different due to optical conditions (light permeability and reflection, direction of looking at the glass). Variation of colour intensity is possible due to the amount of iron oxide in glass, coating process, coating itself, variation of glass thickness, place of glass mounting, and this is not possible to avoid. When additional orders of coated glass are processed, it cannot be guaranteed that the colour will be identical due to the reasons related to manufacturing technology. Variation of glass of such a type should not be considered as a basis for claim.

Damage of glass surface

Damage can be done to the glass surface by mechanical, thermal or chemical factors.

Etching with alkaline substances

Alkaline substances washed away from the concrete surfaces and reaching glass as well as splashes of grout can cause etching of the glass surface.
During construction, the mounted glass surfaces must be protected from such factors. Fresh and not yet stiffen splashes of grout can be washed off with water, and damage of glass can only be eliminated with special cleaners. Old damages are usually not possible to eliminate.

Welding drops and sparks from polishing discs and cutting discs

If welding and polishing works are performed near glass surfaces, welding drops or hot polished particles can penetrate the glass. Micro cracks remain on the glass surface after cleaning them from the glass surface.

Facade cleaning factors

Facades very often become dirty and splashed with construction materials during the construction works, especially in brick buildings. In such cases glass surface is usually cleaned with facade cleaners containing hydrofluoric acid which can make damage to the glass surface. This can be avoided by covering glass units with plastic film when construction works are carried out.
Glazing elements should be protected from such kind of damage, as responsibility for them falls to the construction contractor.
It is impossible to provide a thorough and complete list of protective measures due to the great variety of possible damage to the glass surface. Risk must be assessed and respective protection measures applied in each specific case.

Visual defects of glass packs and their determination

General view through a glass pack, i. e. looking at the background behind the glass pack and not at reflections is the most important when performing checking for defects. Possible unconformities are not specifically described or defied.
Checking of glass products in accordance with the Table presented below should be made at the distance of approximately 1 m from the inspected surface, at a certain angle of looking which corresponds to the conventional use of the premises. Checking shall be made in the dispelled day light (for example, cloudy sky), without direct sunshine or direct artificial lighting.


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