Criteria for appreciation and acceptance of the appearance of the glazing — Gridoma

Criteria for appreciation and acceptance of the appearance of the glazing

1. Observation conditions

The glazing shall be examined as follows:

– observation from the inside of the enclosure to the outside, in natural light (daylight), in the absence of sunlight falling directly on the glass pane;

– examination only in transmission and not in reflection;

– defects will not be marked on the glass pane;

– observation during normal occupancy of the space, the angle of observation must be as close as possible to the angle perpendicular to the glass plane;

– observer at a distance of at least 2.0 m from the glass;

– examination time of each glass pane: 60 seconds/m2 maximum.

Comments : These preconditions exclude as significant defect:

– Elements perceived under special lighting conditions (oblique daylight, artificial lighting…);

– Elements viewed closely;

– Elements viewed from the outside;


R – Edge area of 15 mm, usually covered by frame or corresponding to the perimeter sealing in case of unframed side,

E – Visible edge area with a width of 50 mm,

M – Main area.

2. Points to be considered
2.1. Defects or asymmetries

Punctual defect: localized asymmetry, no preferential dimension. Punctual defects may be, as the case may be, stains, bubbles, stones, dents, impact, deposits of too great a thickness.

Linear defect: linear shape asymmetry. Linear defects can be small cracks or scratches.

NOTE: Inaccessible scratches on the inner faces of insulating glass generally have a lighter shade.

Punctual and/or stain-like residues

NOTE: these defects are materials left on the glass surface. Usually, these residues are from sealants.

NB: visual deformations of the objects viewed, generated by an episodic deformation (example: dilation/contraction of gas in the gap) and/or by shades of materials, are not defects.

3. Acceptability of defects

For mirror, clear and mass-tinted glass, the following shall be observed:

  • scratches;
  • surface damages not perceived at fingertips;
  • closed bubbles;
  • crystalline intrusions (unmelted particles);
  • shallow flat damage on the edges of processed edges;
  • superficial scoring on processed edges that do not impact glass resistance.

The following table shows the tolerated defects, which cannot be avoided due to the additional handling of the glass sheets and the specifications of the technical security process.

Acceptance criteria for defects of float and coated glass

4. Aspect variations

All types of glass, whether tinted, deposited or ornamental, show slight variations of hue from one production batch to another. There is possible to be some difference in shade between a glass pane and the next. This variation is not a defect.

5. Interference fringes

Under certain transient lighting conditions, optical phenomena may occur by combining rays reflected on glass pane surfaces, which may lead to the appearance of tinted fringes called interference fringes (so-called “Brewster fringes”).
This phenomenon is due to the perfect flatness and parallelism of the glass faces.

Interference fringes move when pressure is applied to the center of the glass.

The risk of fringes is reduced in the case of asymmetrical composition glass panes.

This phenomenon of interference fringes is not a glazing defect.

6. Condensation

– on the outside face of the outer glass sheet (face #1).
– on the outside face of the inner glass sheet (face #4);
– on one of the faces facing the gap (face #2 or #3)

Condensation on the face towards the room

Condensation on the inner glass sheet is the result of one of the following situations:

– Poorly heated and/or insufficiently ventilated space

– Very humid space due to room occupancy or important sources of humidity (kitchen…);

– Very low outside temperatures or high outside temperatures and humidity; When using insulating windows, the frequent phenomenon of condensation generally signals the existence of a poorly heated, insufficiently ventilated or very humid space. Only intervention on these parameters is likely to lead to significant improvements.

Condensation on the outside glass sheet

Superficial condensation on face #1 of insulating glass will appear if the temperature at the level of this face of the glass pane is much lower than the temperature of the outside air and if the dew point (the temperature at which water vapour becomes liquid) of the latter is higher than the temperature of the glass. The surface temperature on the outside of a glass pane depends on:

– the flow of heat coming from inside and crossing the glass. It depends on the temperature differences between the inner and outer surfaces of the glass pane and the U-value of the latter;

– by convective transfer with outside air;

– by radiation losses, especially to the sky. Various studies, as well as the measurements carried out, indicate that the heat exchange by radiation is relatively limited by cloudy weather. On the contrary, when the sky is clear at night, there are significant thermal losses to the sky. Also, studies have shown that:

– a simple window never has a surface temperature below the temperature of the outside air, so the formation of condensation on the outside face is excluded;
– improving thermal insulation (low Ug) involves reducing the heat transfer to the outer surface: the outer glazed surface is colder and the risk of condensation increases;
– when the wind speed is high, the glass temperature tends to approach that of the outside air;
– the risk that a glass pane would have a much lower temperature than the outside air decreases as the weather becomes colder.

In conclusion, the condensation on the outside of glass panes is a phenomenon that can sometimes be observed at night and early in the morning on well-insulated windows, when the sky is clear and in the absence of wind. Thermal losses to the clear sky are the main cause of this phenomenon. It is important not to consider this as a criterion for the bad quality of double glazing, but as a proof of good thermal insulation.

Condensation on a face towards the gap

When condensation occurs between glass sheets, it is a defect.

4. Maintenance of glazing
  1. Important preliminary instructions

By its very nature, the glass is recognized as durable, hard and easy to maintain. To ensure that it remains clean and bright for as many years as possible you must follow the instructions in this guide.

  1. 2. General recommendations:

      Be sure to clean the glass safely

Carefully read the instructions concerning the chemical agents and detergents you intend to use and follow the manufacturer’s recommendations exactly

Never use products containing hydrofluoric acid or derivatives of chlorine or ammonia to clean the glass, as they may destroy the surface of the glass or lacquered layer.

Do not use highly acid or highly alkaline products, as they may attack the glass. Brushed (satin) glass surfaces should never be cleaned in spots, but on the entire surface.

Avoid cleaning the glass if it is exposed to heat sources or when the temperature is negative.

  1. Cleaning at construction sites (initial cleaning after the installation is completed)

Glass can get dirty especially during construction works. In particular, contamination with mortar, cement, concrete and alkaline substances that may corrode the glass surface must be avoided.

Following such contacts, the glass should be washed immediately without waiting for the work to be completed. Sometimes it is advisable to apply a protective plastic foil over the installed glass.

Remove labels, cork protections immediately. If necessary, isopropanol-type solvents may be used.
Traces of grease, fingerprints etc., may be cleaned using acetone or methyl ethylketone solvents, provided that these substances do not come into contact with the painted back of the glass.

Rinse thoroughly with clean water. Inspect the glass for dirt.
Inspect the glass for dirt.

  1. Regular cleaning

Do not use products based on hydrofluoric acid, fluorine, chlorine, ammonia or derivatives as they may attack the glass surface.

Cleaning will be done with clean water, in which a little detergent has been dissolved.
Do not attempt to remove impurities from the glass if it is dry.

Do not apply too much pressure on glass. After the glass has been cleaned, rinse with clean water and dry with a glass squeegee.

Make sure to thoroughly dry the edges of mirrors with a soft clean cloth.

  1. Special cleaning

If regular cleaning does not work, special measures can be applied:

Clean grease stains or other organic pollutants with a clean cloth soaked in solvents (isopropanol or acetone), taking care not to touch the painted (silvered) part of the glass.

Clean other stains with a cloth soaked in water in which cerium oxide has been dissolved (100÷200 grams/liter).

If the situation allows, you can try cleaning with pressurized water, taking care that the water is at ambient temperature.

Rinse and dry thoroughly, then resume the steps of regular cleaning.

Type of interior mounted decorative glassCleaning instructions
Satin glass, sandblasted glass
  • Do not use abrasive or strong alkaline substances
  • Do not use abrasive sponges, wire sponges etc.
  • Contamination of the satin surface with silicones or similar substances is generally impossible to remedy.
  • The best results can be achieved with a white soft rubber.
  • Cleaning substances for regular cleaning are based on alcohol (for window cleaning, possibly butoxy-ethanol)
Lacquered glass, mirror
  • Never use ammonia-based products or abrasive products, such as anti-limescale.
  • Use alcohol-based products for glass and windows.
  • Make sure you wipe the edges of mirrors dry.
Decorative laminated glass
  • Be sure to wipe and dry the edges of the glass


  • Any scratch can penetrate the deposition layer, which cannot be repaired.
  • Any excess mechanical action may remove the deposition layer in certain areas.         
  • Avoid any contact with metal objects.
  • Avoid all chemicals that can attack and irreparably damage the glass surface.
  • Always carry out a test on a small area before starting the special cleaning programme. Never wash glass panes when fully exposed to the sun.
  • Avoid washing the glass when it is too cold or too hot
  • Do not use abrasive sponges, wire sponges etc.

Therefore, particular attention should be paid to the instructions and precautions provided in this guide. In areas with a high level of pollution, solutions and products supplied by experienced professionals are essential.


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