Glazing - Safety and Defects


Protection against fire is an increasingly important consideration when designing building envelopes. Two separate concepts should be highlighted: reaction to fire and fire resistance.

Reaction to fire measures the behaviour of a given material when exposed to fire and its contribution to fire propagation. For example, a distinction is drawn between non- combustible materials (which do not release any noticeable quantity of heat), combustible materials (which tend to emit heat depending on the level of heat to which they are exposed)and inflammable materials (materials liable to release gas, the nature and quantity of which are likely to generate gaseous phase combustion, i.e. to produce flames). Each product can be placed in a fire-reaction class.

The fire resistance of glazing is the time in minutes during which that element meets the criteria of loadbearing capacity and/or integrity and/or insulation. Fire resistance concerns the glazing and frame rather than a part thereof.

Safety and Security

Safety is a far-reaching concept covering many areas, protecting individuals from the risk of injury due to sharp, broken glazing and falling (defenestration). In trying to avoid the risk of injury only, it is the breakage pattern of the glazing which is significant: it is important to ensure that if the glazing breaks, it does not produce pieces which are likely to cause injury.

To protect against falling, care must be taken to ensure that the glazing is not obliterated, protecting goods and providing protection from burglary and vandalism. In this context, the glazing should remain in place and prevent anyone or anything from penetrating it. It can also be designed for protection against firearms and explosions.

Very few glazing products meet the break pattern, defenestration and resistance criteria described above. These are toughened and laminated glazing products. Other glazing products, including annealed and heat-strengthened, are not safety glazing.

Optical defects are relatively rare within glazing products and are typically controlled in manufacture. However, when occurring, they can be highly conspicuous with even minimal deviations from a perfectly flat surface distorting reflected images and views. While undesirable, the physical properties of glazing make this unavoidable and, therefore, can be argued to be not strictly defects but part of the limitations of using glazing.

Bowing and Dishing

The effect of bowing of dishing is frequently observed in conjunction with using double-glazed units. As individual panes of double-glazed units are hermetically sealed, a pressure difference between the surrounding air and the gas in the cavity between panes leads to volume changes. This results in convex or concave deformations of the glazing panes with the extent of change determined by atmospheric pressure, temperature fluctuations and the difference in pressure between the manufacturing facility and the place of installation.

Anisotropy and Double Refraction

Glazing is isotropic, meaning it has the same optical properties when measured in different directions. However, the jets of air used in the thermal toughening process can cool the surface unevenly, leading to glazing with anisotropic optical qualities. Incident light on the surface at an angle other than 90 °, or polarised light, is split into two waves resulting in the effect known as double refraction. Under certain lighting conditions, this phenomenon usually maintains itself as a stripy pattern, with the stripes are caused by the arrangement of air jets.

Interference Phenomena

While rare, interference phenomena occur when there are several perfectly parallel panes of float glazing, such as in a double-glazed unit and under certain light conditions. It manifests itself as stripy zones that are split into the colours of the light spectrum, the position of which change as the individual panes change. While described as an optical defect, as its occurrence requires perfectly parallel glazing panes, it also indicates quality.


Coatings, such as low-E coatings, are used to control the optical properties of glazing. They can, however, lead to undesirable coloured reflections dependent on the lighting conditions and angle viewed by the observer.

Roller waves and Lens Effect

A wavy surface is a phenomenon that is particularly prevalent with toughened glazing. As the hot glazing rests during the toughening process on the rollers, deviations can occur, which manifest themselves in a distortion of the reflected image.

Where panes of glazing are further processed to form laminated safety glazing, the so-called lens effect can occur. Caused by the unevenness of the glazing, this optical effect can lead to the convex or concave distortion of the view through the glazing.

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