The specific properties of glass

Assessment of the visual quality of the glazing: This assessment is important for the visual quality of the insulating float glass for the building industry. The light of the built-in glass surface is assessed. The guideline is also valid, however in a limited way, for insulating glazing made as special glazing. It covers insulating glass with muntin or Georgian bars, anti-burglary, and fire-resistant glazing. These glass products should be assessed according to the materials used, production process, and relevant manufacturing guidelines. When checking for deficiencies, the transparency of the glass is appraised by looking at the background, not the glass. The visual quality of the glass should be assessed from a distance of 3 m from the viewed surface at the right angle (90). The checking takes place under diffused daylight conditions (e.g., covered sky) without direct light from the opposite side (e.g., direct sunlight). When assessing an embedded glass product, it should generally be assumed that, in addition to visual quality, the essential characteristics of the glass product should also be considered as fulfilling its function.

CERTAIN SPECIAL CHARACTERISTICS OF INSULATED GLAZING

The physical properties of glass in general and the structure of particular insulated glazing unit determine specific properties of insulated glazing units. These, being the properties, cannot be considered defects, thus they are not subject to complaint: Interference symptoms. Optical interference is the characteristic symptom of overlapping two or more light waves when converging at a single point. They can be observed in the form of weaker or stronger coloured zones (rainbow effect) changing their position when the glass is pressed. This physical effect is enhanced by the quality of parallel glass surfaces. Visual symptoms of interference happen by chance and cannot be controlled in any way.

CONVEXITY AND CONCAVITY OF GLASS

The glazing unit closed with a sealing contains a determined volume of gas in the space of the glazing unit chamber. The properties of that gas are determined largely by the atmospheric air pressure, absolute altitude, and air temperature at the time and place of production. When insulated glazing is installed at different heights, when temperature changes or there is a change of atmospheric pressure, concave and convex bends of individual panes of glasses inevitably arise bringing about some optical distortions. The occurrence of this phenomenon proves that the glazing unit is well-made: It is simply because it is airtight, and this quality should not be considered a defect.

VAPOUR CONDENSATION

Under certain conditions, the insulated glazing unit may also show water vapour condensation on its outside surfaces: in the room or from the outside. The appearance of inside room vapour condensation depends on the UG (W/m2K) coefficient value, humidity, air circulation as well as internal and external temperature. It is worth adding that the basic solution for this type of issue is effective and frequent ventilation of rooms. This is especially true for rooms where, for obvious reasons, there is greater amount of water vapour, i.e., kitchens, bathrooms, and bedrooms. This problem appears especially where old, leaky windows are replaced with new ones, much tighter than the previous ones. When thermal insulation of the whole window set is particularly good, the air humidity and temperature are high, the vapour may condensate on the glass surface outside the room. This effect occurs especially on winter mornings and concerns glass with a very good UG = (W/m2K) coefficient. Generally speaking, this phenomenon shows the high quality of the glazing.

COLOUR DEVIATIONS

The individual colour of the glass (shade) depends on the thickness of the glass, manufacturing process, and composition of the glass raw material mixture. Colour deviations may happen especially when ordering additional windows or panes after some time passed. Even when they are ordered from the same manufacturer, colour deviations may occur, and they are both a result of changes in material itself and of constant change of manufacturing technology of glass. Particularly with panes of glass with the heat transfer coefficient of UG = 1.1 (W/m2K), which is the standard today, there may be observed differences in colour of the low-emission coating (from a difference in shade to even difference in colour). Therefore, when ordering windows, it is worth considering either replacing all windows in the building, or at least replacing all windows in a given wall in a single window order. In this way, we can avoid differences in colour of glass in adjacent windows. Unfortunately, if with the passage of time, one of windows is broken or replaced for other reasons, there is no guarantee of identical colour.

MUNTIN RINGING

This phenomenon is caused by casual vibration of the aluminium muntin when the window is opened, closed or even when the wind blows. The minimum width of the distance frame when using muntin is 12 mm. Failure to meet this parameter means that under certain atmospheric conditions (high air pressure) the glass may touch the muntin bar, causing damage to the low-emission coating or even a cracking of glass under high stress.

CRACKING OF GLASS

Glass, being a cooled fluid, is a brittle material subject to a certain inherent stress. It cannot be visibly plastically changed (like steel). This means that when its elasticity limit is exceeded, it breaks immediately. The residual stress of float glass is very uniform and has a negligible initial value. If there had been any stress imprinted on the glass during manufacturing process, it would not have been possible to cut it and process mechanically. The breakage of glass therefore is only caused by external, mechanical, or thermal factors.

WETTABILITY OF INSULATED GLASS UNIT DUE TO HUMIDITY

The wettability of the glass surface on the outer side of the insulated glazing unit may vary depending on e.g., roller, finger, label, vacuum suction prints. Wet glass surfaces, caused by presence of tarnish, rain, or water — different wettability can be observed in the form of distinct stains, theoretically appearing more transparent.

ANISOTROPY WITH TEMPERED GLASS

Anisotropies are formed on glass subject to thermal toughening processes. Different stress zones of glass create double refraction of the light beams. Polarised light waves make this phenomenon visible through manifestation of spectral colours in the form of rings and clouds.