Technical Information

So many variables impact how glass can be curved and bent – please contact us directly to discuss your needs as even complex curves may be achieved. The Bent Glass Specifications & Tolerances guide provides some direction on tolerances for tight curves – generally speaking, increasingly tighter curves require increasingly thinner and smaller panes of glass.

A tolerance of ± half of the thickness of the glass to a maximum of ± 6mm is the accepted industry standard.

Glasshape® bends glass by either heating and rolling the glass while in a plastic state, or by the process of “sag bending”. Glass is a visco-elastic material whose mechanical properties change rapidly when heated between 600º and 700º C becoming a ‘plastic’ substance. The science of glass bending uses this plastic phase to produce shapes which are both complex yet free from wrinkles and other optical aberrations starting from flat float glass.

Optical distortion sets the limit for most shaping. All bending requires precise control of stress levels to ensure the product meets regulatory, environmental, robustness and optical requirements. A degree of distortion, both when looking through and in reflection, is inevitable in bent glass, particularly when viewing a moving object through the glass.

All bent glass should be site inspected from a minimum distance of 3m and viewed at right angles to the glass. It should also be noted that bent glass will split direct sunlight into striped shadow. Some variation in edgework may be discernible on exposed edges where different machine or hand forming is required for manufacturing. Such variation will be kept to a minimum.

Marine Glazing GuideHeavy Macinery Glazing Guide

“Spontaneous glass fracture” refers to incidents where toughened glass windows shatter without warning. These failures are due to the presence of nickel sulphide inclusions. For more information about mitigating this please refer to the Heat Soaked Glass section below.

Glasshape® is able to provide a Heat Soak service that will test panels of toughened (tempered) glass for nickel sulphide inclusions. In almost all cases, this destructive test will cause an affected panel to shatter within the safety of the testing oven. If the panel emerges from the testing oven in-tact we can be almost 100% confident that there are no nickel sulphide inclusions, and the panel will not spontaneously fail as a result.

Delivery, Storage & Handling of Laminated Glass 

• Insulating Glass Units must be transported and stored so that both panes of glass are equally supported. Therefore units should be stored vertically or stored on a 90° angle rack set at 4° -7° from the vertical.
• Units may be moved and stored on site storage trolleys.
• Insulating Glass Units should not be stacked more than 6 deep. For various size units, stack the largest against the supports. Where size changes occur, suitable interleaving should be used.
• All Insulating Glass Units must be stored in dry ventilated conditions out of direct sunlight.
• Wrapping plastic over glass for weather protection is not advised as it can allow condensation to accumulate, resulting in damage to the glass or unit seal.
• Insulating Glass Units that are transported or installed at heights exceeding 1200 metres above sea level require special precautions and advice must be sought from the manufacturer.
• Strops or slings which support both panes of glass are recommended for moving Insulating Glass Units.
• If it is necessary to rotate large Insulating Glass Units, a corner block may be used to protect the edge of the unit, or use a vacuum lifter with a rotating head.
• Care must be taken if handling large Insulating Glass Units with suckers, as this can put undue stress on the seals.
• Glass grabs which create a crushing force which can damage the spacer are not recommended.

Newton’s rings are named after Sir Isaac Newton who first studied the phenomena. These are interference patterns caused by the reflection of light between two surfaces, a spherical surface and an adjacent nearly flat surface. They appear as concentric rings of rainbow colors and occur only near the center of a unit.

In a large IGU the two glasses may be displaced to touch or nearly touch in the middle by an increase in atmospheric air pressure due to; insufficient pressure equalization during manufacture, heat treated glass bow, incorrect airspace for the unit size, or inadequate glass thickness. This effect is normally a manufacture or specification fault and replacement is required.

Brewster’s Fringes are reflected light phenomena which occur if wavelengths of light meet up at 180 degrees out of phase. This can occur when high quality float glass is used with surfaces which are optically flat and both panes of glass are parallel in the IGU. Light reflected within one glass can combine with that similarly reflected within the other, with such small path differences as to cause interference.

The effect is of faint colored bands or irregular shapes, which can be located anywhere over the surface. It is rarely noticeable in normal lighting conditions.

Brewster’s Fringes are not a manufacturing fault but can be generally avoided by using IGUs of unequal glass thickness.

IGU’s containing heat treated glass may exhibit a similar level of distortion as heat treated monolithic glazing. This is normally visible on an oblique angle perpendicular to the direction of the toughening furnace rollers, and more obvious in reflection than in transmission.

Where IGU’s contain two or more panes of toughened glass this effect may be increased. For this reason, units containing multiple panes of toughened glass particularly where reflective or coated glass is involved should be evaluated for possible visual issues associated with roller wave distortion.

Where thermal stress prevents the use of annealed glass in both panes, the use of heat strengthened glass instead of toughened glass (tempered glass) may be considered as often the roller wave is reduced. Roller wave is not a manufacturing fault but an industry recognized and accepted inherent feature of heat treated glass.

As a standard IGU has four reflective surfaces, a higher level of reflectivity occurs and multiple images in reflection may be created. This will be more apparent when viewed on an angle to the glass and is an inherent property of the unit.

Due to the sealed airspace of an IGU differences in temperature and atmospheric pressure from the time of manufacture will cause the IGU to act as a lens. This can cause significant changes in the images reflected from the windows due to glass deflection. The appearance is of a convex distortion when the glass is bowing outwards and a concave distortion when bowing inwards. The effect will be more noticeable when reflective coatings are incorporated within the IGU, and in larger units.

Haze is the scattering of light rays when visible light passes through a transparent material. The amount of haze in ordinary glass is very low and is not detected by the human eye.

High performing IGU’s often incorporate Low E coated glass. With any coated glass it is possible to see the presence of the coating under certain lighting conditions. When bright sunlight shines directly onto partly shaded, coated glass and there is deep shade on both sides of the glass, haze may be visible and usually has the appearance of a blue-grey film or dust on the glass. The shaded area will be free of the effect, giving a clean appearance in the shadow.

The effect will be more noticeable on some types of coated glass than on others. Vacuum or sputter coated Low E glass products generally have a very low amount of haze, Pyrolytic coatings tend to have higher levels of haze that can be more readily seen by the human eye. Haze is not a manufacturing flaw, rather an industry known and recognized inherent feature of IGU’s.