What are the GGF Guidelines?
The Glass and Glazing Federation (GGF) Guidelines are the official government-recognised standards for the glazing industry for both manufacturers and installers. These guidelines define the methods and conditions under which glass should be assessed to determine whether it is acceptable or unacceptable for installation. We work to these guidelines.
Point 15A of the GGF Guidelines highlights the ‘common sense approach’ we must take when inspecting glass, appreciating that modern processed glass cannot be perfect or flawless. It also points out that glass is for looking through, not looking at. Some degree of visual distortion is considered normal within glass manufacturing. Glass that undergoes additional processing, such as toughened (tempered) glass, may experience slight changes in appearance as a result of these treatments.
While damaged glass is always deemed unacceptable and will never be installed, there are certain visual effects that are well-recognised and accepted as inherent characteristics of processed glass rather than defects.
Inspecting Glass According to the GGF Guidelines
Every glass unit undergoes a comprehensive visual inspection both before leaving the glass manufacturers’ factory as well as at our factory and again at site installation.
When inspecting units for scratches on the outer faces of the glass must be done prior to any rendering, plastering, decorating or any other works adjacent to the glazing as these types of works are the most common causes of those scratches.
The GGF specifies these key requirements for carrying out a valid glass inspection:
The GGF Guidelines recommends an extra tolerance of 25% for laminated glass compared to standard toughened glass, due to the interlayer film and how this makes minor imperfections more visible.
By following these guidelines, we ensure that all glass products meet the highest visual and performance expectations before being handed over to our clients.
Acceptable Imperfections
Toughened, laminated or coated glass is considered compliant with GGF guidelines if the following imperfections are neither obtrusive or bunched:
Unacceptable Imperfections
The following imperfections are deemed unacceptable under GGF guidelines:
Other Visual Distortions
There are also a number of other phenomena that can occur in processed glass worth noting, which are not considered as defects.
Roller Wave
Roller wave is a common type of visual distortion that can sometimes be seen in toughened glass panels. It occurs during the toughening process, when the heated and softened glass moves across a series of rollers as it cools. This process can create a slight undulation or wave-like effect on the surface of the glass.
Although roller wave may be visible under certain lighting or viewing angles, it does not compromise the performance, strength, or durability of the toughened glass.
In general, the presence of roller wave is not considered a defect. However we adhere to the Glass and Glazing Federation (GGF) Guidelines to assess what constitutes an acceptable level of roller wave in each project, which measures how deep the waves are and how separated they are.
Anisotropy (Strain Patterns)
Anisotropy refers to a visual effect sometimes visible in heat-treated (toughened or heat-strengthened) glass when viewed under polarized light. It occurs due to variations in stress or tension across the thickness of the glass that develop during the heating and cooling stages of the toughening process.
During manufacture, the glass is uniformly heated in an oven and then rapidly cooled (quenched) by high-velocity air applied through nozzles. The areas of glass above the nozzles cool at slightly different rates, resulting in variations in surface compression and overall density. These differences cause the glass to become optically anisotropic, meaning that the appearance of light passing through it may vary depending on the viewing angle or position.
When light passes through these stressed regions, it can produce iridescent or rainbow-like patterns on the glass surface. These are most visible in polarized light—such as natural daylight—and when viewed at oblique angles or when glass panels are positioned at right angles to one another.
This phenomenon is also referred to as strain patterning, quench marks, or leopard spots. The visibility and intensity of these patterns can vary from unit to unit, and even within a single unit, depending on factors such as furnace settings, glass thickness, and viewing conditions.
Anisotropy is a normal and expected outcome of the heat treatment process and cannot be completely eliminated. It should not be mistaken for discoloration, nonuniform tint, or colour variation. The presence or variation of strain patterns is not considered a defect or blemish and is not cause for rejection. Importantly, anisotropy does not affect the structural integrity, performance, or safety of the glass.
Brewster’s Fringes (Interference Fringes) – ‘The Rainbow Effect’
Brewster’s Fringes represent another type of interference fringe that can occasionally appear. These visual patterns are relatively uncommon and are usually difficult to detect under standard lighting conditions.
This phenomenon occurs primarily in high-quality glass units with optically flat and parallel surfaces. When both panes are of identical thickness and manufactured with precise tolerances, the light reflected from one surface can interact with the light reflected from the other. Because the difference in the optical path is extremely small, the two reflected light waves can interfere with each other, producing subtle visual effects.
The result is a series of faint bands or irregular shapes, often grey or slightly coloured, which may appear anywhere across the glass surface. These effects are rarely visible in normal lighting but may become apparent when the glass is viewed at an angle or under specific lighting conditions, such as when looking through to a shaded background.
To confirm whether the observed effect is a Brewster’s Fringe, you can gently press or tap on the glass surface. If the pattern shifts or moves slightly, this indicates that the effect is due to the interference between the two parallel panes.
It’s important to note that Brewster’s Fringes are not considered manufacturing defects or faults. Glass exhibiting this pattern remains structurally sound and does not require replacement.
Suction Marks on Glass
During the manufacturing and installation process, rubber suction cups are often used to handle and position glass safely without causing damage. On occasion, these suction devices can leave a slight, invisible residue on the glass surface, which typically fades over time.
This residue may become temporarily visible when moisture or condensation forms on the glass, as the particles interact differently with water compared to the rest of the surface. Due to them only appearing in specific weather conditions, these marks are notoriously difficult to spot during an installation.
These marks are not a defect and do not affect the quality or performance of the glass. Under normal weathering and exposure to the elements, the fine rubber particles will gradually wear away. In some cases, the appearance of suction marks can be reduced or removed using appropriate glass cleaning products, which we can arrange for you.