Glazing - Safety and Defects

Fire Performance

Reaction to Fire

When evaluating fire performance, it's important to distinguish between reaction to fire and fire resistance. Reaction to fire describes how a material behaves when exposed to fire, particularly its contribution to fire spread. Materials are typically classified as:

• Non-combustible: Do not release significant heat when exposed to fire

• Combustible: Emit heat depending on the intensity of exposure

• Inflammable: Release gases that can ignite and produce flames

Each material can be assigned a fire-reaction class to indicate its performance under fire conditions.

Fire Resistance

Fire resistance refers to how long a glazing system (including the frame) maintains its structural role during a fire. This is typically measured in minutes, and based on the ability to retain:

• Load Bearing capacity

• Integrity (preventing the passage of flames and hot gases)

• Insulation (limiting heat transfer)

Glazing used for fire-resistant applications must be designed and tested as a complete system.

Safety and Security

Glazing Safety

Safety glazing protects people from injury due to breakage. The key consideration is how the glass breaks:

• Toughened glazing shatters into small, blunt fragments

• Laminated glazing holds together on impact, preventing fall-out

• Annealed or heat-strengthened glazing, which breaks into sharp shards, are not considered safety glazing

Protection from Falling and Impact

In addition to safe breakage patterns, glazing used in high-risk locations (such as balustrades or overhead applications) must prevent defenestration and withstand impact. Laminated or toughened glazing is essential in these situations and can also be specified for protection against forced entry, firearms, or explosions.

Optical Characteristics and Defects

Optical Quality

Glazing is typically manufactured to high visual standards, but minor distortions can occur due to the material and manufacturing process. These effects, while often seen as defects, are usually within acceptable tolerances.

Bowing and Dishing

In double-glazed units, sealed air spaces can expand or contract due to temperature and pressure differences. This results in slight convex or concave deformations of the panes, known as bowing or dishing. These changes are influenced by environmental conditions and the altitude difference between the manufacturing site and installation location.

Anisotropy and Double Refraction

Thermally toughened glass may exhibit anisotropic properties due to uneven cooling from air jets during processing. When viewed under polarised light or at specific angles, this can cause double refraction, visible as iridescent, stripy patterns. While noticeable under certain lighting conditions, this is a by-product of the toughening process.

Interference Patterns

In rare cases, especially with perfectly parallel glazing in double-glazed units, interference phenomena can occur. These appear as spectral stripes that shift with viewing angle and lighting. Far from being a defect, this phenomenon can indicate high manufacturing precision.

Coatings and Reflections

Low-emissivity (low-E) and other optical coatings enhance performance but can cause coloured reflections, which vary with viewing angle and lighting. These are aesthetic considerations rather than performance issues.

Roller Waves and Lens Effect

Roller waves are surface distortions caused during the toughening process as glass rests on rollers. These waves can subtly distort reflections. Similarly, in laminated glazing, slight unevenness between panes can create a lens effect, distorting the view through the glass into a convex or concave appearance.