Safety glass

Safety glass is a specialized category of glass designed with specific features. These features make the material harder to break than standard glass. If the glass does break, these designs ensure it is less likely to become a hazard.

Engineers use different methods to achieve these safety goals. The three primary designs are toughened glass, laminated glass, and wire mesh glass. In many developed nations, these materials are required by building regulations. This ensures that structures remain safer for the people inside them.

Toughened glass, also known as tempered glass, is created through specific processes. Manufacturers use controlled thermal or chemical treatments to strengthen the material. These treatments create balanced internal stresses within the glass sheet. Because of these stresses, the glass becomes much stronger than normal glass. When tempered glass finally breaks, it does not splinter into jagged shards. Instead, it crumbles into small granular chunks of similar size and shape.

These small pieces are much less likely to cause serious injury.

Laminated glass works by using a layered construction method. It consists of two or more layers of glass held together by an interlayer. This interlayer is typically made of a plastic called polyvinyl butyral, or PVB. When an impact occurs, the glass layers may crumble into small pieces. However, the PVB interlayer keeps the broken pieces bonded together. This prevents the glass from falling out of its frame.

This process often creates a characteristic "spider web" pattern of radial and concentric cracks.

Laminated glass is chosen for specific safety needs. It is used when there is a high possibility of human impact. It is also used where glass might fall if it were to shatter. Common examples include automobile windshields and skylight glazing. In areas prone to hurricanes, it is used for exterior storefronts and curtain walls. The PVB interlayer also provides extra benefits beyond physical safety. It offers a higher sound insulation rating due to its damping effect. Additionally, it blocks a large amount of UV radiation. It blocks 88% of UV radiation in window glass. In windshields, it blocks 97.4% of incoming UV radiation.

Wire mesh glass, or wired glass, contains a metal grid embedded inside. This grid consists of a mesh of thin metal wire. This design is used primarily for its fire-resistant abilities. The wire helps the glass withstand intense heat and water from hose streams. This makes it useful for service elevator shafts to prevent fire ingress. The wire also prevents glass from falling out of the frame during thermal stress. However, the wire mesh does not actually strengthen the glass. In fact, wired glass is often weaker than unwired glass. The incursions of the wire into the structure create weaknesses. This can actually cause more injury because the wire makes fractures more irregular.

History shows how these different technologies were developed over time. Toughened glass was invented in 1874 by Francois Barthelemy Alfred Royer de la Bastie. Wire mesh glass followed later, invented by Frank Shuman in 1892. Laminated glass was the next major development, invented in 1903 by the French chemist Édouard Bénédictus. These inventions changed how we use glass in our daily lives. Today, the use of wired glass is being debated globally. The US International Building Code effectively banned wired glass in 2006. In Canada, building codes are currently being reviewed to restrict its use. Australia has not yet implemented a similar review.

While these three types are distinct, they can sometimes be combined. A single pane could be toughened, laminated, and contain a wire mesh all at once. However, combining a wire mesh with other techniques is quite unusual. This is because the mesh often betrays the individual qualities of the other methods. Each type of safety glass serves a specific purpose in modern engineering. From car windshields to shower doors, they protect us in many different ways.

Understanding these mechanisms helps us see how technology improves our safety.