<P> Glass block, also known as glass brick, is an architectural element made from glass used in areas where privacy or visual obscuration is desired while admitting light, such as underground parking garages, washrooms, and municipal swimming baths . Glass block was originally developed in the early 1900s to provide natural light in industrial factories . </P> <P> Annealed glass is glass without internal stresses caused by heat treatment, i.e., rapid cooling, or by toughening or heat strengthening . Glass becomes annealed if it is heated above a transition point then allowed to cool slowly, without being quenched . Float glass is annealed during the process of manufacture . However, most toughened glass is made from float glass that has been specially heat - treated . </P> <P> Annealed glass breaks into large, jagged shards that can cause serious injury and is considered a hazard in architectural applications . Building codes in many parts of the world restrict the use of annealed glass in areas where there is a high risk of breakage and injury, for example in bathrooms, door panels, fire exits and at low heights in schools or domestic houses . </P> <P> Laminated glass is manufactured by bonding two or more layers of glass together with an interlayer, such as PVB, under heat and pressure, to create a single sheet of glass . When broken, the interlayer keeps the layers of glass bonded and prevents it from breaking apart . The interlayer can also give the glass a higher sound insulation rating . </P>

What type of glass is used in windows