ANNEALED GLASS Annealed glass is a basic product formed from the annealing stage of the float process. The molten glass is allowed to cool slowly in a controlled way until it reaches room temperature, relieving any internal stresses in the glass. Without this controlled slow cooling, glass would crack with relatively little change in temperature or slight mechanical shock. Annealed glass is used as a base product to form more advanced glass types.
HEAT STRENGTHENED GLASS Heat Strengthened Glass is semi tempered or semi toughened glass. The heat strengthening process involves heating annealed glass back up to about 650 to 700 degrees Celsius and then cooling it quickly, although not as fast as with toughened glass. The heat strengthening process increases the mechanical and thermal strength of annealed glass, making it twice as tough as annealed glass. When it breaks the fragments are similar in size to annealed glass, but with a greater likelihood of staying together. This glass is not often used in balustrades or similar structural applications because of its limited strength compared to tempered or toughened glass, although is sometimes specified when there is concern about tempered glass fracturing into thousands of small pieces.
TEMPERED OR TOUGHENED GLASS This is the most common type of glass used in balustrades or similar structural applications. Annealed glass is heated to about 700 degrees Celsius by conduction, convection and radiation. The cooling process is accelerated by a uniform and simultaneous blast of air on both surfaces. The different cooling rates between the surface and the inside of the glass produces different physical properties, resulting in compressive stresses in the surface balanced by tensile stresses in the body of the glass. The counteracting stresses or surface compression gives toughened glass its increased mechanical resistance to breakage, and when it does break, causes it to produce small, regular, typically square fragments rather than long, dangerous shards that are far more likely to lead to injuries.
LAMINATED GLASS Any one of the above types of glass can be laminated. Laminated glass offers many advantages. Safety and security are the best known of these, so rather than shattering on impact, laminated glass is held together by the interlayer. This reduces the safety hazard associated with shattered glass fragments, as well as, to some degree, the security risks associated with easy penetration. If a glass panel breaks or shatters it is highly unlikely that both laminated panels will break at the same time, which means the remaining panel and interlayer will support the broken glass, keep it in place as edge protection until it is replaced or secured suitably.