Heat-strengthened glass is a semi-tempered glass which retains the normal properties of ordinary float glass. It is similar to tempered glass except that the cooling is done slower than toughened glass but faster than annealed glass.
The heat strengthening process is similar to that of toughened glass but with lower levels of surface compression, resulting from a lower cooling quench.
Heat Strengthened Glass has a mechanical strength of about twice that of annealed glass and if broken, breaks into large pieces from edge to edge and does not fracture into small fragments like toughened glass. This breakage characteristic allows the glass to remain in the window frame as the large pieces support each other in the opening.
Heat Strengthened Glass also has high thermal strength like toughened glass and is resistant to thermal stress breakage associated with high performance tinted, reflective, and/or Insulating Glass Units.
Heat Strengthened Glass also provides resistance to thermal breakage caused by heat build-up in spandrel situations and the probability of nickel sulfide inclusions inducing spontaneous breakages in heat strengthened glass is practically non-existent.
Heat Strengthened Glass is often used in laminated glass, as it allows better interlayer adhesion than toughened glass due to the flatter nature of the heat strengthened product. This flatter finish also results in less optical distortion in building façades. In addition it has excellent breakage characteristics and prevents fallout and collapse.
As a result of the reasons above, heat strengthened glass is often used in curtain wall façades of buildings, in both the vision and spandrel applications.
However, in some situations Heat Strengthened Glass is not suitable. For example; where safety glass is required in human impact safety situations, for mechanical fixing or for higher strength requirements. In these situations Toughened Safety Glass is recommended.
