In order to improve the safety aspects of glass (both float glass, figured glass or coated glass) this standard glass is treated thermally. As a result of this thermal treatment, the glass becomes stronger, not harder, as you may think when you hear its name. Not only standard glass but also insulating glass or laminated safety glass can be treated thermally. Depending on the process, the finishing process of standard glass results in two thermal glass variants:
These are the official names for reinforced glass types. However, we also know them as:
The glass may also be thougened chemically (not for constructions).
The cause of the stresses in the glass is not the thermal but the chemical treatment which implicates that the Na + ions in the glass are replaced by K+ ions, that are larger. Chemically toughened glass breaks in the same way as float glass.
Toughened or tempered glass is glass that is made much stronger than ordinary glass due to a special treatment. The strength of toughened or tempered glass can be up to five times the strength of ordinary single glazing.
The procedure for turning glass into toughened or tempered glass is above all a thermal treatment. The glass is heated until more than 600°C.
After that, it is cooled very quickly with the result that the glass shrinks.
Therefore the outside of the glass cools off before the core and this causes permanent stresses in the glass. The core is under tensile stress, while the zones close to the glass surfaces are under compressive stress.
When bending the glass, the compressive stresses on the surface therefore must be compensated before tensile stresses occur at the surface, which can cause breaking of the glass. The resistance of this type of glass against mechanical and thermal loads is bigger than the resistance of annealed glass. The result is that the structure of this glass is much stronger than ordinary glass.
Toughened or tempered glass has another advantage. Besides being much stronger than ordinary glass, toughened or tempered glass is also much safer. Not only does it break much not as fast as ordinary glass, and even if it does, it's not falling apart into shards but in harmless blunted glass beads.
Thermally toughened or tempered glass is also used for the insulation of glazing, laminated safety glass, doors, glass facades, entrances, escalators, elevator shafts, bathroom elements and as glass for furniture applications.
Glass may contain inclusions of nickel sulphide (NiS). The inclusions have a diameter of a few microns (µm) to several millimeters (mm) and another particular property is that their crystal structure is different at a low than at a high temperature. Their volume is bigger at low temperature. If the glass cools down slowly (annealed glass), all NiS particles have the time to reach their structure at low temperature during the cooling of the glass. The volume fluctuations of the inclusions can still be absorbed by the pasty condition of the glass and they do not cause any risk of glass breakage.
On the other hand, when thermally toughened glass is heated till about 650°C, the NiS reaches its stable structure at high temperature at the start of the curing. The following abrupt cooling allows the NiS to reach its stable structure at low temperature before the glass has become completely solid. Therefore the transformation will continue in the operating temperature of the glass and the associated volume increase can cause spontaneous breakage. In order to restrain the risk on glass breakage, they apply the heat-soak treatment. With this treatment the glass is heated in an oven for a certain period of time in order to accelerate the transformation reaction of the NiS. The possible fracture as a result of the presence of critical NiS particles will occur during the treatment. Depending on the use of the thermally toughened glass, it should be mentioned in the specifications whether the Heat Soak treatment should be applied or not. In case of load-bearing structural components (beams, SGG, SVG, ...), all glass elements must be treated.
Semi-toughened or heat-strengthened glass did have a thermal treatment similar to the thermal curing, but where the obtained voltage is lower than the one of toughened glass, because the cooling was slower.
Semi-toughened or heat-strengthened glass doesn't have the same characteristics compared to the commodities which it was made of: