Glass may be enhanced or strengthened to become more resistant to physical impact and heating. Strengthening glass can be achieved by three primary processes:
• heat strengthening
• chemical strengthening
Toughened glass cannot be cut, drilled or polished, and can be in single sheets, laminates and insulating glazed units.
Toughened glass usually stamped (in a corner) with a small label.
Toughened glass is manufactured from annealed glass via a thermal toughening process.
The annealed glass moves through a furnace on a roller table while being heated to temperatures between 564℃ and 620℃, where it is held for some time.
The glass is then rapidly cooled with jets of cold air, which forces the outside surface into compression while the inside remains free to flow for a time.
Toughened glass is under compressive strain on the outside and under tension on the inside. Glass is much stronger under compression than under tension. Toughened glass is up to 6 or 7 times stronger than ordinary annealed glass, and requires the application of a robust and focused force to break it. If the glass breaks, it shatters into thousands of small, 'harmless' pieces.
One issue with toughened glass can be roller wave, which may be visible when glazed into a building.
The heat soaking process will reduce the incidence of spontaneous breakage in the toughened glass caused by nickel sulphide inclusions.
These inclusions may be introduced randomly as part of the typical glass manufacturing process.
The heat soaking process subjects toughened glass to a temperature of 290℃ for up to eight hours in a heat-soaking oven. During this time, the nickel sulphide inclusions undergo a significant change and reverse back to its original low-temperature state (i.e. the nickel sulphide inclusions expand in volume). If these inclusions are near the central tension core of the glass, the change in size will generate sufficient stress to break the glass.
Heat-strengthened glass is intended for general glazing, where additional strength is needed to withstand wind load and thermal stress.
It is generally twice as strong as annealed glass of the same thickness and configuration and ideal for applications that do not require a safety glass product.
Heat-strengthened glass has been subjected to a furnace heating cycle and is approximately twice as strong as annealed glass of the same thickness.
Chemically strengthening of glass is done at the time of manufacturing. The glass's strength is a result of the ion exchange process the glass undergoes during production.
Once the glass has been manufactured, it is dipped in a bath of molten salt.