Today, flat glass comes in many highly specialized forms intended for different products and applications. Flat glass produced by way of the float process is often further processed (see below) to give it certain qualities or specificities. In this way, the industry can meet the various requirements and needs of the construction, automotive and solar-energy industries:

• Annealed glass
• Toughened glass
• Laminated glass
• Coated glass
• Mirrored glass
• Patterned glass
• Extra-Clear glass

Toughened Glass

Toughened or tempered glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering puts the outer surfaces into compression and the inner surfaces into tension. Such stresses cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards as plate glass (a.k.a. annealed glass) creates. The granular chunks are less likely to cause injury.

As a result of its safety and strength, toughened glass is used in a variety of demanding applications, including passenger vehicle windows, shower doors, architectural glass doors and tables, refrigerator trays, mobile screen protectors, as a component of bulletproof glass, for diving masks, and various types of plates and cookware.

Toughened glass, as the name itself suggests, is glass material is tougher than ordinary glass that has low resistance power. These are stronger varieties that are used in various places, whether residential or commercial constructions. In fact, the strength of the toughened glass is up to five times more than that of annealed glasses. Toughened glass is also used to manufacture glassware and these products are comparatively safer and offer more durability. However, the strength of these items should not be confused with any wrong perception of being unbreakable. If products made of toughened glass are handled with proper care, they can last even for a lifetime.

Annealed glass is heated above 720°C, which is the highest annealing point and after the extreme heating procedure, rapid cooling is conducted with air. Taking the surface lower than the annealing point cools off the glass. This causes the surface to become hard and contracts while the core still flows for a while. Finally, the core cools into numerous layers under tensile stress, balanced by compressive surface stresses. Because of these stresses, the toughened glass is strong and tough. However, a scratch on the surface or even a small cheap can weaken the layer and sometimes it may lead to breaking into small pieces.

There is an alternative process of creating toughened glass, which is done by using certain chemicals. Here the surface layer of the glass needs to be forced at least 0.1mm thick into compression by the exchange of sodium ions in the surface of the glass with the potassium ions that are larger by 30%, by immersing the glass into a molten potassium nitrate mixture. This chemical process of making toughened glass results in more toughness. This process of manufacturing can be used with complex shaped glass objects.

Toughened glass items have wide usage. Where the chances of damage and stress are high, this glass variety is used. Typical usages of this glass include counter tops, shelves, glass table tops, doors, window panels, window glazing, etc. Therefore, it is suitable for use in residential and commercial purposes.

Toughened glass is a type of soda-lime-silica glass with a sheet thickness 4-12 mm. The sheet has a central tensile stress of 500-1200 kg/cm2 and a ratio of surface compressive stress to central tensile stress of 2:1 to 4:1. The article is toughened by heat exchange with an oil (or chilled air) in which these are maintained from 0.01 - 0.07 % liquid. The boiling point liquid may be an organic liquid such as carbon tetrachloride, methanol, benzene, toluene, trim ethyl alcohol, ethyl alcohol or Xylene etc.

The toughened glass is described as a type of glass which has been mechanically strengthens by a thermal process in which a layer of compression has been introduced at the surface.

There are several glass items introduced into early ages of the century.

Toughened safety glass (float or polished) is produced by subjecting glass to a process of heating and rapid cooling so as to induce high compressive stresses in the surface sones balanced by high tension in the central place. This treatment endows the glass with greatly increased resistance to external forces, such as mechanical loading and thermal shock. If toughened safety glass is fractured, it gives fragments which are less liable to cause severe cuts than fragments of ordinary glass.

Thus toughened safety glass, if fractured, disintegrates into small pieces and in addition, its liability to fracture under the action of external forces or changes of temperature is greatly reduced.

Characteristics of Toughened Glass

Toughened glass is four to five times stronger than annealed glass of the same size and thickness against impact. Toughened glass has higher thermal strength and can withstand a high temperature differential upto 250°C.Toughened glass is considered as safety glass. It is difficult to break and even in the event of a breakage, disintegrates into small globules, which are relatively harmless.

Toughening does not alter the basic characteristics of glass such as light transmission and solar radiant heat properties.

After heat treatment, the surface of toughened glass has the same resistance to surface damage as annealed glass. Toughened glass cannot be cut, drilled or altered.

Heat soak test is recommended to be carried out on Toughened glass that is used for overhead and horizontal application, to prevent the spontaneous breakage caused due to nickel sulphide inclusion.

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