1. What is Glass Strengthening ?
2. What is heat Tempered Glass ?
3. How to process Tempered glass ?
4. What are the methods of heating?
5. What are the advantages of a Tempered Glass ?
6. What is a Float Glass ?
7. What is a Strengthened Glass ? ?
8. What is a Toughened Glass ?
9. What is Thermal Breakage ?
10. Examples of Thermal Breakage
11. What is weathering?
12. What is Thermal Shock Risk ?
13. What is Absorptions ?
14. What are the Properties of General Glass ?
15. What is Chemical composition ?

What is Glass Strengthening ?

In order to improve the tensile strength characteristics, it is possible to "pre-load" the surfaces of the glass with compressive stresses. This can be done either by heat strengthening, heat tempering or chemically strengthening the glass. When a glass plate is bent, the convex surface is placed into tension while the concave surface is compressed. By adding compressive stress to the glass, the compressive force must be overcome before the surface becomes tensile in nature. This can dramatically improve the amount of bending force that may be placed on a plate of glass before it breaks.

What is Heat Tempered Glass ?

Heat tempered glass, or sometimes referred to as "tempered glass," is the name given to glass products that have been strengthened by heattreating the glass to increase the surface compression and/or edge compression beyond a specified level. When the surface compression introduced is above 10,000 psi (69 Mpa), it may be considered "Kind FT" per ASTM C 1048. In order to prevent confusion, the term "Heat Strengthened" is normally used to define glass heat-treated to a level below 10,000 psi surface compression.

How to process Tempered glass ?

Heat tempered glass or sometimes referred to as "tempered glass" is the name given to glass products that have been strengthened by heat-treating the glass to increase the surface compression and/or edge compression beyond a specified level. When the surface compression introduced is above 10,000 psi (69 Mpa), it may be considered "Kind FT" as per ASTM C 1048. In order to prevent confusion, the term "heat strengthened" is normally used to define glass, heat-treated to a level below 10,000 psi surface compression.

What are the methods of heating ?

There are two primary ways to heat temper glass - horizontally and vertically:

-Horizontal heating is accomplished by placing the parts on ceramic rollers in a furnace that heats both surfaces of the parts with the quenching occurring sequentially on either metal ringed or polymer ringed rollers in the quench zone of the equipment.

-Vertical heating may be accomplished by two major processes:

By placing the parts vertically in holding racks on overhead?type roller conveyors or by tong-type carriers, which hold the parts by clamping bars against the surface on overhead?type roller conveyors. Quenching is accomplished while the parts are still in the same racks or carriers by transferring the rack or carrier to the quench zone of the equipment.

The horizontal heating method is preferred over the vertical heating method because in the vertical method a slumping of the glass may be caused or a slight mark will be introduced from the rack. In addition, the tong-type equipment will cause a dimple and stress point to be formed on the surface of the glass.

What are the advantages of a Tempered Glass ?

1)  Tempered glass is four times stronger than annealed glass as the typical breaking stress for annealed glass is 6000 psi whereas for tempered glass 24000 psi.
2) The typical impact velocity causes fracture of annealed glass at 30ft/sec. whereas for tempered glass at 60ft/sec. (ΒΌ" light 5 gm missile, impact normal to surface) .
3)  Tempered glass withstands higher wind loads which is a must for high-rise buildings. Tempered glass has a high resistance to uniform pressure and can thus be used on tall buildings where one must consider the impact of high wind pressure.
4)  Tempered glass can take heavier weights than annealed and ordinary glass of similar thickness.
5)  Tempered glass will to resist temperature differences (200? F - 300? F).
6)  On breakage, tempered glass fractures into small relatively harmless fragments thus reducing the likelihood of injury to people as there are no jagged or sharp edges.
7)  The tempering process does not affect the clarity and transparency of glass.

What is a Float Glass ?

Float glass does not resist high stresses from the impact of an object. Its breakage pattern is uneven and when broken, it shatters into large uneven pieces with sharp / jagged edges. This is not desirable as the risk of injury to human beings is greatly increased.

What is Strengthened Glass ?

Heat strengthened glass is about twice as strong as annealed float glass and is generally used as a protection against thermal breakage. However, its strength is lesser than the strength of tempered glass and the breakage pattern is still uneven. Strengthened glass is not classified as safety glass.

What is a Toughened Glass ?

Toughened glass is four times stronger than float glass and offers the highest resistance to impact. It is ideal for application such as indoors. If broken, the whole pane of glass shatters into small pieces of blunt granules that are relatively safe. Additionally, the shattered glass falls out quite easily. This is classified as a safety glass.

What is Thermal Breakage ?

Glass which is not heat treated (heat-strengthened or toughened) can experience thermal breakage. In those applications where thermal stress may be a concern, the glass should be toughened. In non-heat-treated glass, the risk of thermal breakage is greatest when the central area of glass becomes hotter than the edge. This condition can occur when the centre of the glass is heated by the sun and the edges remain cool. Under these conditions, the centre of the glass expands but is restricted from natural expansion by the cool edges. This results in stress within the glass that could cause thermal breakage. The degree to which the central area of the glass becomes hot is largely dependent on the solar absorptance of the glass, which varies between different types of glass as shown in the following table.

Examples of Thermal Breakage

For those applications where thermal breakage is a concern, toughened glass should be specified. Toughened glass, because of its compressive stresses, resists thermal breakage. The crack in thermally broken glass is initially perpendicular to the edge and glass face for 2-5cm and then branches out into one or more directions. The number of branches or secondary cracks is dependent on the amount of stress in the glass. The potential risk of thermal breakage on major contracts can be estimated by a computer-aided thermal stress analysis for those areas of high risk. Manufacturers recommend heat-strengthened or toughened glass in high risk areas. Where heat strengthened or toughened glass is specified, i.e., spandrels, entranceways or sloped glazing, a thermal stress analysis is not necessary and not offered as a common practice.

What is Weathering ?

Contrary to general belief, glass can be spoiled by static water or moisture, if allowed to stay for a long period. In such a situation, sodium ions are leached from the glass surface. Normal exposure to rain or cleaning will easily remove these ions and they do not cause any problem. However if the same water particles remain on the glass surface, they combine with sodium ions to start a chemical reaction very slowly but accelerating quite fast subsequently. The defect can be seen as white powderish patches with glass surface losing its polish. On cleaning, the defect appears as rainbow coloured areas when seen from an acute angle in bright light. Normal care during storage or at the time of receipt of material will eliminate all possibilities of this defect.

What is thermal shock risk? ?

The risk that occurs due to breakage of glass due to rapid, uneven temperature change is known as thermal shock risk. Below is a table stating the percentage of thermal risk of various types of glass.

Glass TypeSolar AbsorptancePercentage of Risk
Coloured or Tinted Medium/High 45-55%
Light transmitting coating on Colour or Tinted Medium/High 60-70%
High reflective coating Very High 80-85%

What is Absorptions ?

(S*)Absorptance is the percentage of energy incident on the glazing not directly transmitted or reflected. (S*)%Absorptance = Direct Transmission - % Reflected.

What are the Properties of General Glass ?

What is Chemical composition ?

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