Tempered Glass :

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Tempered Glass

Tempered glass is made from normal annealed glass via a thermal tempering process in which the glass is subjected to heat till its softening point and then rapidly cooled. This gives the glass its strength. A fully tempered glass is 4 to 5 times stronger then an annealed glass of similar thickness. A fully tempered glass is regarded as a safety glass and when it breaks it disintegrates into small blunt pieces which greatly reduces the chances of injuries and if there are any then they are superficial in nature.


A fully tempered glass as required in ASTM C 1048 is generally 4 to 5 times stronger than annealed glass and twice as strong as heat strengthened minimum surface compression is 10000 psi and for a heat strengthened glass is 4000-7000 psi for 6 mm glass.


When broken by impact, fully tempered glass immediately disintegrates into relatively small pieces thereby greatly reducing the likelihood of serious cutting or piercing injuries in comparison with ordinary annealed glass.
The fully tempered glass has a high edge strength as compared to normal annealed glass. This gives freedom to designers to use the tempered glass in spider glazing and point fixed glazing.

Thermal Breakage:

When direct sunlight falls on a pane of glass then the glass surface tends relatively cooler. This creates temperature difference inside the same thermal breakage. But a fully tempered glass has significantly higher edge thermal breakage. But a fully tempered glass has significantly higher edge strength to withstand chances of thermal breakage.

Manufacturing Process

The two different types of furnaces are: -

  1. Radiation Furnace
  2. Forced Convection Furnace
  3. And they can produce: -

  1. Tempered Glass
  2. Heat Strengthen Glass
  3. Tempered Low e Glass

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Key Quality Areas:

Roller Marks:

Roller marks are small whitish hue or marks along the line of glass movement. The only reason for this is poor maintenance inside the furnace chamber. Whenever the glass breaks during the production process the small glass fragments remain inside the furnace on the surface of the rollers. glass.

Waviness and Bending:

Since heat-strengthened and tempered glass are reheated to their softening the resulting stress. Although warp and bow is not generally a significant factor to the design professional, it may appear as distorted reflected images under certain viewing conditions. For instance, it will be more noticeable in reflective glass. However, it is an inherent characteristic of heat-treated glass and is not considered a defect.

Edge Strength:

Edges of a tempered glass are the weakest point but significantly better as compared to the edges of annealed glass. Edge strength is important to avoid thermal breakages and for point fixed glazing. Quality of edge improves performance by up to 3 times giving glass superior edge strength. As the edges heat up and cool down faster operator skills and operating procedures are key. Grinding is not only about looks but it also serves a very important purpose.


Tempered glass is primarily used for strength and safety. Although fragmentation is a destructive test, it reveals significant information about the tempered glass. The fragmentation should be even and should not have large chunks because a large piece means weaker glass. So it is important to conduct fragmentation for determining the quality of tempering.

Coating Burns:

Coating Burns are a major cause of concern, during tempering especially in reflective glasses and Low e glasses. As each manufacturer has different variants of coatings, which behaves differently when exposed to heat inside the furnace, some are better and more heat resistant than others. So, if there are high temperature pockets inside the furnace then the probability of coating burns are quite high in those areas. Reflective glass are designed to reflect radiation (heat) and take longer to temper. As a result, many people increase the temperature throughout and in turn inflict damage on the coating. A protective method that can be adopted to reduce the chances of coating-burns which primarily occur on the edges is to remove a small area of the coating by the CNC machines before scoring and cutting coated glass.