Glass Tempering or Toughening Process

Annealed float glass when breaks is a safety hazard, just because it breaks into sharp pieces and can injure. Imagine, if an annealed glass applied to a window in the 6th floor of a building, and if it breaks by accident, the sharp pieces of glass falls down, the magnitude of the danger increases when there are many people present in the ground floor. 

Toughened glass when it breaks

Glass is toughened so as to make it more stronger and less prone to breakage. Moreover, toughened or tempered glass, unlike annealed glass, breaks into small circular fragments, thereby decreasing the possibility of injuring anyone. Glass is toughened by heating it over 600°C in a furnace and then rapidly cooling by blowing air over a period of 3 to 10 seconds. Initially I was also not sure how glass could become more stronger just by heating and sudden cooling, but I gradually learned that this does work, mainly has to do with the physical properties of glass. 

Toughened glass becomes stronger and gains its safety characteristics mainly because of the compressive and tensile stress built up caused by snap cooling. The sudden cooling of hot glass results in the cooling of the surface of the glass much faster than the center of the glass, this results in the contracting of the surface and building up of compressive stresses in this area. At this point, the center part of the glass expands and develops tensile stresses. During an impact, the force has to first penetrate the compressive stress region, when this layer is penetrated, the built up energy is released suddenly, as a result, glass breaks into small circular fragments instead of large sharp pieces. Toughening of glass increases the compressive stress region of the glass there by making it less penetrable to impact forces, and making it 4 to 5 times stronger than annealed glass.  

A Glass Toughening Plant

Another process of making the glass less prone to breakage is heat strengthening process. Here, the hot glass is not suddenly cooled as compared to what is done while toughening, but it is slowly cooled. A heat strengthened glass is said to be 2 to 3 times stronger than annealed glass, but it does not break into small pieces like toughened glass. Heat strengthened glass breaks just like annealed glass, and hence it is considered to be unsafe just like annealed glass (when it breaks).

Quality of toughened glass can be mainly assessed by comparing the size of broken pieces. In a good quality toughening process, the broken pieces are small and consistent in size, whereas, in poor quality toughening the broken pieces are of comparatively larger in size and there will be variation in their sizes.

(Image courtesy:

 http://www.breakglass.org/How-is-glass-made.html , http://bhatiaglass.com/images/inner/broken_glass.jpg&imgrefurl=http://bhatiaglass.com/about_toughened_glass/about_toughened_glass.htm&h=160&w=200&sz=19&tbnid=mJdvM6Od066GlM:&tbnh=83&tbnw=104&prev=/images% )

What is Float Glass?

Initially, window glasses were made by cutting from large discs of Crown glass. Crown glass is nothing but a large globe of glass, made by blowing molten glass into a crown or a hollow globe. The crown glass is further reheated and spinned out of the globe into large discs, which were then flattened using centrifugal force. These glasses were then cut into desired sizes. There were other methods of making glass as well, like the blown plate, broad sheet, polished plate and cylinder blown sheets, which were all in practice up till the end of the 19th century. 20th century witnessed more advanced processes like  rolled plate, machine drawn cylinder sheet, flat drawn sheet, single and twin ground polished plate and float glass.


The latest, advanced and widely used in our times is the float glass manufacturing process. Since I'm not from an engineering background, I would be able to explain this process only like how I learned it like a common man. The float glass process was invented by Sir Alastair Pilkington and Kenneth Bickerstaff. The major raw material used for making glass is Silica Sand, along with it are added soda ash (sodium carbonate), dolomite, limestone, and salt cake) etc. Each of these raw materials have a significant role to play and contribute to the formation of glass as the end product. Broken glass, is processed and added to the raw material batch (also called cullet), acts as a flux, bringing down the energy required to make molten glass. The furnace is heated up to 1500°C, the molten liquid inside this furnace is then let to flow through molten Tin, it floats due to the difference in density. This floating is what renders the flatness to the glass. The molten glass is then cooled in a controlled manner, this stage is known as annealing, any sharp decline would cause distortion to appear on the glass, that is why the cooling happens in a controlled manner. After all this, the glass is cut to sizes and then packed. 

Major companies that manufacture float glass in India are Asahi India Glass Ltd (AIS), Gujarat Guardian Ltd (Modiguard), Saint Gobain Glass, Hindustan National Glass, Sejal Architectural Glass Ltd (now Sezal), and Gold Plus Glass Industry Ltd. Modiguard was the first float glass brand in India. 


http://www.modiguard.co.in/
http://www.asahiindia.com/float.htm
http://www.saint-gobain.co.in/
http://goldplusgroup.com
http://www.sejalglass.co.in/float.htm
http://www.hngfloat.com/history.html

A Glass Is a Glass Is Not Just a Glass!!!

To be honest, my exposure to glass and glazing industry is brief, and the same was my knowledge about glass. Glass never had more than a decorative value in my mind, as it had to be for me since I was from a decorative product segment (paints). It was just after I got inducted into this industry that I had to invent this statement- "A Glass is a Glass is not just a Glass!!!".


At first, I realized that there are different types of glasses being manufactured, each with different chemical properties and functions. But I could superficially assimilate the knowledge and as a new comer, it was enough for me to classify them as Soda-lime glass, lead glass, borosilicate glass and glass fibre. Since I'm only concerned with glass as a building material, I would be writing only about flat glass. Flat glass industry includes both float glass and sheet glass manufacturers. Basic difference between these two are the process by which they are made. Sheet glass is made by using iron rolling pins to flatten the molten glass and to get it into desired shape, and further grind and polish it to make it clear. Float glass on the other hand is flattened by making the molten glass to float over molten tin. I also learned that the float glass manufacturing process was invented by Alastair Pilkington and Kenneth Bickerstaff in 1957. 


Further, my knowledge on glass started to expand, I learned about various applications of float glass. The first of these, obviously, automobiles. Then there came the application of glass as a building material and as a decorative product. Finally, the effective usage of glass in Green buildings (honestly, I couldn't digest this info in the beginning!).


Then I learned that for all the above applications, float glass cannot be used as such, it has to be processed in many ways depending on the respective applications. Some of these process were tempering or toughening, heat strengthening, lamination, double glazing, etc.


After all these I was introduced to the concept of heat reduction and energy saving using glass, during which I came to know about heat absorbing glasses, heat reflective or solar control glasses, solar control thermal insulating glasses, and various combinations of glasses and processing used for this purpose. 


With all these information injected into my mind, initially it was suffocating and very difficult to decode them at the right time. But as I got exposed to the market, and to more real life situations, I'm now able to relate all my knowledge (what ever learned so far) and use them at application level. But there is more to learn and it keeps on increasing as the industry grows (would be agreed by my bosses and their counterparts in other glass manufacturing companies !). 


This blog is nothing but my journal on whatever I've learned so far about float glass and a note book for glass and glazing enthusiasts to read, scribble and share.