When we talk about security in aluminum windows, doors, and glass cabins, we always suggest tempered glass because it is 5 times stronger than ordinary glass. However, many people ask a very interesting question. If we place tempered window glass directly over fire, it shatters like a firecracker in a short time. On the other hand, with electric stoves, as shown in the picture, intense red fire (the heating coil) burns right beneath them and we place heavy pots on top to cook food, yet that glass neither cracks nor shatters. Why is that? In today's post, I will explain the technical difference between these two types of glass, their manufacturing methods, and the scientific reason in simple language.
Tempered Glass for Windows and Doors
The tempered glass used in windows and doors is basically soda-lime glass.
How it is made: First, ordinary glass is measured with a measuring tape and cut with a cutter pen, then it is heated inside a tempering furnace in a factory up to approximately 620 degrees Celsius. After that, it is cooled within seconds by sudden blasts of sharp, cold air.
Why does it shatter when placed on fire? Due to this sudden cooling process, extreme compression is created on the outer surface of the glass and tension is created in the inner part, which makes it strong. However, when you place it directly on a stove's fire, the part of the glass right above the fire heats up and tries to expand, while the remaining part stays cool. The internal stress cannot withstand this sudden and uneven difference in temperature inside the glass, and the glass shatters into tiny pieces with an explosion.
Electric Stove Glass
As you can see in the picture, the black or red shiny top installed on an electric stove is actually not ordinary glass or tempered window glass; rather, it is a special material called glass-ceramic or Pyroceram.
How it is made: While making this, special chemicals and minerals such as lithium, aluminum, and silica are mixed inside the glass material, and it is manufactured using a highly technical process that forms fine crystals inside it. This is a magical blend of glass and ceramic.
Why does it not break on fire? The biggest technical feature of this glass is that its thermal expansion rate is approximately zero. This means that no matter how intense the fire or heat of 700 to 800 degrees Celsius it is subjected to, it does not expand at all upon heating. Since it does not expand from heat, no internal stress or tension is created inside it. This is the reason why, despite the intense red coil burning below and a cold pot or water falling on top, this glass neither cracks nor shatters.
Fabricator Pro-Tip
As an aluminum craftsman, always remember never to fit tempered window glass directly behind a kitchen stove or as a replacement for a hot plate at a client's request or as a temporary hack. Due to the intense heat or direct heat from kitchen utensils, that tempered glass can shatter in the customer's face at any time. For kitchen tops or stoves, specific ceramic glass is always used.
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