Tempered glass, also known as toughened glass, is a type of safety glass, which is glass that is modified in some way to become less likely to break and/or less likely to cause harm when broken. Other glasses in this category include laminated glass, engraved glass and wire mesh glass. Tempered glass is known for its strength; though it may appear no different than standard float glass used to make window panes, it is actually four to five times stronger. In addition, if and when tempered glass breaks, instead of breaking into jagged and sharp shards like regular glass, it instantly shatters into small, thumbnail sized pebbles that are relatively harmless. This is a fact thanks to a heat treatment process that balances and the center and outer stresses of the glass, thus changing the internal structure of the particles within.
Tempered glass is produced by a heat treatment process that balances the center and outer stresses of the glass and changes the internal structure of the particles. If tempered glass is broken, it will instantly shatter into small, relatively harmless thumbnail sized pebbles, instead of sharp, jagged shards, which could seriously injure a person. Tempered glass is widely used within the construction, automotive, electronic and kitchen appliance industries. Storm doors, window panes, glass in building entrances, shower doors, sliding doors, coffee maker carafes, oven windows, tableware, computer screens, cell phone screens, diving masks and microwave oven screens are all made of tempered glass, as they are often used and surrounded by human activity. Building codes often require public buildings to use tempered glass in construction of their windows or decorative features. Tempered glass breaks at about 24 thousand psi, but shattering will still occur at that point. One downside of tempered glass is the tendency to shatter without warning if a small knick occurs. Instead of partially breaking or becoming shards that stay in place, tempered glass explodes into thousands of small pieces instantly, even if a small section of the glass pane breaks.
History credits the first development of tempered glass as a result of the efforts of a Parisian named Francois Barthelemy Alfred Royer de la Bastie. He patented his method of quenching nearly molten glass in grease or oil in 1874 and, thus, tempered glass is sometimes alternatively referred to as Bastie glass. Later, in 1877, a German named Frederick Siemens invented another method of tempering glass, known as the compression method or the Siemens method, which involved pressing the glass in cooled molds. Years later, an Austrian chemist named Rudolph A. Seiden, who moved to the United States in 1935 to escape the Holocaust, secured the first patent for tempered glass processing as a whole.
To create tempered glass today, glass fabricators typically put the glass through a thermal tempering procedure. To begin the process, manufacturers cut, wash, sand and inspect a pane of standard glass. Note that the glass must be treated in the aforementioned ways before it is tempered because any attempts to alter its shape afterwards will result in immediate shattering. Once the pane of glass is ready, fabricators place it onto a roller table and then roll it into a furnace, or tempering oven. where it is heated to extremely high temperatures (between around 1047? and 1148?) for a short period of time. Once the glass is thoroughly heated, manufacturers remove it from the heat and rapidly cool its outer portion using forced, high pressure air drafts. Meanwhile, the inner portion remains heated continues to flow freely for a little while. This results in the exterior going into compression as the center remains in tension, thereby balancing out the glass tension and substantially increasing its strength and impact resistance. Another way to create tempered glass is by using a chemical toughening process that enlists the powers of compression and potassium nitrate. To work, the surface of a glass pane must be immersed at least 0.1 mm deep into a bath of molten potassium nitrate. Here, the sodium ions in the surface of the glass participate in an ion exchange with the potassium ions, the latter which are 30% larger. This exchange results in the compression of the the immersed surface layer and, thus, the glass is chemically tempered. Chemical tempering results in stronger results than thermal tempering and it can be used on more complex glass shapes. After it is created, in order to increase its resistance to scratching, tempered glass can be finished with laminates and/or coatings.
Tempered glass offers many advantages to its purchasers. One advantage is, of course, its strength. Tempered glass is four to six times as strong as annealed glass and and has an extremely high temperature resistance rating. In addition, unlike many glasses, it can be microwaved without harm. Another important feature of tempered glass is that, if it does break, it will shatter into harmless granules. It also does not suffer distortion or loss of stiffness when it is tempered. It is important to note, however, that if it has a small knick, tempered glass sometimes shatters without warning. Also, if it does break, it is not repairable, as it explodes into literally thousands of pieces.
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