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Bob Crager
Bob Crager of Lewis Street Glass is a 26 year veteran in the glass business. Lewis Street Glass is a leading Wichita Glass company, serving the entire Wichita/Sedgwick County area since 1919. They do anything and everything having to do with glass, both residential and commercial. They also do Auto glass. They are located at 743 South Market, facing Kellogg on the South, and you can reach them by phone at (316) 263-8259. You can email Bob Crager at bcrager@lewisstreetglass.com
Glass
2005-11-01 16:19:00
What is 'tempered' glass?
: What is tempered glass?     
ANSWER: I found some interesting information this month on the internet regarding the production of tempered glass. I thought I would share it with some of you, as it relates to interesting things the average person doesn't know about tempered glass. Tempered glass is utilized in most commercial applications, such as doors or windows that are subject to being in contact with consumers to avoid injuries from breaking and causing cuts, etc. In the production of flat glass, the molten silica-based mix is cooled slowly under carefully controlled conditions. This annealing procedure removes undesirable stresses from the glass. Cooling occurs in an annealing "lehr". As a result, the glass is termed "annealed" or "ordinary" glass. Annealed glass which has been heated to a temperature near its softening point and forced to cool very rapidly under carefully controlled conditions is described as "heat-treated glass", a term we've all heard before.  The heat treating process produces highly desirable conditions of induced stress (described below) which results in additional strength, resistance to thermal stress, and impact resistance. Heat-treated glasses are classified as either fully tempered or heat strengthened. According to Federal Specifications, fully tempered glass must have a surface compression of 10,000 psi or more, or an edge compression of 9,700 psi or more. Heat-strength glass must have a surface compression between 3,500 and 10,000 psi, or an edge compression between 5,500 and 9,700 psi. The fracture characteristics of heat- strengthened glass vary widely from very much like annealed glass near the 3,500 psi level to similar to fully tempered glass at the 10,000 psi level. The principle of heat treating is that glass can fracture when its surfaces or edges are placed into tension. Under those conditions inherent surface or edge fissures may propagate into visible cracks. The basic principle employed in the heat treating process is to create an initial condition of surface and edge compression. This condition is achieved by first heating the glass, then cooling the surfaces rapidly. This leaves the center glass thickness relatively hot, when compared to the surfaces. As the center thickness then cools, it forces the surfaces and edges into compression. Wind pressure, missile impact, thermal stresses or other applied loads must first overcome this compression before there is any possibility of fracture. In the "heat-treatment" process the key procedure is application of a rapid air quench immediately upon withdrawal of hot (approx. 1200 ° F) glass from the "tempering furnace." The immediate and sustained application of an air quench produces the temper. As air direction against hot glass from arrays of fixed, reciprocation or rotating blast nozzles, it is important to extract heat uniformly from both surfaces (uneven heat extraction may produce a bow or warp) and to sustain the quench long enough to prevent reheating of the glass surfaces from the still-hot glass core. A quenched condition becomes stable when the glass is reduced to a temperature of approximately 400-600 ° F.There are two principal manufacturing methods for producing heat-treated glass. One process heat treats the glass in a horizontal position while the second method moves the glass through the furnace in a vertical position with each light of glass held by metal tongs. Under wind pressure, tempered glass is approximately four times as strong as annealed glass. It resists breakage by small missiles traveling approximately twice as fast as missiles which break annealed glass. Tempered glass is also able to resist temperature differences (200 ° F - 300 ° F) which would cause annealed glass to crack. More next month...
 
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