This text is copied from:

Alumax Bath
http://www.alumaxbath.com/tech/tgp.htm

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Tempered Glass

DEFINITIONS

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"; hence, 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 rapidly under carefully controlled conditions is described as "heat-treated glass." The heat treating process produces highly desirable conditions of induced stress (described below) which result 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 Specification DD-G-1403B, 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.


HEAT TREATMENT PRINCIPLE


Glass can fracture when its surfaces or edges are placed into tension. Under these 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 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.


MANUFACTURING PROCESSES


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 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.


STRENGTH


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.

Typical Breaking Stress (large light 60 sec. load)
  Annealed Glass: 6,000 psi
Tempered Glass: 24,000 psi

Typical Impact Velocity Causing Fracture (1/4" light 5 gm missile, impact normal to surface
Annealed Glass: 30 ft/sec
Tempered Glass: 60 ft/sec


SAFETY


Fully tempered glass is used in many applications because of its safety characteristics. Safety comes from strength and from a unique fracture pattern. Strength, which effectively resists wind pressure and impact, provides safety in many applications. When fully tempered glass breaks the glass fractures into small, relatively harmless fragments. This phenomenon called "dicing," markedly reduces the likelihood of injury to people as there are no jagged edges or sharp shards.


Fully tempered glass is a safety glazing material when manufactured to meet the requirements of the ANSI Z97.1 Standard and Federal Standard CPSC 16 CFR 1201. Federal Standard CPSC 16 CFR 1201, as well as state and local codes, require safety glazing material where the glazing might reasonably be exposed to human impact. This includes doors, tub and shower enclosures, side lights, and certain windows. Applicable building codes should be checked for specific information and requirements.


USES FOR TEMPERED GLASS


Fully tempered glass is used traditionally in place of other glass products in applications requiring increased strength and reduced likelihood of injury in the event of breakage. The building industry, motor vehicle industry and certain manufacturing industries find tempered glass is effective and economical in a wide range of applications.


Fully tempered glass can satisfy federal, state and local building code requirements for safety glazing in such applications as doors, side lights, shower and tub enclosure, and interior partitions. It is also used in storm doors, patio-door assemblies, and escalator and stairway balustrades. As a glazing product it is used in windows and in spandrel areas (for wind pressure, small missile impact and thermal stress resistance). Special building applications include sloped glazing, racquetball courts, skylights (see below), and solar panels. Any conditions or requirements imposed in the applicable safety glazing laws and building codes limiting such special uses should be determined prior to glazing.


The domestic motor vehicle industry employs tempered glass as side and rear windows in automobiles, trucks, and multi-purpose vehicles. Manufacturing industries use tempered glass in refrigerators, furniture, ovens, shelving, and fireplace screens.


Tempered glass should not be used where building codes require wired glass for fire-spread resistance. Tempered glass should not be used, alone, where the objective is to provide security against forced entry or bullet passage. Combinations of annealed and tempered glass can be effective barriers against forced entry and bullet impact, if properly designed and constructed. When using tempered glass in fireplace screens, provisions must be made for expansion and edge insulation.


TEMPERED GLASS IN SLOPED GLAZING AND SKYLIGHTS


Because of its high resistance to thermal stresses and small missile impact, tempered glass is used in skylights and sloped glazing. On rare occasions when tempered glass in these applications fails, it may fail completely from the opening, individual fragments from tempered glass are relatively small and harmless. A number of these fragments may be loosely joined and fall in this manner. Such pieces do not have the sharp edges normally associated with broken glass but may have significant weight. Some building codes may require the use of screens under skylights. The use of screens may also be dictated by considering the risk of breakage and the resulting consequences.


HANDLING AND INSTALLATION


Tempered glass should receive the same care as annealed glass. Unfortunately, familiarity with the greatly improved strength of tempered glass may mislead people to exert less care in handling it. Careless handling and improper installation sometimes produce edge damage. Delayed breakage can ensue when edge-damaged tempered glass is subjected to a moderate thermal of mechanical stress. Full penetration of the compression layer will likely produce instantaneous total fragmentation of tempered glass. Hence, tempered glass cannot be cut or modified following heat treatment.


IMPERFECTIONS


Inclusions in glass originate from impurities in th batch or cullet, or are combined from furnace refactories. Common forms of inclusions include aluminous stones, iron stones, and silicon. Nickel sulfide stones are uncommon, microscopic defects in glass, and may cause breakage. Delayed breakage may occur when a nickel sulfide stone is present near the center of the glass thickness.


The tempering process rarely introduces imperfections into glass. The basic glass may contain bubbles, vents, chips, and inclusions which, if accepted or not revealed by inspection before tempering can cause breakage in the initial heating or final quench operations. If inclusions are not eliminated by self destruction during the tempering process, in rare cases they may lead to failure at a later time.


VISUAL APPEARANCE


Tempered glass possesses the basic optical qualities of annealed glass. The induced stress condition sometimes produces a slight bow in tempered glass lights. Tempered glass that has been manufactured in a vertical tempering oven contains small surface depressions resembling dimples along one edge. These marks are caused by the pointed metal tongs which support the glass during its passage through the oven. Glass which is passed horizontally through an oven may contain a very slight surface wave caused by contact with the rollers. The waviness can sometimes be detected when viewing reflected images from a large distance. Finally, the air quench nozzles discharge air in a fixed, reciprocating or rotating motion. The area of air quench can be seen through polarized glass as arrays of irridescent spots or lines. Under some lighting conditions these patterns can be seen in ordinary light.

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This text is copied from:

Alumax Bath
http://www.alumaxbath.com/tech/tgp.htm

Ok,,, thought this was best place to puit this info,,

I have had my PJ running for about three months now,,

I used a glass panel retrieved from an old scanner they have glass about the right size,

I have since pulled about 8 of them apart, for the glass

So far I have had no problems with it,, so in effect the heat shield cost me zero dollars

to remove glass from plastic case is fairly easy if you take care, I used a flat blade screwdriver, carefully slid under glass, then gently lifed glass away from plastic case, then worked all away round the glass. Ihave done 8 of them without one breakage,,, mind you I recomend wearing safety googles, just in case...

just an alternative to paying lots of money for tempered glass,, mind you I suspect scanner glass is tempered anyhow..

Tempered glass isn't really that much money... I was talking to a friend who is a glass cutter here in Vancouver. We have lots of glass shops, I am also friends with the Glaziers Union. I have ins when it comes to glass.

Anyways, my glass cutter friend told me that if tempered glass is continually heated and cooled (as with the projectors) it will eventually lose its temperment. But in reality, he wasn't sure the tempered glass is really doing anything special for us anyways.

I am going to specify tempered glass in my design for the sole purppose of safety. I'd rather work with and have tempered glass break in my hands or in the projector than untempered glass.

If we really want HEAT RESISTANT glass then we need to use NEO CERAMIC heat glass, but that will run you about $75 per square foot. A pretty price, but it will handle temperatures of 1300 degrees.

Not for me. What I am going to do is to use tempered glass in a sealed unit, perhaps even a triple sealed unit. I also may have them use a special UV protective coating that will be sealed inside of the unit (otherwise air eats the coating). Excellent thermal protection and UV protection all in one.

Just so you know, glass is avalailabe in all sorts of thicknesses. Common to most high capacity commercial glass shops are the sheet thicknesses of:

3mm
4mm
5mm
6mm
8mm
10mm
12mm
15mm
19mm

Larger thickness will now adays be harder to find as it is a product that the market no longer demands. I may go with 19mm for better thermal characteristics.

One more thing, products will have different names in different areas but here in Vancouver he said the most Optically Perfect glass contains No Lead. It goes by the name of OPTI WHITE, or STARFIRE glass. You would have to specify and source this if you want it.

Regards,
Lucky

The short version:

Glass like alot of material expands when it gets hot, this principle is used to make tempered glass.

Tempered Glass is glass that has had the surface rapidly cooled/hardened while in it's expanded state, when the inside of the glass cools at a slower rate it starts to EVENLY pull on the hardened glass shell created by the rapid cooling (aka tempering). All of these forces constantly pulling inward hold the glass together and make it much stronger than normal untempered glass. The downside is when you chip tempered glass or try and drill a hole in it the well balanced forces pulling inward may become unstable or unbalanced enough to produce an effect like popping a balloon.

I found one Matal halide fixture (flood light)
I guess the glass used over that must be a tempered glass.
I am still not sure that i should go for it or not, coz its hardly 4 or 5 mm.
Any suggestions ??

Does tempered glass have any protection from UV rays? I use lexan with UV protection, but have a piece of tempered glass out of a printer that I would like to use. any thoughts on this?