Heat-Treated Glass                  Back to Glass Textbook
In order to provide greater resistance to thermal and mechanical stresses and achieve specific break patterns for safety glazing applications, annealed float glass products may be subjected to a heat-treating process. The most commonly used process for heat-treating architectural products calls for glass to be cut to the desired size, transported through a furnace and uniformly heated to approximately 1150° F (621° C). Upon exiting the furnace, the glass is rapidly cooled (quenched) by blowing air uniformly onto both surfaces simultaneously. The cooling process locks the surfaces of the glass in a state of high compression and the central core in compensating tension. Heat-treated glass has two compression layers or zones, one starting at each surface, plus an interior tension zone centered in the middle of the glass. Each of the two compression zones is approximately 20% of the glass thickness. The middle 60% of the glass thickness is the tension zone.

The color, clarity, chemical composition and light transmission characteristics of glass remain unchanged after heat-treating. Likewise, hardness, specific gravity, expansion coefficient, softening point, thermal conductivity, solar transmittance and stiffness remain unchanged. The only physical properties that change are improved flexural and tensile strength and improved resistance to thermal stresses and thermal shock. Under uniform loading, heat-treated glass is stronger than annealed glass of the same size and thickness. Heat-treating glass does not reduce the deflection of the product for any given load.

Heat-Treated Glass is separated into two products, heat-strengthened glass and fully tempered glass, by definition of the degree of residual surface compression or edge compression. Most furnaces can produce both. A furnace and its quench must be adjusted by its operator for one or the other of a product “run.” The adjustments may include changes in furnace temperature, exit temperature of the glass, residual time in the furnace, and volume and pressure of the quench air.

Production of Heat-Treated Glass
There are two basic methods for producing air-quenched heat-treated glass. The most commonly used heat-treating furnace, a horizontal roller hearth, transports glass on horizontal rollers through the heating and quench processes. A limited amount of heat-treated glass is produced in vertical furnaces, which call for the glass to be held in a vertical position by tongs as it is transported through the heating and quench processes.

Each method produces some degree of bow and warp, which is an inherent characteristic of all heat-treated glass. Tong-held glass, the vertical process, may exhibit a long arc or “S” curve plus some minor distortion at the tong points. Horizontally heat-treated glass will have characteristic waves or corrugations caused by the transport rollers. Industry fabrication requirements, product tolerances and testing procedures for heat-treated glass are defined in the ASTM International (ASTM) document C 1048 Standard Specification for Heat-Treated Flat Glass - Kind HS, Kind FT Coated and Uncoated Glass.

Heat-Strengthened Glass
Heat-strengthened glass is produced with surface and edge compression levels less than fully tempered glass, as specified by ASTM C 1048. The lower compression levels yield a product that is generally twice as strong as annealed glass of the same thickness, size and type. The size and shape of the break pattern of heat-strengthened glass varies with the level of surface and edge compression achieved in the heat-treating process. Heat-strengthened glass with low compression levels will tend to fracture into large fragments, similar to annealed glass breakage. As the compression levels increase, the size of the particles of broken glass tend to become smaller.

ASTM C 1048 requires that heat-strengthened glass have a surface compression level between 3,500 pounds per square inch (psi) to 7,500 psi. The break pattern of heat-strengthened glass is relatively large. The glass pieces typically remain engaged in the glazing pocket, decreasing the probability of fall out. Broken glass should be removed and the opening boarded up or reglazed as soon as possible.

Heat-strengthened glass does not meet the safety glazing requirements of the American National Standards Institute (ANSI) Z97.1 American National Standard for Safety Glazing Materials Used in Buildings - Safety Performance Specifications Method of Test or the federal safety standard Consumer Products Safety Commission 16 CFR 1201 Safety Standard for Architectural Glazing Materials.

Fully Tempered Glass
Fully tempered glass is required in ASTM C 1048 to have either a minimum surface compression of 10,000 psi (69 MPa or an edge compression of not less than 9,700 psi (67 MPa) or meet ANSI Z 97.1 or CPSC 16 CFR 1201. The higher compression levels yield a product that is generally four times stronger than annealed glass and twice as strong as heat-strengthened glass of the same thickness, size and type.

When broken by impact, fully tempered glass immediately disintegrates into relatively small pieces thereby greatly reducing the likelihood of serious cutting or piercing injuries in comparison with ordinary annealed glass. To qualify as a safety glazing material as defined by ANSI Z97.1 and CPSC 16 CFR 1201, the ten largest particles taken from a broken fully tempered lite of glass shall weigh no more than the equivalent weight of 10 square inches (64 sq. cm) of the original specimen when tested according to the standards. Fully tempered glass that meets ASTM C 1048 does not automatically qualify as a safety glazing material.

Note: The GANA Glazing Manual should be consulted for additional information on Safety Glazing in Hazardous Locations (Section V, page 33) and Design Considerations (Section II - Fabricated Architectural Glass Products, page 9) when specifying and using heat-treated glass.

The above information is from the GANA Glazing Manual, 2004 Edition - the most frequently referenced resource in the architectural glass and glazing industry. The Glazing Manual is an excellent addition to any technical library. Go here to order a copy of the manual or CD-ROM. For further information on this and other GANA reference documents visit the PUBLICATIONS section of the GANA website.

Chemically Strengthened Glass
Chemical strengthening of glass is produced through a process known as ion-exchange. One of the methods used to chemically strengthen glass calls for the lites to be submersed in a molten salt bath at temperatures below the strain point of the glass. In the case of soda-lime float or soda-lime sheet glass, the salt bath consists of potassium nitrate. During the submersion cycle, the larger alkali potassium ions exchange places with the smaller alkali sodium ions in the surface of the glass. The larger alkali potassium ions “wedge” their way into the voids in the surface created by the vacating smaller sodium ions.

Chemically strengthened glass production requirements and test procedures are defined in ASTM C 1422 Standard Specification for Chemically Strengthened Flat Glass. The specification covers the requirements for chemically strengthened glass products, which originate from flat glass for use in building construction, transportation and other specialty applications.

Under the specification, chemically strengthened glass is classified on the basis of independent levels of surface compression and case depth. Increasing levels of surface compression permit an increasing amount of flexure. Greater case depths provide increased protection from strength reduction caused by abuse and abrasion. Consumers should consult with chemically strengthened glass fabricators regarding the recommended surface compression and case depth levels required for their individual application. Product classification levels may be confirmed through laboratory testing in accordance with the specification.

Chemically strengthened glass can be significantly stronger than annealed glass, depending upon the glass product, strengthening process, level of abrasion, and the application. Chemically strengthening glass is often the alternative to thermal tempering when applications call for glass that is very thin, small in size, or complex in shape.

Although chemically strengthened glass can be cut after treatment, it is not recommended, as edge strength will be reduced to that of annealed glass.

When broken by impact, chemically strengthened glass exhibits a break pattern similar to annealed glass, and therefore, does not meet safety-glazing requirements in a monolithic form. When safety-glazing performance is required, chemically strengthened glass should be laminated.

While chemically strengthened glass is often used monolithically, product usage has increased in laminated constructions for security, detention, hurricane/cyclic wind-resistant, blast and ballistic-resistant glazing applications.

The above information is from the GANA Glazing Manual, 2004 Edition - the most frequently referenced resource in the architectural glass and glazing industry. The Glazing Manual is an excellent addition to any technical library. Go here to order a copy of the manual or CD-ROM. For further information on this and other GANA reference documents visit the PUBLICATIONS section of the GANA website.

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Last modified: 11/09/08