Heat-Treating for Safety & Durability
Many of the world’s most dramatic, visually breathtaking buildings involve large expanses of glass. But these architectural masterpieces often require that the glass be heat-treated to provide durability and/or safety. There are two processes for heat-treating glass: heat-strengthening and tempering. Both processes take place on the same processing equipment and involve heating the glass to approximately 1,200 degrees, then force-cooling it to create surface and edge compression. Which process is required depends on the specific application of the glass.
Tempered glass, which also may be called safety glazing, is typically specified to provide security or to keep occupants safe wherever there is the potential for broken glass to hurt people if it becomes a projectile, such as in a fire, explosion, tornado or hurricane. It must be used in patio doors, entrance doors, side lites and other hazardous locations. When shattered, tempered glass breaks into thousands of small particles.
To make tempered glass, the glass is reheated to just below the melting point but air-cooled (quenched) quickly, which creates higher surface compression and/or edge compression in the glass. Tempering makes the glass four to five times stronger and safer than annealed or untreated glass. As a result, tempered glass is less likely to experience a thermal break.
Heat-strengthened glass is typically specified when additional strength is needed to resist wind pressure, thermal stress or both, but the strength or safety break pattern of fully tempered glass is not required. A common use of heat-strengthened glass is in spandrel glass. An advantage of heat-strengthened glass is that, when broken, the glass fragments are more similar in size and shape to annealed glass fragments and thus tend to stay in the opening longer than fully tempered glass particles. Although heat-strengthened glass is not a safety glazing by building code, this breakage pattern prevents the glass from falling and injuring someone.
To make heat-strengthened glass, the glass is reheated after forming to just below melting point and then air-cooled more slowly than tempered glass. This slower cooling forms a compressed surface, which increases its strength to approximately twice as strong as annealed or untreated glass.
The industry standard specification requirements for heat-treated or tempered glasses are ASTM C1048 and ANSI Z97.1, as well as the Consumer Products Safety Commission 16 CFR section 1201. For heat-strengthened glass, the requirement is a surface compression of 3,500 to 7,500 psi with no requirement for edge compression. Fully tempered glass will have either a minimum surface compression of 69 MPa (10,000 psi) or an edge compression of not less than 67 MPa (9,700 psi).
Vitro Architectural Glass (formerly PPG glass) processes tempered and heat-strengthened glass at its plants in Carlisle, Pennsylvania; Wichita Falls, Texas; and Barrie, Ontario, Canada.
Effects of Heat-Treating Glass
With both heat-strengthening and tempering, optical image distortion is a possibility. Distortion occurs in glass for many reasons, including glazing pressure, wind load, temperature and barometric pressure changes—or even changes in altitude between where a glass is made and where it is installed. Glass distortion may also occur due to strain patterns in heat-treated glass or interference fringe patterns in the fabrication of insulating glass units. Because of its fluidity at higher temperatures, glass also is inherently susceptible to roller wave, bow and warp while it is being heat-treated.
To minimize the potential impact of the glass distortion that is inherent with heat-treating processes, we:
- Produce all heat-treated glass for a given project on the same equipment, using the same processing parameters
- Use thicker glass, as it is less prone to distortion
- Orient heat-treated glass so that roller wave (the periodic wave imparted to glass during heat-treatment, measured by the peak-to-valley distance) is parallel to the window sill/header
Optical Quality & Flatness of Heat-treated Glass
While flatness of heat-treated glass is a factor of both how glass appears and its optical quality, other factors, such as roller wave, will weigh much more heavily. That is why there is no industry standard for flatness of heat-treated glass. A common specification for flatness of heat-treated glass is .005 inches (five-thousandths of an inch) or less over a 12-inch area; however, with the advent of online electronic inspection, Vitro Glass recommends specifying flatness of heat-treated glass with a formulaic calculation or millidiopter rating. The millidiopter rating is calculated by factoring in roll diameter and the distance, on center, between rollers to arrive at a rating expressed in a +/- tolerance. A tolerance of +/- 120 over 90 percent of the glass surface would be an example of an aggressive flatness requirement for large commercial buildings and would narrow the pool of potential glass fabrication suppliers.
Heat-soaking is a process used to help reduce the incidence of spontaneous breakage due to rare nickel-sulfide (NiS) inclusions or fabrication damage in tempered glass before it reaches the field. It involves placing tempered glass inside a chamber and raising the temperature to approximately 550 degrees to accelerate nickel-sulfide expansion and cause the glass to break in the heat-soak chamber, thus reducing the risk of potential breakage in the field.
Most experts agree that heat-soaking does not guarantee 100 percent elimination of potential spontaneous breakage. Further, the process may create other risks. Please see TD-138 for a more complete discussion of the topic.
Herculite® & Hestron® Heat-Treated Glasses
Herculite® glass describes any glass product expertly tempered by Vitro Glass. All Vitro Glass is available with the Herculite® brand. Only glass tempered by Vitro Glass can be identified with the exclusive Herculite® brand.
Hestron® glass describes any glass product expertly heat-strengthened by Vitro Glass. All Vitro Glass is available with the Hestron® brand. Only glass heat-strengthened by Vitro Glass can be identified with the exclusive Hestron® brand.
Herculite® and Hestron® are registered trademarks of Vitro Glass.
Vitro Glass has several Technical Documents (TD) related to heat-strengthened and tempered glass:
- TD-113 - Why Annealed, Heat-Strengthened and Tempered Glass All Deflect the Same Amount: Discusses the stiffness of glass and the deflection characteristics of annealed, heat-strengthened and tempered glass
- TD-115 - Strain Pattern in Tempered and Heat-Strengthened Glass: Discusses visual strain patterns in tempered and heat-strengthened glass, and how and why they occur
- TD-124 - Fabrication of Heat-Treated Glass: Includes our recommendations concerning further fabrication of heat-strengthened and tempered glass
- TD-138 - Heat-Treated Glass for Architectural Glazing: Discusses the appropriate use of heat-strengthened and tempered glass, including the occurrence of spontaneous breakage, and the use of heat-soaking of tempered glass; offers our recommendations
Heat-Strengthened vs. Tempered Glass
Visit the Vitro Glass Education Center to learn the difference between heat-strengthened and tempered glass.