Volume 37, Issue 4, April 2002

Rolling Forward
        Advancing the Use of Heat-Treated Glass
by Ren Bartoe

Soon, roll-wave distortion requirements will be specified by ASTM to ensure distortion-free buildings such as this one. (Photo Courtesy of Pilkington North America.)

Responding to the architectural design community, the Tempering Division’s Roll Wave Subcommittee of the Glass Association of North America (GANA) is developing procedures for the measurement and quantification of roll wave distortion in heat-treated architectural glass.

GANA is addressing the mutual interests of architects, contractors, consumers and glass fabricators. The Tempering Division’s Standards and Engineering Committee chartered a subcommittee to address roll-wave distortion in heat-treated architectural glass in direct response to market interests. Ultimately, the subcommittee intends to propose a roll-wave specification for inclusion in the American Society for Testing and Materials (ASTM) document C 1048-97b–Standard Specification for Heat-Treated Flat Glass—Kind HS, Kind FT Coated and Uncoated.

While various types and causes of distortion in heat-treated glass are referenced in ASTM C 1048-97b Section 7.4 Distortion, roll wave is a fundamental form of distortion that may be manifested in heat-treated architectural glass. It has been defined as “a repetitive wave like distortion from flatness related to the heat-treating process, excluding edge effects and distortion influenced by assembly or installation.” Roll-wave distortion is usually recognized as a washboard effect through the central plane of the glass (see figure 1).

The subcommittee has conducted extensive studies and has developed and refined numerous documents on roll-wave distortion. These include a document titled Standard Test Method for In-Plant Measurement of Roll Wave in Heat-Treated Architectural Glass and a glass information bulletin, Quantifying Roll Distortion in Heat-Treated Architectural Glass. Both documents are in the final stages of review prior to submission to division membership for acceptance and publication.

Previous attempts to quantify roll-wave distortion involved measuring the peak-to-valley depth of the wave. This measurement alone was found to be insufficient as the peak-to-peak spacing has a significant affect on visibility of optical distortion. Optical distortion in the form of roll wave is measured as an optical power, similar to the power of a curved mirror or lens. Optical distortion is expressed in diopters (dpt), or in the case of roll wave distortion, millidiopters (mdpt). One millidiopter is the distortion produced by reflection or lens with a focal length of 1,000 meters.

Roll-wave distortion is calculated as: D = 4p² W / L² x 106 .

In this equation, D represents distortion (mdpt), W is peak-to-valley (in.) or (mm) and L is the average peak-to-peak wavelength (in.) or (mm).

Distortion (D) is also referred to as the RW Factor.

The standard test method is comprehensive and requires measurement of peak-to-valley using either a flat bottom or three-point contact gauge (see figures 2 and 3). Other methods and devices (such as lasers, coordinate measurement machines, video, etc.) are acceptable, but should be calibrated against the flat bottom or three-point gauge. The standard test method also calls for the elimination of one roll circumference from the leading and trailing edge when measuring peak-to-peak. This area is commonly referred to as edge kink or edge effect and will likely be studied and addressed in a separate specification. (Edge kink is visible on the leading and trailing edge of the glass shown in Figure 1.)

Fig. 1

Fig. 2    Fig. 3

Figure 1 (top) shows a piece of tempered glass with an edge kink and roll wave. Figures 2 (above left) and 3 (above right) demonstrate tools used to measure roll-wave distortion, such as the dial indicator in Figure 2 and the chart recorder in Figure 3.  

The subcommittee respects that safety takes precedence over aesthetics and wants to ensure that any roll wave specification will not compromise safety. Separate from the subcommittee efforts, a Center-Point Fragmentation Test has been proposed for inclusion in an update of the American National Standards Institute (ANSI) Z97.1 American National Standard for Safety Glazing Materials Used in Buildings–Safety Performance Specifications Methods of Test currently under development. Heat-treated architectural glass produced on a wide range of horizontal furnaces has been evaluated and the RW Factor determined following the Standard Test Method. Additionally, the roll wave distortion data has been correlated with center-point fragmentation tests to evaluate the distortion relative to the break pattern and particle count of the heat-treated glass. Tempering Division member companies have contributed to the subcommittee efforts by providing an extensive amount of this data from normal day-to-day production. The results have been tabulated and validated for subcommittee review and discussion in anticipation of proposing roll wave specifications.

With the oversight of the Standards and Engineering Committee GANA’s Tempering Division, the Roll Wave Subcommittee is responding to the architectural design community based on the science and technology of glass fabrication. It is our mutual objective to apply this knowledge to advance the use of heat-treated architectural glass.

For more information on roll wave distortion and the activities of GANA, contact Greg Carney, GANA technical director, by phone at 910/596-2209, fax at 910/592-9261 or e-mail cgcarney@aol.com

Ren Bartoe is product line manager, Americas, for Vesuvius McDanel, and chairperson of the Roll Wave Subcommittee within the Tempering Division of GANA. He has more than 25 years of tempering roll and refractory experience in the glass-tempering and float glass industries.

USG