Edge Grinding: Safety versus Aesthetic

Bethany Stough and Katy Devlin
November 1, 2018

glass railing
Editor’s Note: This article is based on the presentation, Edge Grinding: Safety versus Aesthetic, given by Bill Lingnell, owner of Lingnell Consulting Services, and Urmilla Sowell, director of technical services and advocacy for the National Glass Association during Fall Conference 2018. 

As architects and building owners continue to seek glass systems with as little metal as possible, the glass industry responds with solutions for minimally supported glass applications in everything from façades to skylights, to interior partitions and glass balustrades. This trend toward more glass and less metal often results in using laminated glass with exposed edges. However, this creates a challenge for glass fabricators as they work to balance safety and aesthetics, according to Bill Lingnell, owner of Lingnell Consulting Services, and Urmilla Sowell, director of technical services and advocacy for the National Glass Association. 

Glass railings—one of the most common applications for exposed-edge glass systems—require laminated glass with fully tempered or heat-strengthened glass to meet requirements of the 2015 International Building Code. 

With the edge of laminated lites visible, fabricators must also meet a range of aesthetic demands from owners and architects. Fabricators must ensure clean edges; precise alignment of the glass plies within the laminated assembly; and a clear, smooth interlayer edge. “The architects want to see nice, smooth edges with laminated glass,” says Lingnell. 

“The expectation from designers and customers is for the same high-quality finished edge they can consistently receive with monolithic glass,” says Sowell. “When the edges are exposed, the alignment of glass plies and overall appearance of the laminate edge are likely to be critical to the designer and building occupant in most applications.” 

The breakage concern

In working to meet the stringent aesthetic requirements for exposed-edge laminated glass, fabricators might consider polishing and edge grinding the glass. However, if the glass plies within the laminated lite have been heat-treated, the glass can break down the line as a result of polishing and grinding, Lingnell says. 

The cause of the breakage requires a close look—a microscopic look, in fact—at the physical effects of the heat treatment process on glass (Fig. 1, right, Source: GANA Glazing Manual 50th edition.). The process of heat treatment typically creates a 20-percent compression layer on the surfaces of the glass and a
60-percent tension layer at the center. The process not only provides additional strength, it also ensures the glass will break into many small pieces, rather than dangerous shards.

“When the compression layer is made thinner, damaged or compromised in some manner, the potential for glass fracture is increased. Also, when the compression layer is made less than the original depth, the overall strength or stress carrying capacity is unknown, and without physical test data, the factor of safety in many engineering designs may not be accurate. This is precisely the concern of post-fabrication edge grinding or polishing that potentially reduces the edge compression,” says Lingnell.  “When grinding the edge encroaches on the limit of the compression stress depth, it can cause a penetration of a flaw, micro-crack or other condition that the tension layer will be reached. This makes the glass susceptible to breakage after installation, or significantly reduces the load carrying capability of the heat-treated glass product.” 

What’s in the specs

To avoid these concerns, edge fabrication should be performed prior to heat treating, says Lingnell. This is reiterated in the specifications. 

ASTM C1036, the Standard Specification for Flat Glass, provides the cut size tolerance for each ply of glass in the laminated construction. ASTM C1048, the Standard Specification for Heat-Strengthened and Fully Tempered Flat Glass, provides guidance that fabrication techniques that alter the glass surface, thickness or edge shall be performed prior to heat treating to avoid a reduction in glass strength. ASTM C1172, the Standard Specification for Laminated Architectural Flat Glass, also states that fabrication techniques should be performed prior to heat treatment. The standard says, “After the glass has been strengthened or tempered, it shall not be modified except as recommended by the fabricator.” However, this is intended to allow only minor alterations, such as coating edge deletion or logo application, says Sowell. 

Unfortunately for glass fabricators, the specifications offer little guidance for heat-treated laminated glass in exposed-edge applications, according to Sowell. The industry currently has no generally accepted exposed-edge tolerances to offer as guidelines beyond what appears in ASTM C1036, C1048 and C1172. 

The standards don’t adequately address the aesthetic concerns surrounding exposed-edge laminated glass, Lingnell says. “What happens when the product meets specifications but is rejected by the architect? This is a concern for fabricators,” he says.

Because of this concern, fabricators have developed techniques to produce laminates with alignment tolerances tighter than those listed in ASTM C1172, Sowell says. This includes edging glass prior to heat treating and employing techniques during lamination to ensure tighter edge alignment on multiple edges. Doing this helps companies create laminates with edge alignment intended to meet customer expectations without requiring post-fabrication processing, she says. 

Communication and improvement

The trend toward exposed-edge glass assemblies isn’t likely to wane anytime soon. As a result, fabricators and their glazing contractor customers should work closely with architects and owners early on to ensure that everyone understands exposed-edge specifications, Sowell says. 

“Until the industry establishes acceptance guidelines or criteria for mismatch tolerances and interlayer appearance, the glazing contractor and glass specifier should continue to discuss the project needs with the glass fabricator on a case-by-case basis,” Sowell says. 

For additional guidance, the NGA recently published a Glass Informational Bulletin, “Heat-treated Laminated Glass Exposed Edges.” Access the GIB at glasswebsite.com/publications.