March 3, 2016
by Lindsay Hampton
Modern architectural designs favor open spaces and natural light throughout large buildings. Now, even enclosed interior areas like offices, corridors, and stairwells are using interior glass to open up otherwise windowless spaces. This requires fire-protective glazing that offers not only life safety, but also visual and color clarity.
Architects have relied on glass ceramics as a fire-protective window glazing for more than 30 years. Originally, glass ceramics were used as cooktop surfaces because they have no thermal expansion. In other words, they do not experience thermal stress when heated locally and will not break. Untempered non-ceramic glass products will crack when heated, creating an opening through which smoke and flames can pass. When used as part of a fire-rated window assembly, glass ceramics can effectively prevent fire and smoke from moving between rooms for 20 to 180 minutes, depending on the application and code requirements.
If cold water is spilled on the hot surface of a glass ceramic cooktop it will not shatter as normal glass would. Likewise, falling raindrops or cold water from a fire hose will not crack or shatter the glazing of a fire-heated glass ceramic window. This unique property makes glass ceramics a safe choice for compartmentalizing building sections designated by local code officials as requiring hose stream tested ‘fire-protective’ openings or windows.
Glass ceramics, like other fire-protective building materials, do not prevent the penetration of radiant heat. Some codes prohibit glass ceramic use where ‘fire-resistive’ partitions and walls—which block heat as well as smoke and fire—are required. Additionally, codes typically restrict ‘fire-protective’ glazing materials’ area to 25 percent of a wall.
Still, according to a number of experts and authorities having jurisdiction (AHJs), fire-rated glass ceramics meet or exceed building codes and life-safety requirements for fire protection. Additionally, these products allow the opening of fire separations and exterior walls, permitting natural daylight to penetrate further into a building’s interior. Exposure to natural light has been shown to have significant health and performance benefits for occupants, and its presence can greatly reduce the need for electric lighting—an advantage for building owners who aim to meet energy efficiency or utility cost goals.
Even with its fire- and life-safety performance, glass ceramic glazings have a tradeoff. The glass is traditionally amber-colored cast and has higher levels of distortion and haze compared to float glass. However, over the last decade, the various manufacturers of glass ceramics have incrementally improved the transparency of the once-heavily tinted fire-rated products. The natural, but unwanted color is now nearly invisible except when placed against a stark white surface or paired with the more transparent float glass, which has a slight greenish tint. Even so, some architects and building owners worry the yellow-hued panes might appear discolored when compared with nearby windows or doors containing more transparent glass.
The yellow earth-tone tint feared by architects and owners is a byproduct of the manufacturing process. During the production of glass ceramics, components of the glass partly crystallize and leave fine-grain pieces embedded in the matrix. When these beneficial crystals are heated they contract in proportion to the scope and volume of the expansion of the heated glass. These two actions within the matrix negate each other, so there is no thermal expansion or cracking.
The inherent chemical composition of the glass matrix results in a final product with a slight amber hue, which can distort the colored appearance of objects on the opposing side of the glass. Generally, the darker or more colored the glass tint, the greater the deviation from the actual color of objects viewed through the glass.
However, manufacturers have recently introduced fire-rated glazings with substantially improved color rendering, reflectivity, and clarity, along with less distortion, greater ultraviolet (UV) absorption, and better light transmission. Through various technologies, manufacturers have produced glass-ceramic glazings that measure 85 to 97.1 out of 100 on the color rendering index (CRI). This scale determines how ‘true’ a color appears when viewed through the glass. The color of an object viewed in natural daylight without any glass between the eye and the object would rate 100.
These high ratings for color clarity would have been unheard of a decade ago when manufacturers began their quest to improve the transparency of glass ceramics. Many of these products can now meet the aesthetic goals of building design professionals and decision-makers at schools, hospitals, and commercial buildings who require products that comply with increasingly stringent fire codes while still demanding visual quality comparable to float glass.
To rank higher on the CRI, manufacturers have altered their processes in several ways. For example, one maker of fire-rated glass ceramic has offset the amber cast by introducing more neutral gray tones, which the human eye perceives as unnoticeable. However, an unfortunate side effect may include an increase in the glass’ hazy appearance. Another manufacturer has achieved the industry’s highest CRI (i.e. 97.1) by incorporating refined raw materials into the manufacture of its glass ceramic product and improving surface-polishing technology. The unique chemical composition and manufacturing process contribute a glass ceramic that can more closely match the visual qualities of standard architectural float glass.
As manufacturers have improved the color clarity of their glass ceramic glazing, they have also addressed other critical factors of visual and aesthetic performance, including surface quality, distortion, blurriness, haze, visual light transmission (VLT), and shadow. The relatively new practice of polishing the surface on both sides and edges improves the brilliance and reflectivity of the glass, so it appears clearer, allows more light to pass through, and improves color clarity. This results in an appearance more closely matching the non- or fire-rated intumescent products that may be located nearby. Polishing removes the subtle, dull, ‘orange-peel’ texture that has been a signature of large panes of fire-rated glass ceramics.
Sharpness and brilliance, including the accurate and undistorted definition of objects seen behind the glass, are components of surface quality—a measureable value. Standards covering surface quality include ASTM C1036-11e1, Standard Specification for Flat Glass, and ASTM C1652, Standard Test Method for Measuring Optical Distortion in Flat Glass Products Using Digital Photography of Grids. (A national standard, Federal Specification DD-G-451 Rev D, Glass Float or Plate Sheet Figured, previously covered this variable, but it has been eliminated from use.)
Other standards specifically cover heat-treated flat glass (ASTM C1048, Standard Specification for Heat-strengthened and Fully Tempered Flat Glass) and laminated architectural flat glass (ASTM C1172, Standard Specification for Laminated Architectural Flat Glass).
Distortion, clarity, and sharpness
Similar and related to surface quality is the measure of visual acuity through a glass product. Naked eye or side-by-side comparisons of one piece of glass with another can prove valuable in evaluating visual distortion in windows and doors. A quantitative review can include comparisons of the peak-to-valley depth of the typical sine curve or roll-wave surface of the glass, measured with a three-point or flat-bottom gauge. The results of recent tests by manufacturers show new glass ceramic products have parallel glass surfaces with minimal distortion.
Blurriness, haze, and light transmission
Glass with even a slightly rough surface diffuses the transmitted light and results in partial light transmittance. Smooth, polished glass surfaces provide the sharpest display of objects viewed through the glass.
Haze describes how ‘cloudy’ a view through one or more glass panels appears to be. Its primary cause is a lack of homogeneity, or uniformity within the glass composition as well as non-optimal ceramization process settings. The degree of haze, or ‘haze value,’ is stated as a percentage—the lower the percentage, the greater the transparency of the glass.
The industry’s most impressive haze value for a ceramic glass product is 0.5 percent, although most ceramics on the market have haze values ranging from 0.9 to 2.5 percent—a visually significant differential.
The capacity of light to pass through a glass panel is measured with a spectral photometer. Recent studies and user experience show high VLT levels—which indicate greater amounts of natural daylight within the building’s interior—contribute to the improved comfort of occupants. Higher VLT introduces the potential to use natural daylighting to offset the necessity of electrical lighting.
Building design professionals who specify and work with the color-corrected new versions of fire-protective glass are likely to find most manufacturers have resolved many of the issues previously hindering the acceptance of glass ceramics. Architects and building owners unwilling to compromise on aesthetics such as visual and color clarity may find the latest versions of glass ceramics are the best options when complying with fire-safety codes and desiring open interiors awash with daylight. After all, fire-rated glazing—unlike a concrete wall—can both protect against fires and invite natural light deep into a building.
Transparent glass ceramics are suitable for use in lobbies, retail settings, hospitals, hotels, public safety buildings, research and development sites, or wherever visual acuity through fire-rated glass walls is essential. Improvements in the chemical composition and manufacture of the glass have made it such a close aesthetic match for non-fire-rated glazing systems the two could be specified next to each other if fire safety codes allow. Additionally, the glazing works well in hard-to-light spaces like stairwells and corridors, and on a building’s exterior perimeter when a fire-rated glazing system is needed.
When evaluating the benefits of a glass ceramic product, architects, specifiers, façade consultants, and other building design professionals should perform a close visual examination of glass samples or mockup assemblies to determine whether they are close enough to match any standard float glass specified for the same project. Among the most important qualities to consider include:
Specifiers with particular requirements for fire-rated, design-friendly glazings are advised to work with manufacturers to customize a suitable product.
Fire-rated glass has progressed significantly since being developed in Europe three decades ago. Longstanding aesthetic limitations are no longer a burden to the architect or specifier as novel fire-rated glass ceramics closely rival the visual, color, and reflectivity attributes of fire-rated and non-rated glazing products.
Lindsay Hampton serves as the Keralite product manager and south central regional sales manager for Vetrotech Saint-Gobain North America, a manufacturer of innovative fire-rated glass products. She works within the architectural and glazing communities to advance building safety through fire-rated glass solutions. Hampton can be contacted via email at firstname.lastname@example.org.
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