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As seen in the January 2009 edition of Patio & Hearth Products Report.

In Case of Fire, Don’t Break Glass – Glass-ceramic provides enhanced performance for hearth appliances

By Jeff Razwick

Advances in technology are responsible for improving many aspects of our lives. For example, the science behind ceramics has accelerated rapidly in recent years, with the material now used in specialty applications where its strength and heat-resistant properties are well suited. These include cooktops, cookware and glass for microwave ovens. Increasingly, ceramics are also used in the hearth industry for glazing in high efficiency gas and wood-burning fireplaces and stoves.

Glazing Materials Comparison

The quality of components in a hearth appliance is a major factor in a customer’s satisfaction with the appliance, as well as its overall performance and safety. When selecting glazing for hearth appliances, either during the manufacturing process or when a call for replacement glass is received, manufacturers and dealers have several options.

Tempered soda lime glass (traditional tempered window glass) is one material commonly used. It generally has a blue-green hue when viewed from the edge, and is frequently found in relatively low-heat applications. In tempering, the glass has been heated then cooled with air. Like dipping red-hot steel in a bucket of water, glass tempering alters its molecular structure, strengthening it. While tempered glass can withstand temperatures above 400° F, the higher direct heat levels generated by modern high efficiency appliances may cause it to crack or shatter. For manufacturers and dealers, that could mean costly replacements and safety issues.

To address this concern, transparent ceramic sheets can be manufactured to look like ordinary glass. These clear materials have become a product of choice for gas and wood burning stove applications because they can withstand very high temperatures, allow for heat transfer from the appliance into the surrounding room, and still retain aesthetic qualities that appeal to consumers.

Ceramic glass has an amber tone when viewed from the edge. This difference in coloration from tempered glass is due to the inherent raw materials used. Like tempered glass, however, the slight coloration is generally not perceptible once installed.

Ceramics work well in high-heat applications because they have a nearly zero expansion coefficient. In other words, they do not expand or contract when heated or cooled, making them nearly impervious to breakage from temperature change.

Performance Characteristics

There are several performance criteria for manufacturers and dealers to consider when choosing or recommending glazing materials for high-efficiency gas and wood-burning stoves and fireplaces.

Long-Term Durability

A glazing material’s ability to retain strength over time despite repeated heating is an important factor. Traditional soda lime glass will lose its temper over time when heated, increasing the possibility of breakage. After only one year, it can lose up to 30 percent of its temper when repeatedly heated to 600° F. At regular heatings to 800° F, it can lose virtually its entire temper after one year. Such glass is more susceptible to thermal stress and therefore breakage.

By comparison, glass-ceramics retain their heat resistance indefinitely, remaining strong year-after-year even when repeatedly heated to nearly 1,300° F. A glass-ceramic familiar to many in the hearth industry is Neoceram®.

Some glass-ceramics like Neoceram also include a silica-based surface coating applied during manufacturing. This nearly invisible layer reduces the glazing’s susceptibility to damage from acids generated during hearth appliance operation. Without such protection, the glass surface can more easily become permanently etched – a process called “crazing.”

Thermal Shock Resistance

Another consideration when evaluating glass for hearth product applications is the ability to withstand thermal shock. When glazing materials are heated to high temperatures then rapidly and unevenly cooled, they are placed under high physical stress and can crack or shatter. This is an especially important safety consideration for hearth appliances used in outdoor settings, where hot glass can come in contact with cool rainwater. Also, indoors or outdoors, uneven heating resulting from hot spots in a fire can cause tempered window glass to break from thermal shock.

Common window glass approximately ¼” thick breaks at about 230° F when plunged into 50° F water. Tempered glass can withstand temperatures to about 428°F in the same testing condition. Glass-ceramics, on the other hand, can be heated to nearly 1,300° F and rapidly cooled without shattering – almost three times greater heat resistance than tempered glass. As a result, glass-ceramic will not break during normal usage in a wood stove or gas fireplace.

Rapid Replacement

Unlike tempered glass, which must be cut to size prior to tempering, an appropriately trained person can cut glass ceramic with an ordinary glasscutter. This difference can be critical for quickly servicing customers. Hearth appliance dealers can either work with a local glass company, or stock product at their own facility to be cut on-demand.

Aesthetics

Glass-ceramic is well suited for many different hearth appliance designs, as it is available in many shapes and sizes, including bent configurations. Glass-ceramic sheets are available in sizes up to 42” x 78”, and come in clear, white and black. They also can have surface applied colors to fit with nearly any stove and fireplace design.

Advances in technology have resulted in more options for hearth appliance glazing, with glass-ceramic now popular for many applications. The material meets a variety of design needs while offering higher heat-resistive performance than tempered glass. As such, it is an ideal choice for today’s high-efficiency heating appliances, providing the long-term durability and good looks that manufacturers, dealers and consumers require.

Jeff Razwick is the Vice President of Business Development for Technical Glass Products (TGP), a Snoqualmie, Washington-based supplier of fire-rated glass and framing systems, along with specialty architectural glass products. www.fireglass.com, 800-426-0279

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