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Opal glasses are a type of historic glass that can be between semitranslucent to opaque. It is made of small, clear particles held in a clear atmosphere. Transparency is reduced by the reflection and scattering of the light caused by these small particles. This type of opal glass was traditionally used for illuminating glassware. The emergence of colored opal glass was first seen in Egypt. Ancient Romans used translucent glass to imitate marble. The 16th century saw the reemergence of this technology. Venetian artists began producing semi-translucent glass by adding cryolite fluorides into the matrix. Imitation porcelain was also created by the Chinese using this technique. In the late 19th century, the development of the regenerative furnace and an abundance of natural gas reserves in particular states led to exponential growth in U.S. domestic flat glass production, which would later lead to innovative adaptations in glass manufacturing. Opaque structural glass slabs emerged in 1900 and served as an alternative to white marble slabs for wainscoting and table surfaces. Sani-Onyx was the first structural glass and was created by the Mariette Manufacturing Company. Pittsburgh Plate Glass Company soon followed with Carrara glass, which was available in black and white, and Penn-American Plate Glass Company presented Novus Sanitary Structural Glass. Though eight American firms were producing structural glass, the most prevalent were Pittsburgh Plate Glass’s Carrara and Libbey-Owens-Ford’s Vitrolite. By 1929, production of opaque structural glass surpassed 5 million square feet, causing a demand for new colors and finishes. Structural glass saw a decline in popularity and by the 1950s was just a fraction of what it had been in the 1940s. The development of porcelain enamel and other competing materials proved too much for structural glass to compare with. Marketing structural glass became more aimed at interior spaces for utilitarian use, as structural glass faded out of popularity for storefront use.
 Manufacturing Process
Opaque structural glass was usually made by heating silica, fieldspar, fluorspar, china clay, cryolite, manganese, and other materials in a large pot or tank. After heating the materials at nearly 3,000°F, the sheets were cast then rolled into the desired thickness. The glass was then cooled for three to five days, significantly slower than modern plate glass. Glass sheets were then fire polished to finish then process. Glossy finishes were created by grinding the surface with fine sand and rollers. The material is then polished to a mirror like finish with felt blocks and ferric oxide powder. Slabs were cut to size and the edges of the material could be cut and drilled and the edges finished before leaving the factory. For more opaque glass, the addition of fluorides was the first option. Cooling the fluorides would cause them to precipitate, which caused a dense mass of particles to become suspended in the clear matrix. The fluoride particles reflected and trapped light until the glass was semitranslucent or opaque. Coloring oxides could be added before firing. Novus Sanitary Glass could be made in a variety of colors but was seen on buildings predominately in black and white until the late 1920s. In the 1930s, colors were offered in more than 30 hues of pastels, jewels, and solid tones as well as striated agate and dendritic patterns. The most predominant structural glass in the beginning was the fire-polished and satin finishes. Colorful mirror-like finishes became popular in the 1930s.
 Uses and Installation
Successful structural glass manufacturing procedures emerged around 1900 and was compared to statuary marble. Structural glass was also marketed as a highly sanitary alternative for utilitarian use. The glass was also ideal because it was nonporous and noncrazing, two qualities that made it suitable for production in large sheets. Structural glass soon replaced marble and tile in hospitals. In the beginning, structural glass was used primarily in locations that required durable, nonstaining slab materials. Some of the earliest application of structural glass include: wainscoting, table- and countertops, switchboards, and refrigerator linings. Structural glass’s light diffusion properties made it popular for use in operating rooms and laboratories. Due to its variety of colors, structural glass was at its peak in popularity during the Art Deco and Modernism phases of the 1930s and 1940s. The material was also popular because it could be bent, carved, sand blasted, and painted with gold or silver. It was a popular material in modern office buildings, theaters, bathrooms, and restaurants. Structural glass could be used to modernize any exterior of commercial buildings. New construction was also capable of being clad in structural glass that was set in aluminum framing. The lustrous finish on both the aluminum and structural glass made the two products aesthetically compatible. In an attempt to make a complimentary product to its Carrara structural glass, Pittsburgh Plate Glass introduced Pittco-Carrara Glass Store Fronts. These store fronts included metal window sashes to overlap and protect the edges of Carrara glass. The introduction of Glastone (a prefabricated Vitrolite-faced concrete masonry unit) by Libbey-Owens-Ford was meant to encourage the use of Vitrolite in new construction. Structural glass could be bought in a thickness from ¼ to 1 ¼ inches with the panel size being determined by the end use. The exterior location of the glass determined how large the panel could be made. If it were going to be hung higher than 15 feet, it could not be more than 6 square feet; if the piece was going to be installed lower than 15 feet high, it could be 10 square feet. Interior wall panels could be produced at no more than15 square feet. Toilet partitions were commonly produced at a size of 25 square feet by laminating two 7/8 inch slabs with bituminous adhesive. Structural glass’s versatility depended heavily on its substrates. It could be applied to most flat surfaces but wood substrates should be avoided because of their potential to warp. The backing was prepared and sealed with a bonding coat and mechanical fasteners were used to secure the substrate. The prefabricated panels were usually attached with asphaltic mastic. 3 inch daubs were applied to 50 percent of the back glass panel. The glass was rocked into place until the mastic could be forced into the backup surface providing a keying action. Joints were pointed when the cement was set with a pointing cement commonly provided by the glass manufacturer. Cork tape was used to protect the edges. The use of cement instead of mastic was necessary if the material was being installed where high levels of moisture were expected, like tub surrounds. When being installed as a ceiling application, small screws and felt washers, with an addition of large mastic daubs, were usually strong enough to hold the structural glass safely in place.
Structural glass is durable but is still capable of breaking. The lack of domestic production makes conservation of historic pieces essential.
Structural glass will not warp, craze, or fade easily and is impervious to most acids. Impacts can cause structural glass to break. Thermal expansion and contraction can cause movement and breakage. If structural alterations have been made, cracks and chips may be visible. Hardening of mastic adhesive can allow for movement between the glass and the substrate and may affect the structural integrity. Narrow joints leave little room for movement. Movement, therefore, can cause breakage if the slab detaches. Mastic is also damaged by water infiltration because it accelerates the hardening process. It is common for pointing cements to deteriorate due to weathering and other stresses. Darker shades of structural glass obtain more heat, which will cause the panels and walls to endure greater thermal stress. Most structural glass failure is noticeable due to cracked, damaged, delaminating, joints appear damaged, or water penetration is obvious. Because structural glass was rarely installed more than 15 feet above the ground, it is easy to reach. Gently pushing on the panel is an easy way to test the adhesion to the substrate. If the piece seems to have been water damaged, removal of the piece may be necessary to determining the stability of the mastic and substrate.
 Conservation Techniques
Opaque structural glass is no longer produced and repairs should be made when possible. The glass can be cleaned with water and ammonia or detergent. Joints are easily repaired with traditional joint cement with a watertight surface integrated, latex caulking, or a glazing compound. Silicone sealants prove harder to use with fine joints. Joint cement was traditionally matched to the glass. Repairs should be done in a manner that compliments the historical joint patching material. Hairline cracks can be filled with a caulking that had been specifically tinted to match the glass. Chips can be repaired with a polyester resin adhesive that has been tinted to match the glass. The surface should be polished with fine sandpaper and buffed. An alternative method for repairing chips is filling the hole with a glazing compound and painting the area with a computer matched color. Reinstallation of structural glass panels that are detaching is often difficult because of the hardening of the mastics and the find joints between panels. The most common problem with these repairs is glass breakage. Solvents should be injected behind the glass to soften the mastic. After the mastic softens, a piano wire can be slipped behind the panels to cut through the mastic. Directly steaming the face of the panel for approximately ten minutes can also soften the mastic but is time consuming. Cutting edge joints should prevent oystering of the glass edges. The panels should then be carefully pried or sawn off. Prying glass panels require wooden blocks to protect the material from the crowbar. The slab should be cleaned and then reattached with a hot-cup asphaltic mastic in daubs. Existing shelf angles should be evaluated and horizontal edges should be protected with recessed cork tape. Caulking or joint cement is used to repoint joints.
Replacing these pieces is difficult because they are no longer produced in the United States. It may be possible to find pieces at a glass shop and architectural salvage yard but the color is unlikely to match the existing piece. A Czechoslovakian kiln still produces black, white, beige, and mint green structural glass and is distributed by Floral Glass and Mirror of Hauppauge in New York but the size and finish are limited. The panels are ¼ inch thick, thought the thickness can be adjusted with mastic and mechanical fasteners. In Japan, NEG Industries produces NeoClad, which is an opaque glass that comes in white, beige, and gray. ASAHI Corporation makes an opalescent structural glass in white or light gray. With all of these different glasses, the colors and sizes are limited and the cost of shipping proves problematic. Different materials have been proposed for use as replacement pieces for historic material but none have been able to perfectly replicate structural glass.
- Dyson, Carol J. "Structural Glass." Twentieth-century Building Materials: History and Conservation. By Thomas C. Jester. New York: McGraw-Hill, 1995. 200-04. Print.
- Douglas A. Yorke, "The Preservation of Historic Pigmented Structural Glass (Vitrolite and Carrara Glass)," Preservation Briefs 13 (Washington, DC: National Park Service). Available online at http://www.nps.gov/hps/tps/briefs/brief12.htm
- Carol J. Dyson and Floyd Mansberger, "Structural Glass: Its History, Manufacture,Repair,and Replacement" in CRM, Cultural Resource Management vol. 18 no. 8 (1995): 15-19. Available online at http://crm.cr.nps.gov/issue.cfm?volume=18&number=08
- Vitrolite Specialist (St. Louis, MO), Preservation, repair and maintenance services.