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THE CHEMISTRY AND MANUFACTURE OF Am a.m..a TURAL GLASS
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& R. A. MILLER )L\ Pittsburgh Plate Glass Company, Pittsburgh, Pennsylvama
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UPPER LEFT: The electrical insulating qualities of glass used advantageously. The facing of this master switchboard is composed of structural Carrara. Being inorganic, it will not deteriorate a s other materials might. LOWER LEFT; Glass is used for the walls of this brewery room where condensation of moisture is excessive. UPPER RIGHT: Resistance of glass to chemicals makes it suitable for laboratory desk tops. LOWER RIGHT: Structural glass in a coal mine. The iron bottom of the upper half of this chute is warn out, while the glass bottom of the lower half is practically unaffected. (Photos courtesy o f the Pittsburgh Plate Glass Company)
ONE of the outstanding developments in the glass industry during the last half century was the production of opaque structural glass. Since its introduction to the building market around 1900 by the Pittsburgh Plate Glass Company under the name "Carrara," the uses of struotural glass have multiplied a thousand-fold. Originally used in commercial buildings, this versatile material is now found on the fronts of stores, in bathrooms and kitchens of homes, in tanks, work areas, and laboratories of industry, in the chemical laboratories, as
plaques, and in a hundred and one other places serving both utilitarian and decorative needs. The original Carrara structural glass was pure white. In many ways it closely resembled the fine white marble obtainable only from the quarries located near the town of Carrara, Italy. Thus, the natural trade name for the product was suggested in this interrelation and fine structural glass became known as Carrara. Carrara is made up of a mass of crystals suspended in a matrix of glass so that when one looks a t a piece of
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Carrara,one actuallylooks into the glass,which accounts for the depth of color not obtainable in a coated snrface. With Carrara it is the body of the material which is of interest; therefore the surface of Carrara is ground and polished to reveal the interior of the sheet and to permit one to see beneath the surface, a beautiful feature. THE CHEMISTRY OF CARRARA
In general, the Carrara glasses may be classified with the basalts and feldspars. All except black belong to the family of soda-alumina-silica glasses, which are opacified principally by the addition of sodium silicofluoride to the melt. This compound tends to become milky while the glass is being annealed, due to the action of the fluroine which has an opalizing, opacifying effect. Actually, the colors are variations of the white batch. (1) White Carrara, the original product, is a sodaalumina-silica glass, opacified with sodium silicofluoride. (2) Tranquil green has the same composition as white with chromium, cobalt, and nickel. (3) I v o y is obtained when selenium and iron are added to the white batch as the colorants. (4) Gray is obtained when nickel, cobalt, and selenium are added to the white batch. (5) Beige has selenium added to the white batch. (6) W i n e is the white batch plus cupric oxide. (7) Forest green is the white batch plus chromic oxide, calcium fluoride, and cuprons oxide. (8) Rembrandt blue has cobalt, nickel, and calcium fluoride added to the white batch. (9) Orange presents some deviation from the basic soda-lime-silica mixture. It is a soda-potash-aluminasilica glass, opacified with sodium silicofluoride and colored by the additionof cadmium sulfide and selenium. (10) Black Carrara is an entirely normal soda-limesilica glass to which the coloring agents, manganese add chromium, have been added in considerable amounts. Manganese absorbs certain portions of the spectrum, and chromium absorbs certain other portions. Both of these oxides are oxidizing agents in the glass melt and tend to maintain in the ferric condition any iron which may be present. The intensity of color is so great that black Carrara is entirely opaque to visible light in thicknesses over one-quarter inch. If an intense light source is placed behind a piece of black Carrara one-quarter inch thick or less, a very considerable portion of the visible red light from the lisht source wiU be transmitted by the glass. I t is highly transparent to the infrared in all thicknesses, and infrared photographs may readily be made through it. Black Carrara is actually a very deep red-purple color and is not opacified in the sense that the other Carrara glasses are opacified.
CHARACTERISTICS OF CARRARA
The characteristics of Carrara glass determine its range of use. Some of these characteristics are peculiar to glass alone; some are possessed by glass to a greater degree than by other comparable materials. Carrara structural glass is one of the most useful, intriguing, and versatile glass products. The wide choice of colors permits ready adaptation to almost any color scheme and opens wide the portals to creative artistry in design. The range of available thicknesses of these structural glasses enables the mchitect or engineer to use glass of adequate strength for the purpose in hand. Variations of surface finish may be adapted and combined in almost infinite variety. The modernization of "Main Street" with new or remodeled stores for customer eye-appeal began in 1934 with an array of colors of Carrara and polished plate glass set in metal, comprising a complete tailor-made line of new and beautiful store-front setting members of extruded alumilited aluminum and bronze. Signs and decorative features of Carrara are a must in store-front work. The long-lasting, attractive appearance of these units is a valuable asset to any commercial orgauization. These signs are of two types: laminated and sandblasted, or a combination of the two. The laminated sign panel consists of a Carrara glass background to which Carrara letters of various types and sizes are laminated or cemented. Letters, trademarks, or design features of virtually any character may be produced. The all-glass construction is obviously superior, since the colors cannot fade and the sign is easily cleaned with a damp cloth. The sandblasted sign is prepared by sandblasting the letters or design into the glass about inch deep. The blasted area is given a prime coat and enameled. Bronze, aluminum, and gold leaf are used with equally satisfactory results. As with the laminated treatment, design possibilities are limitless. Decorative panels produced by this process are frequently used in bathroom or powder-room installations. Permanent beauty and freedom from maintenance combine to make the Carrara plaque a very attractive medium for honor rolls and war memorials, small signs, and directories. Various treatments are possible: sandblasted letters, painted or unpainted, or raised letters where the background is blasted. Borders, seals, trademarks, and decorative features may be sandcawed and executed in gold leaf. The richness of color throughout the glass insures a permanence of decorative value which is enhanced by the almost indestructible character of the surface.
