Launch Slideshow

Float Glass

Float Glass

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    Broken glass to be recycled piles up at the mixing or "batching" end of PPG's float-glass factory in Wichita Falls, Texas. Conveyors in the background carry sand and other materials from storage to the plant's furnaces.

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    Batching begins when raw materials are combined on a conveyor belt to be weighed, blended, and carried to storage silos.

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    Then they travel, by a different conveyor belt, to the main factory building (the gray structure visible in the background), then enter the melter.

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    A hopper feeds the mixed materials into the front end of the 36-foot-wide melting furnace, where temperatures reach 3,000 F.

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    The melting process is completed in the refiner, where air bubbles are removed before the molten glass is poured onto a tin bath.

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    On the red-hot forming line, the glass is poured in a thin layer atop a bath of molten tin, from which it stays separate as it takes shape. T-shaped electrodes hang above to convey heat, and a guide wheel controls the stream's width, which in turn determines the glass's thickness.

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    Because of the terrific heat of the glassmaking process, operators on the forming line work largely on computers in airconditioned consoles, yet wear protective clothing in case they need to approach the machinery. Even packing-line operators wear protective gear to shield them from broken glass.

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    After exiting the lehr, or cooling chamber, at about 300 F, a "ribbon" of fully formed glass drops in temperature further as it moves down the cooling line. As it cools, a laser checks it for flaws.

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    Next, it passes under automated cutters that score it into preprogrammed widths before cutting it lengthwise. The cutting blades are computer-adjusted to fulfill the specified dimensions for different sizes of prefabricated glass sheets.

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    The finished product: Workers in the wareroom transport bulk loads of Azuria aqua-blue glass (made at PPG's Wichita Falls plant, though not in Carlisle). Each load weighs about 16,000 pounds.

Pick your analogy: Milky Way bars (but not Snickers), molasses, maple syrup, ice cream. Food is the handiest way Edward M. Kapura knows to explain the finer points of making glass—in his case, mass-producing the big sheets of glass that architects specify for the windows or walls of buildings.

Kapura is the senior engineer of manufacturing programs at PPG Industries' Works No. 6 factory near Carlisle, Pa. PPG is one of eight makers of flat or “float” glass (often imprecisely called “plate” glass) in North America; the company has six float-glass facilities in North America operating 10 glassmaking lines. The Carlisle plant alone makes 350 million square feet of float glass per year. A typical commercial building has about 10,000 square feet of float glass per floor, so Carlisle's yearly output is enough for 3,500 double-glazed, 10-story office buildings. (This article was reported from Carlisle. Owing to technical concerns, however, the photographs were taken at PPG's Works No. 4 in Wichita Falls, Texas, which has nearly identical processes.)

Glass first gave us the advantage of being able to see outside without feeling the elements. Today, with new low-emissivity coatings, it can help reduce a building's heat loss and gain, making it crucial for improving energy efficiency.

Glass is believed to have first been made by humans more than 4,000 years ago. The materials, their mixtures, and the types of labor evolved through the late 19th century, when mass-production techniques first took hold. In 1902, Emile Fourcault, a Belgian, patented a breakthrough machine for making flat glass by drawing a continuous sheet of it upward from a tank of molten material. A modified form of Fourcault's process, called the Pennvernon process, was introduced by PPG (then known as Pittsburgh Plate Glass) in the mid-1920s.

At Carlisle, that's all pretty much history. The float-glass process that PPG uses today is a modified form of the one invented by Sir Alistair Pilkington in Great Britain and patented in 1962.

Pilkington's process and others like it have proved to be something of a holy grail in making architectural glass because, as Kapura says, it is fast, continuous, and suitable for high-volume production. The huge, infernal tanks in which the glass is made are shut down only once every 10 to 12 years for major maintenance. “There is no off button,” Kapura explains. “No Christmas. No New Year's.”