Volume 35, Number 9, September 2000
the facts about float glass
by Dez Farnady
Most of us in the glass business have become accustomed to float glass references. While the phrase has some very specific meaning and suggests some general ideas to others, there are a lot of people who have no conception of what it means. Even those who know what float glass is and understand its concept may not be familiar with the float glass-making process.
Although I have seen several float glass plants, the one single thing that struck me the first time is its overwhelming scale. The plants are impressively large, incredibly hot, and produce an unbelievable volume of glass.
Formerly called Pilkington Brothers Ltd., Pilkington developed float glass manufacturing about a half a century ago. There are float plants all over the world now, producing probably more than 95 percent of the worlds glass. Each float line is slightly different from the others and the capacities vary depending on plant and product.
While each plant is different, there are a few things they all have in common regarding the basic process. Pilkington calls it a five-step process. The first step is the batch mixing. The basic ingredients, silica, sand, soda, ash and limestone, are mixed with small amounts of various components, depending on the product, to form the batch. Cullet is added for flux to begin the glass making. In the second step, all of these ingredients are melted at what is called the hot end of the lehr at a toasty 2,800 degrees Fahrenheit, and is pushed to the furnace lip to flow over the tin bath. In step three, the molten white-hot goop oozes over, spreading out on top of the tin. This is where I can honestly say that the bath may be hotter than hell.
This tin bath is the most amazing part. Did you ever float cream over a glass of Kahlua? In the tin bath, the molten glass pours out onto the tank of white-hot liquid and floats, one liquid over the other, much like the cream and Kahlua, spreading out to form what becomes an 11-foot wide river of glass. This river flows nearly half a mile in big factories like Pilkingtons Laurenburg plant. All I know about the speed of the flow is that it varies with glass thickness, slowing for heavy glass or drawn for thin glass, by the assister-wheels. It was probably running a ¼-inch-thick glass when I visited, and I was not able to keep up with it at a crisp walking pace. They say a normal walking speed is about three miles an hour. I dont want to do the math, but at three to four miles an hour, a ¼-inch river of glass 11-feet wide makes a lot of glass.
Step four is the annealing process in which the glass is carried over a long conveyor system through the annealing lehr and cooled by air jets of steadily decreasing temperature. Washing and drying the glass in the process, the conveyor carries the river to step five for capping or automated cutting. With the volume of glass running down the line, even the cutting and packing is far from routine. Because plants are so different, and capacities vary by plant and by product, it is hard to put numbers to it. I was told that Pilkingtons Lathrop plant produces about 600 tons per day and I read somewhere that PPGs Meadville can run as much as a million square feet on a given day.
If you consider that a large multi-story commercial building complex may only require about 40,000-square-feet of glass, then you get an idea of what it means to measure a plants output in tonnage. This all translates into a boon for the consumer for two reasons: the first is that the quality we have come to take for granted borders on the amazing. The second is that due to the capacity of the float process, glass is the only commodity I can think of that sells cheaper today than it did 20 years ago.
Dez Farnady is manager of architectural products for ACI Distribution in Santa Clara, CA. His column appears monthly.
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