National Geographic : 1935 Jul
THE NATIONAL GEOGRAPHIC MAGAZINE than in the cement industry. A pilgrimage through a cement plant 20 years ago was like working at the "bunghole" of a thresh ing machine before the days of the straw blowers. There was dust everywhere. As one surveyed the horizon of Lehigh and Northampton Counties, it seemed that there were a hundred whirlwinds perpetually blowing and marking the sites of the cement plants scattered over the countryside. Today it is different. Now the rock is crushed under streams of water and the final powdering of the stone produces a sludge of about the consistency of mush. This is introduced into the big rotary kilns-some of them as much as 120 feet long and 15 feet in diameter. Here it meets a stream of powdered coal under a flame that gives a temperature of from 2,500 to 3,000 degrees Fahrenheit. The coal has been so finely ground that 95 percent of it will pass through a screen that has 10,000 meshes to the square inch. When the powdered coal, the sludge, the fiery heat, and a regulated amount of air meet, glass hard clinkers are formed. These clinkers in turn are the intermedi ate materials between cement rock and the finished product. They are mixed with heavy steel oval-shaped globules and con veyed into rotary grinders. Round and round these big machines turn hour after hour until all the clinkers have been ground almost to impalpable dust, in which form it is Portland cement. Some of these big plants bag and load 40,000 sacks of cement a day, and yet the air around them is so free of dust that the flower gardens adorning the homes of the employees and the lawns of the company's plant match those of many a city park. The Lehigh Portland works at Sandts Eddy is one of the many modern cement plants in the vicinity of Easton. GLASS MAKING IS A "HEAVY INDUSTRY" Among all of Pennsylvania's dramatic industries there is none possessing greater fascination than plate-glass making. Such opaque substances as salt cake, pure lime stone, and quartz sand go into a furnace in 3,500-pound batches, become liquid, and then pass out as a continuous sheet of plate glass which is cut, ground, and polished until it is as transparent as thin air. Up the Allegheny River from Pittsburgh stands the little village of Creighton. On its outskirts is the largest plate-glass plant in the world. The company owns at its back door the coal mine that supplies its fuel, for coal is used in such quantities that such a plant is always located near its fuel supply rather than close to its raw material. Here are huge bins for storing salt cake, soda ash, glass sand, limestone, and other ingredients. There is the giant furnace that holds 1,200 tons of molten glass. Come with me and, with a colored glass shield before your eyes, have a peep into this fiery furnace. Here are little hills and tiny mountains, survivals of the last 3,500-pound mouthful of material dumped in. There you see a miniature lake of incandescent molten mix ture. Between the little hillocks of un melted batch run rivulets of white-hot glass. Over all roars a tremendously hot flame of 2,700 degrees Fahrenheit that melts sand and limestone and salt cake about as fast as a July sun would melt snow. Twenty-one days of warming are re quired to bring the temperature of the fur nace up to operating requirements. The marvel is that its linings can be made heat resistant enough to stand temperatures that convert sand and limestone into liquid and to take that punishment for months on end. At the rear of the furnace is a giant lip out of which the molten glass flows. Glow ing hot, of doughy consistency, it passes under tremendous rollers, which convert it into a ribbon about 7 feet wide. Along this it travels through an annealing lehr for 400 feet. By now it is cool enough for the cut ters who trim off the edges, cut it into lengths, and mark the defective spots. Then a sort of mechanical spider with vacuum-cup feet swoops down on each piece, lifts it high overhead, and deposits it in a plaster-of-paris film on the six-ton cast iron car that is to be its bed while passing under the grinding machines, where sand and emery smooth it down. From these grinders the plate passes under the felt footed polishers where enough rouge to color the lips and cheeks of an army of women is used to produce that perfection of smooth ness which gives perfect vision through your motor window (see page 3). After the glass has traveled 125 feet in the fiery furnace, 400 feet on the cooling lehrs, 400 feet under the grinders, and 400 feet under the polishers, it is ready for its trip through the Duplate works where two pieces are cemented together with a Du Pont prod uct and become safety glass.