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Glass making has come to be one of the fine arts. Not only is much of our domestic ware made of glass, but bric-a-brac costing thousands of dollars, exquisite colored windows, and even delicate scientific instruments come from the hands of the glassmaker. The minerals used nowadays for glass manufacture consist principally of silicates, such as lime, potassium, lead and soda, and other ingredients are used, including phosphorous, magnesium, tin, iron and bismuth. These ingredients are mixed after a proper formula and are heated for weeks in great porcelain crucibles, specially prepared for the occasion and ending their service with one baking. For weeks, a terrific heat is kept up within a great furnace, and this mixture is finally brought to its proper molten consistency. The master workman repeatedly tests the glass by means of a stirring paddle, through what is called the "glory" hole. When the mixture is just right, a large iron mold, which is to receive the fiery mass, is brought up ready for the pouring, and placed between the heating furnace and another furnace known as the "cooler." The inside of this iron mold is dusted with a quantity of fine sand, to prevent the absorption of impurities from the iron by the glass. After these preparations have been made, a signal is given, a number of workmen tear down the walls of the furnace, and by means of a huge pair of tongs on wheels, the crucible is lifted and drawn from the furnace. The workmen, lest the glass lose too much of its heat, cover the crucible over with a mat of asbestos until it has been brought to the iron mold. The grappling irons on the crucible are changed so that by means of pressure on a bar, the pot may be overturned. The second signal is given, and gently, without great splutter or noise, the fiery liquid flows into the mold. This mold is covered by an iron lid, and a crane picks up the whole thing and running it along a portable tramway, slides the cast glass into the cooling furnace. Gradually, during several weeks, the glass gives up its heat within this furnace. When it is removed, it looks more like sanded glass than a future object glass for a great telescope. Polishing now must be done until the lens becomes clear. Even then it is not ready for scientific uses, for, after the testings to which it is put, it must go back once more to the furnace for a better heating, and be perfected to anneal. The next cooling takes about two months. Then the real lens makers set to polishing it to a degree of extraordinary fineness. When the bare glass, free from serious flaw, reaches the hands of the lens maker it is worth about $5,000; when it leaves him it has grown in value very much—sometimes as much as $25,000. Of other kinds of glass manufacture, that of making thermometer tubes is very interesting. The heating. of the glass is much the same as in other methods. The furnaces are within a long corridor. At the right temperature, a workman plunges a blow pipe into the glass, attaching a small lump of the molten material to the end of it. This for some time is blown and whirled, until it grows to about the size of an apple. A little more glass is then added, and the lump is rolled and kneaded on an iron kneading board. When the proper amount of rolling has been done, another workman quickly attaches the end of his blow pipe to the glass, and runs rapidly backwards away from the other, down the corridor. Both men all the time blow fiercely into their pipes. In a trice they have a small glass tube about the size of one's finger, and perhaps 300 feet long, lying on the floor of the corridor. This can readily be broken up into desired lengths for use in thermometers, barometers and other scientific instruments. IRRIGATION OF THE NILE REGION |