Number Io. THE حیح -- حس سےےے The cracked open cylinder is then carried to a flattening oven. It is laid on an iron carriage and pushed into the oven, where it is heated, lifted with an iron tool from the carriage and laid on the flattening stone-a large, flat fire clay tile with a carefully leveled and highly polished surface. As the glass softens the cylinder is spread out and then rubbed down flat with a wooden block mounted on a light steel bar. The stone with the sheet lying on it is then moved out of the flattening compartment into a cooler one, the sheet is gradually cooled down and when it is sufficiently hardened it is lifted by means of a fork with smooth steel times and laid on a conveyor which carries it slowly out of the oven. This part of the oven, called “ lehr,' is for the purpose of annealing the glass by cooling it slowly and gradually, for otherwise it would be too brittle for commercial use. The ordinary type of flattening oven has four flattening stones set on a circular table, the ' wheel,' which is carried on a vertical shaft, turned by hand from the bevel gear drive. It will be readily appreciated that blowing window glass requires unitsual judgment and manual skill as well is more thau ordinary physical strength and endurance; also that the size and thickness of sheets that can be produced by hand is quite limited. In single strength (1-12 inch thick) the limit is about 14 inches diameter by 60 inches long; in double strength (, inch thick) about 19 inches diameter by 70 inches long. Regenerative tank furnaces are now almost universally employed for window glass melting. A tank furnace consists of a long, rectangular hearth or 'tank' constructed of massive fire clay blocks and kept filled with molten glass. The firing is through ports in side walls above the blocks, the flame sweping across between the glass level and the crown from the end where the batch is filled in to a point usually slightly more than half way to the working WINDOW GLASS MACHIN E. rnamJr Tr SO3 _______ end. The batch floats on the surface of glass bath, exposed to the heat of the fire above and the molten glass below, and is gradually melted, refined and cooled to proper consistency as it is drawn down by the continuous working out of the glass at the other end. The temperatnre. of the melting end is about 2,600 Fahr., and at the working and about 2,150-2,250" Fahr. in machine plants. Glass does not have a definito melting point in the senso of a fixed temperature at which it Strips of window glass under test have shown a slight deflection at 840 Fahr, while they lost stiffness at 920° and bent freely at 980°. The glass becomes softer as more heat is applied passes from a solid to a liquid state. and becomes entirely liquid, so as to assume readily the shape of a vessel into which it is poured, at about 1,700. With the application of still more heat the glass becomes, of course, more and more fluid, until finally a point is reached where a slight decomposition and loss of alkali takes place, which manifests itself hy bubbles of gas rising through the heated mass. Somewhere around l896 John II. luubbers, a window glass flattener by trade, began experimenting with a na chine to make glass cylinders. He was a man of unusual natural ability and in working at his trade, he had acquired a general practical knowledge of the physical properties of glass. He believed that he had devised a practical method of drawing glass cylinders from a bath of molten glass, but, being of limited means and realizing that considerable money would be required for his experiments, he disclosed his proposed method to James A. Chambers, in whose factory Lubbers was working, and one of the ablest window glass manufacturs in the United States. Mr. Chambers was much impressed with the practicability of Lubbers' proposed method, crude though the details must have been at that time, and advanced the necessary money to cover the cost of some preliminary experiments.
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