A pane of glass is not a complicated structure. Were an inventor however called upon to devise a method of making window glass, having no previous acquaintance with the subject, the method in use would seem to be the very last he would be likely to hit upon. It is usual to speak of common window glass as being blown. To one unacquainted with the mode of its manufacture, the term blown, conveys a very imperfect idea of it. It is true that blowing is-a large and important part of the operation, but it would be impossible to pro-luce a flat pane of glas3 by blowing alone. There arc three ways in which flat tables or plates of glass may be obtained, one of which does not involve blowing at all. Glass made by the latter process is called plate glass. The glasses made by the two systems which require blowing are called respectively " crown glass " and " cylinder glass," terms derived from peculiarities in the methods of producing them. We shall first describe the method of producing CROWN GLASS The arrangement of the furnaces for manufacturing glass has already been described in an article entitled " Glass Blowing," published on page 90, current volume. The workman takes up on the end of his pipe a large mass of glass by successive operations exactly as described in the article referred to, except that for making crown glass, he takes up a much larger weight. Pie also rolls it upon the " marver" in the same manner as he would for making bottles; but in the present operation greater care is required to prevent the occurence of inequalities, as the mass of plastic glass is so much larger. Having formed the ball upon the marver until it assumes the shape of a pear, an assistant—generally a boy—meanwhile slightly distending it by blowing, it is next softened in a small fnrnace and again rolled on the marver to correct any iusciualities that may have formed during the last heating, and at the same time to collect the greatest mass of glass at the extreme end technically called the " bullion." The bulb is now further extended by a blast from the lungs of the workman, it being meanwhile supported by resting the pipe upon a horizontal bar called the " bullion bar," until it as-sumc3 nearly the form of a sphere. In some instances the i; bullion bar" is dispensed with, other equivalent apparatus bein,1? substituted for it. Its use is to aid the workman iu rotating the bulb, so as to keep it as nearly as may be to the globular shape. It is next taken to the bottoming hole. This is a circular hole in a furnace before which, at a short distance, is built up a screen of mason work which extends far enough to protect the workman from the heat. His face is also protected by a raask, having apertures with glass plates to permit his seeing his work. The pipe with its expanded bulb is laid across a hook projecting ftt ni the screen of mason work. Here the bottom of the bulb (bullion) is exposed directly to a high heat and the pipe being rapidly rotated by the workman, the centrifugal force thus generated soon reduces the bulb from the form of a globe to that of a prolate spheroid, that is, it spreads laterally until the side opposite the fire becomes nearly flat. An assistant now attaches to the center of the side opposite the pipe, an iron rod called a " pontil " or " punty " so that it snail stand as nearly as possible in the axis of rotation. The first workman then immediately detaches the pipe by touching the neck of hot glass with a cold iron wetted with water. The side to whiuli the pipe has hitherto been attached is now presented to the furnace. Rotation being continued the glass begins to expand, the hole left by the detachment of the pipe becoming larger and larger until finally the whole bulb suddenly expands into a single flat table. Before this result is attained however,it becomes necessary to remove the bulb to a large furnace called the " flashing furnace," which has an opening similar to the " bottoming hole,"except that it is larger. The glass has now the form of a flat circular plate, but at the point where the punty is attached there will be left upon it a projection called the " bull's eye." In order that the form of the plate may not be altered rotation is kept up until the glasa is sufficiently cool to support itself. It is then taken upon a large iron fork by another assistant and the punty being cracked off, the plate is carried to the annealing furnace wherd it remains twenty-four hours. After the annealing the glass may be cut into proper sizes and packed. The above outline gives but a faint idea of the skill required to produce crown glass. Necessarily a description of all the minor manipulations requiring the greatest dexterity have been omitted. Enough however has been said to give the reader correct ideas of the general principles of the method. Crown glass is not manufactured now in this country or in Europe, as extensively as CYLINDER GLASS In the manufacture of cylinder glass the blowing extends only so far as to produce a cylinder open at both ends. In fact the blowing is during the entire process accompanied by manipulations of a peculiar character; blowing alone would not produce the desired result. The furnace holes are somewhat elevated, and platforms extend out from the base of the furnace, one for each hole or pot, upon which the workman stand. These platforms are of considerable length, and have pits of considerable depth dug between them to permit the workmen to swing the bulb during the process of elongation. The manipulations are the same as for crown glass up to the point where the globe is expanded, and have so far been already described. When the blast is first forced into the plastic glass, the expansion takes place in that portion of the pear-shaped mass called the neck. The workman now holds the 1'P.ll over his head, and the weight of the thick portion of the mass presses down the expanded portion until it assumes the shape of the top of a bottle like the large ones used by druggists to contain their tinctures, etc. The expanded portion being now permitted to cool, which it does readily on account of its thinness, becomes rigid. The workman then commences a complex manipulation. He blows through the pipe at the same time he rotates it to keep up the cylindrical form of the expanding portion, and also swings the ball in one of the pits above described, thus elongating the cylinder. The glass as it becomes thin cools oflf and becomes rigid, and one of the most difficult parts of the operation consists in expanding the glass to the required size at precisely the time when it becomes so cool as to remain in the proper form. When the walls of the cylinder have become everywhere of uniform thickness, and the proper length has been attained, the end farthest from the pipe being closed has a hemispherical form. This end is now subjected to a quick heat at the mouth of the furnace, and burst open by a strong blast through the pipe ; the pipe being now rotated the part thus, burst open is expanded by centrifugal force to the size of the cooler parts of the cylinder. The cylinder is now laid in a frame and the pipe detached. The end from which the pipe has been separated lias now the form of the upper part of a druggist's bottle. This portion has therefore to be removed. To effect this the workman takes from the furnace, with small iron rods, a small wad of plastic glass and separating the two rods draws out tlie glass into a red-hot plastic cord which lie winds about the cylinder just where it begins to contract towards the neck. The cylinder being thus heated entirely about,'cracks off in the direction of the heated line, upon being touched with cold water. The cylinder has now to be opened. This is accomplished by placing a bar of hot iron longitudinally along the side of the cylinder; when sudden cooling cracks it from end to end. It is now passed to the annealing furnace and flattening kiln. The construction of this furnace is peculiar. It consists of a vault in which revolves an iron frame supporting platforms, called flattening stones. Openings are placed | around this furnace at which workmen stand. A cylinder of I glass is laid in at the first opening, the cracked side upward, and allowed to heat until it becomes plastic As soon as this takes place the workmen by means of a winch revolves the platform and puts in a second cylinder. The partial revolution brings the heated cylinder opposite the next opening, where a workman spreads it out with an instrument consisting of a block of wood upon an iron handle. By this time I another cylinder is ready and the platforms again revolve. The first cylinder—no longer a cylinder, but a plate— is now operated upon by a third workman who smooths it out with a tool similar to the one above described. The glass is then passed through several chambers having gradually diminishing temperatures until it is sufficiently annealed. It is then cut into panes, and packed for sale. No verbal description Gan give an idea of the facility with which these operations go forward, or the skill required to accomplish such results by the simple methods employed. The labor is severe, and, commands wages equal, if not I superior to any manual labor performed. Some of the cylinders blown are very large, being from four to five feet in length, and from twelve to fourteen inches in diameter. The process is one of the most interesting to witness in the whole range of mechanical operations, and will well repay some extra trouble and time to those who may never have seen it performed, should chance bring them in the vicinity of a glass manufactory.
This article was originally published with the title "How Common Window Glass is Made" in Scientific American 20, 8, 114 (February 1869)