In each plate-glass manufactory, there are different arrangements and different processes. The following description does not refer to any one in particular, but is a selection from various manu factories. The aim of every arrangement and process is the production, with the greatest economy of labour, fuel, material, and time, of the largest number of plates, of great size, strength, and purity of composition, with surfaces parallel to each other, and perfectly reflecting.
In the accompanying plan and section of a casting-house (Figs. 754 and 755), A represent oblong furnaces ; B, annealing-kilns ; T t t', tramways for various purposes; (1) is the roller; (2) the easting-table; (3) a smell movable table, on a level with, and interposed between, the casting table and the bed of the annealing-kilns. This last works on the tramways t, and can be brought opposite to the mouth of any of the six annealing-kilns B. It ie intended to facilitate the removal of the hot plate from the casting-table to the bed of the kiln. To avoid the necessity of pushing the glass, it may be furnished with an endless band of copper gauze, revolving upon two rollers.
A wide carriage or truck (4) runs upon a tramway, the rails t= of which are respectively placed on the summits of the two sets of kilns, at right angles to the direction of the kilns ; (5) is a crane on wheels, movable from one end to the other of the large truck (4). The crane supports the crucible from which the molten glass is poured, and moves with the crucible in advance of the roller. The large truck (4), with the travelling crane, can be moved so as to impend over the casting-table, placed at the mouth of any of the kilns B.
The casting-table (2) is of cast-iron, with a highly polished surface. It must exceed in size 3 r 2 the largest plate of glass it may be desired to produce. The average size is 20 ft. by 8-10 ft. The table rests upon a carriage, the wheels of which move upon a tramway. The roller (1) is usually of cast-iron, hollow, well-polished, and slightly concave. Its length corresponds with the width of the casting-table. It can be moved over the table either by handles at each end, or by spur-wheels working into gearing at one side of the table. It is placed at the extremity of the casting-table furthest from the mouth of the annealing-kiln. The thickness and width of the plates of glass are regulated by two plates of cast-iron, bolted together at their ends, and so adjusted to the curvature of the roller, that the roller and the two plates form three impenetrable resisting sides to the molten glass. The thickness of these plates determines the thickness of the glass, and the distance between them determines the width. The windows and doors of the casting house must be so fitted as to exclude the possibility of sudden currents of cold air, which might interfere with the process of casting. The time required for the complete fusion of the raw materials, and for the refining of the glass, varies, with the construction of the furnace, from 12 to 21 hours.
The size and number of the annealing-kilns are determined by the quantity of glass produced by the furnaces. When very large plates are cast, It is necessary to anneal each plate in a separate kiln, the plate resting on the bed of the kiln. Smaller plates may be placed on movable trucks, running on a tramway in the kiln ; in this case, the number of plates in each kiln is regulated by the number of trucks the kiln can contain. Small plates may also be stacked on their edges, so soon as they have cooled sufficiently not to bend. The time required for annealing plate-glass depends upon the size and thickness of the plates ; 5 days may be considered as the average time. The process of cooling
may be hastened by gradually admitting cold air, to circulate in channels running under the bed of the kiln. The doors of the kiln also are so arranged as to open piece by piece, with a view to the gradual admission of the outer air. The doors, however, should not be opened at all, until the glass has been in the kilns for 2-3 days. Before oasting commences, the kiln or kilns are heated to dull-redness, and a layer of sand is spread over the bed of the kilns, or the bottom of the trucks. The casting-table and roller are cleaned, polished, heated, and placed opposite the kiln, and the " carrying "-table is interposed between the mouth of the kiln and the casting-table. The large truck, carrying the movable crane, is wheeled up, so as to be over the side of the casting-table farthest from the furnaces. The end of the crane projects over and beyond the end of the casting table farthest from the mouth of the kiln. In the meantime, the screen or door protecting one of the crucibles has been removed, the glass has been skimmed, and the base of the crucible has been loosened from the bed of the furnace. The crucible is raised, and moved from the furnace by a large iron projecting fork, supported on wheels. The opening into the furnace is closed. The crucible, having been scraped flee from incrustations, is carried to the end of the casting-table, and immediately under the crane. The " claspers," attached to the crane by chains, are fixed firmly in the grooves, or under the projections on the outside of the crucible. The crucible is raised by the crane from off the forked chariot, carried over the easting-table, and immediately in front of the roller. It is tilted sufficiently to allow the glass to pour, and is moved by the forward motion of the travelling crane, so as to supply a continuous flow of molten glass immediately before the advancing roller. The crucible, so soon as the best of the glass has been poured out, is replaced on its chariot, and, if still uninjured, carried back to the furnace, re-set, and re-charged. By the time the pouring is finished, and the roller and regulating-plates are removed, the end of the plate farthest from the kiln is sufficiently solid to resist pressure. Pressure in the direction of the mouth of the kiln is applied by means of chains attached to the two ends of an iron bar, placed against the solid end of the plate, and passing through pulleys fastened to the front wall of the kiln. It is thus gradually moved forward upon the intermediate table, and thence, by means of an endless revolving sheet of wire gauze, into the mouth of the kiln. Here it is conveniently placed either on the bed of the kiln, or on a movable truck, or on its edge. When a sufficient number of plates have been inserted in the kiln, the doors must be securely closed, and sealed with fire-clay. At the same time, the fire is allowed gradually to die out. That side of the plate which was in contact with the casting-table is always rough, while that over which the roller passed is slightly undulating and polished. The undulations are probably duo to the action of the air upon the glass, while still liquid. Plate-glass, exactly in the condition in which it was placed in the annealing kiln, having undergone no mechanical processes of grinding or polishing, is sold at a comparatively low rate, as " rough east plate," and is largely used for roofing, and pavements, and for all purposes where translucency, strength, And cheapness are essential.