The object of Dr. Siemens' system is to render the process of glass-making continuous and more uniform. Fig. 783 is a longitudinal section of his arrangement of tanks A B C. The raw mate rials are received and partially fused in A, whence the liquid glass flows into the clarifying-com partment B; on leaving this, it passes into the working-compartment C, from which it can be withdrawn in the ordinary way through the openings D D. The compartment A is charged through the aperture E, at the back of the furnace, and is separated from B by a party-wall F, in which are formed a series of passages, one of which ie shown at a. Through these passages, the melted glass flows, and from B, it passes to the tank C, through the passages c in the division wall G. The sides and bottom of the tank are perforated with sir-passages d, through which cold air is made to eiroulate by the draught produced in the chimney H ; thus the tank walls are kept cool, and enabled to withstand the action of the melted glass. The gas-ports are shown at K ; the heated sir issues from corresponding openings, passing in diverse directions over the upper edges of the tank. By this means, is produced an effectual intermixture of the combustible gas and the heated sir, and the air is prevented from coming into immediate contact with the surface of the melted glass.
Fig. 784 shows a vertical cross-section, and Fig. 785 a horizontal section of this combined arrange ment of furnace and tanks. The principal advantages to be derived from the use of the continuous melting-furnaoe are :—(1) Increased power of production, as the full melting-heat may be employed, without interruption in one part; whilst in another part, through the perfect control of the gas and air supplied to the furnace, the glass may be allowed to settle and cool : with the old method, these results could be attained only by the heating and cooling of the entire furnace ; (2) economy of labour in the melting operations ; (3) durability of tank and furnace, owing to uniformity of temperature ; (4) economy of fuel, by the consumption of gas and air on the regenerative system, already explained.
The composition of bottle-glass is very varied. It is, speaking generally, a silicate of soda or potash, and lime, together with alumina and oxide of iron ; it is to this latter oxide, present as an impurity in the ()heap materials employed, that the glass owes its green colour. An analysis of a sample of French bottle-glass gives the following result :—Silica, 53.55 ; potash, 5.48 ; lime, 29.22 ; alumina, 6.01 ; oxide of iron, ; total, 100.00. It may be represented by the formula 27(CaO), 2(Fe200, 45(SiO2).
The materials employed for the preparation of bottle-glass are common coloured sand ; the resi dual alkaline and lime salts from gas-works, eoap•works, and alkali works ; common salt, salt-cake, and ashes from fires of wood or charcoal ; clay, basalt, and other rocks containing felspar ; and lastly the slag from blast-furnaces. This slag accumulates as refuse at the rate of nearly 8000 tons per annum, and great credit is due to Bashley Britten for hav ing worked out a prac tical method of utilizing it in the manufacture of glass. Both the heat and the material of the slag is made use of, and it is upon the possibility of utilizing the former that the economy of the process depends. The manufactory, at Finedon, in Northamptonshire, is in close contiguity to the blast-furnaces of the iron works ; and as the mol ten slag is run from the furnaces, it is conveyed on " carriers " to its des tination. The ingredi ents of the glass, of which the larger portion is molten slag, the remainder being sand and alkalies, are fed into the tank of a Siemens' continuous melting-furnace.
Bottles are made by a " set " of " hands," which usually consists of five persons, respec tively known as the "gatherer," the " blower," the " wetter-off," the " workman," and the " boy." " Medicals " require four, and other
bottles five manipulations. The glass being ready melted in tanks or crucibles, the " gatherer " inserts the end of a long hollow iron tube through the opening D into the working-tank C, Fig. 785, or, if an ordinary furnace is in use, through an opening in the furnace, opposite the mouth of a crucible, and into the crucible. The melted glass adheres to the heated end of the tube, and the gatherer, by revolving the tube, is able to collect as much glass as he judges will be sufficient to form the bottle required. The blower now takes the tube, with the glass attached, blows through it, and trundles it on a smooth iron slab. The mass of glass is slightly hollow, and conical in shape. It is placed in a mould, and distended by the blower's breath, until it acquires the internal form of the mould, both externally and internally. The blower now has a shaped bottle at the end of his tube. If it is a small one, such as a " medical," he taps his tube on the edge of a tray, and the bottle drops off, ready to be carried to the annealing-oven. If it is large, it is handed to the " wetter-off," who runs a wetted iron round the neck, and, in this manner, severs the bottle from the blowing-tube. The bottle is still without a lip, which it is the " workman's " business to make. The " workman" holds the bottle, either by an iron rod attached to the bottom by a seal of melted glass, or by a support with four prongs which surround and clip the body (Fig. 786). He heats the neck of the bottle, at an opening in the main furnace, or at a separate small furnace, which is especially arranged for his work, coils a small piece of molten glass round the neck, and then fashions it, when in a plastic condition, with a tool, which is best understood by reference to Fig. 787: A represents a rounded projection, which regulates the shape and size of the inside of the neck; C is a bottle in position ; B B can be compressed upon the hot glass by means of H, which acts as a spring, and thus form the rim of the lip. The bottle is turned by means of the rod attached to its base, the tool is compressed, and the rim is finished. Fig. 788 is a modification of Fig. 787, and provides for the formation of an indented ring in the inside of the neck, in which, indiarubber can be fixed as an adjunct to the stopper. One side of this tool is shown open, and the other shut. By compressing the arms of the tool, the spring S forces in one end E of a curved im plement, which turns on the screw K, and drives the other end C into the plastic glass. Before the tool is compressed, the end C is contained in the body of the rounded projection A, which is thus enabled to enter the neck of the bottle. When A is inserted, the tool compressed, and the bottle turned, an external rim and an internal indentation are produced simultaneously. When indiarubber is attached to the stopper itself, as in Lamont's patent, the stopper is dropped into the bottle, a tube is thrust after it, and firmly seizes one end, and the indiarubber ring is forced over the tube and stopper, by a second tube sliding over the first. When the rim or lip is finished, the " workman " either takes the bottle out of the holder with a pair of wooden tongs, or separates it from the iron rod by sharply striking the latter. The " boy," finally, carries the finished bottle on fork to the annealing-kiln, which is kept at a temperature rather below the melting-point of glass, until stacked full, when it is allowed to cool gradually. The process of making bottles is exceedingly rapid. In a day of ten hours, one " set " of workmen will turn out 130 dozen of finished bottles.