In the mean time another blower, having formed a smaller ball, opens it by a sharp cut, and presses it while red hot against the end of the stalk held by the former workman, to which it immediately adheres; the workman then with a piece of iron, which he wets with his mouth, touches the globe intended for the bowl of the glass, which is still very hot, although so much chilled as to retain its shape; in a second or two it cracks all round, and by giving it a gentle knock it is detached from the blowing pipe. The workman then instantly heats it, and with a pair of shears cuts the mouth smooth and even; but as the shears have put the glass out of the circular form, he heats it again, and by a dexterous twirl and swing round his head gives it the desired shape almost without the use of any tools. The wine glass now finished, the iron at the foot is detached by a smart blow, and the glass is carried by a boy, upon a long forked iron, to the annealing oven. After the glass is annealed it may be required to be cut before it is ready for sale: this, when the articles are small, and can be easily held in the hand, is performed upon small grit grindstones, or by circular plates of iron, revolving in troughs of sand and water; and for beaded mouldings, as upon decanters, a corresponding groove is formed upon the periphery of the plate.
The operations are concluded by polishing the cut parts by means of revolving straps covered with polishing powder.
In the manufacture of bottle glass, after the metal is brought into fusion, which requires an intense heat during eighteen hours, it is reduced to the working temperature, which is steadily maintained by a due supply of fuel, and being scummed, is ready for blowing. For carboys and similar articles it is merely blown into a globular form; but for common wine bottles, or for square or octagonal bottles or jars, it is blown within moulds. Each piece, as it is blown, is handed over to the finisher, who forms the ring at the mouth of the neck, and delivers it to a boy, to be placed in the annealing oven.
In making crown glass, as soon as the metal is ready for blowing, a workman gathers on the blowing pipe a quantity of metal sufficient for forming a sheet or table of glass, which he blows, and, by frequent rolling on a polished table, brings it into a globular or cylindrical form; he then inflates it gently into an oblong ball called a parisienne, and immediately heats it again at the mouth of the furnace, in order farther to expand it; in this stage a solid iron rod, charged with melted glass, is made to adhere to the centre of the expanded part opposite the extremity of the blow pipe, and this latter is detached from the glass by a cold iron, leaving an orifice, which is gradually enlarged by heating the glass at the small hole of the flashing furnace, and afterwards at the larger hole of the same furnace, the workman all the while wheeling the rod which supports the glass on a hook in a cross wall, built to defend him from the heat. By degrees, in consequence of the heat and the centrifugal motion, the opening of the glass is continually enlarged, until it expands suddenly with great violence into the form of a large circular plate, four or five feet in diameter, and of uniform thickness, except at the centre, where it is attached to the supporting iron; from this iron it is separated by the application of cold, leaving in the centre a thick nodule called the bull's eye; and when the plate is sufficiently firm to prevent warping, it is carried to the annealing oven and placed on its edge in a proper frame, to keep it flat till all the tables are ready for removal into the crates in which they are kept.
Plate glass is formed either by blowing or casting: by the former method is made the glass for carriage windows; and by the latter, the large plates for looking glasses. Of late it is said that the art of blowing plate glass has been so much improved, that plates measuring 60 inches in length rby 21 in breadth have been produced by this method; but by casting, much larger plates are produced, and the establishment at Ravenhead has exhibited at their warehouse near Blackfriars' Bridge plates of the extraordinary dimensions of 12 feet by 6. The most perfect furnace for manufacturing plate glass appears to be that which is employed in France. It consists of a central melting furnace of an oblong form, with a double arched roof; it is capable of containing, upon a raised bench on each side of the fire grate, two large melting pots and a cistern; in each angle of the furnace, and on each side, are two or three openings for introducing the materials, transferring the melted metal from the pots to the cistern, and withdrawing the latter when filled with metal: at each angle of the principal furnace is another of an oblong form communicating with the central one by flues, through which the flame reflecting from the arches of the melting furnace reverberates on their contents.
Each of these smaller furnaces has two openings, a smaller one for producing a current of air, and a larger one for introducing and removing what is placed within them. Three of them are usually employed for burning the melting pots and cisterns, which are made of fire-clay; and the fourth is used for preparing the frit. Around the plate glass house are several annealing furnaces like baker's ovens, with capacious mouths. For receiving the melted metal from the pots, square cisterns of sufficient capacity to contain melted metal enough for one plate are used, and when these are filled they are withdrawn from the furnace by means of large iron tongs, supported upon an axle running upon two wheels; another pair of tongs made to encompass the cistern by entering a groove on its sides, and furnished at each end with a handle for the more convenient management of it, is attached by four chains to a sort of crane for suspending and raising the cistern into a proper position with regard to the table. The casting table consists of an oblong frame of wood, covered on its upper surface with a thick sheet of smooth copper, having at its sides two iron rulers of the thickness of the intended plate, their distance asunder being regulated by the proposed breadth of the plate.
An iron roller of considerable weight, furnished with a handle at each end, resting on the iron rulers at one end of the table, is made to pass over the melted metal when poured upon the table, in order to form it into a plate of uniform thickness. The apparatus just described is used only for easting plate glass, that for blowing being so simple as to need no particular description. In preparing the materials for plate glass they are first reduced to powder, well mixed together and calcined in the fritting furnace before described; they are then removed as quickly as possible into the melting pots previously heated, and after being melted, which generally requires about ten hours, the heat is continued at its utmost degree till the metal becomes perfectly fine; it is then ladled from the pots into the cisterns, which are then drawn out on projecting ledges, and conveyed to the casting table, over which they are suspended by the crane, and by means of the handles of the tongs they are inclined so as to allow their contents to flow over the table: the iron roller is then immediately passed steadily over the surface of the melted metal, sweeping off the superfluous matter into troughs placed at the sides of the table; and when the roller reaches the further extremity of the table, it is expeditiously lowered on a tressel, which prevents its interfering with the plate.
The operation of casting is performed before the mouth of the annealing oven, in which it remains exposed to a moderate heat for fourteen days, the heat being at last suffered to die away as gradually as possible. When quite cool it is withdrawn, carried to the magazine, examined, and cut square by a glazier's diamond, and is then ready for the operations of grinding and polishing. The casting table and tressel, as also the crane, are all mounted upon wheels, for the convenience of removing them from one annealing furnace to another. When plate glass is made by blowing instead of casting, after the materials have been fused as before described, and become fine, the further admission of air is prevented by closing all the openings of the furnace, and every thing is suffered to remain stationary for nine or ten hours. This gradual cooling has been found necessary, to enable the melted glass to adhere sufficiently to the blowing pipe. The mode of blowing plate glass greatly resembles that used in the manufacture of table glass, except in the quantity of metal gathered on the blowing pipe, which is sometimes nearly l00lbs.
When plates of the largest size are to be formed, the metal is gradually blown and worked into the form of a cylinder, which is cut open at one side with a pair of shears, and the soft glass is spread on a heated floor, covered with a thick stratum of sand, and from thence is speedily conveyed to the annealing furnace. In grinding plate glass, two plates are always ground together, one being imbedded in plaster of Paris upon a table, whilst the other, also imbedded in plaster, is placed upon the former, and being loaded with great weights, is moved uniformly but pretty quickly over its surface. Sand moistened plentifully with water is from time to time sprinkled between the plates, and grinds away all the prominences of the glass until both plates become smooth and even. As the grinding proceeds, sand of greater fineness is employed; and towards the conclusion, it is exchanged for emery, also of varying fineness. As by grinding the surface is roughened and rendered incapable of transmitting the rays of light, it becomes necessary to restore its lustre by polishing, which is performed by rubbing the surface with a block of wood, covered on the lower side with a woollen cloth.
The workman keeps it supplied with fine polishing powders, as tripoli and putty, changing from coarse to fine, as the polishing advances to a conclusion. To regulate the pressure a springing pole is put on the back of the block, which, being bent to a curve, is supported from the ceiling of the workshop.