Harper's New Monthly Magazine, No. IX.—February, 1851.—Vol. II.. Various
thin for his purpose by a single blowing, he reheats it by holding it within the furnace, and then blows again, repeating the operation till he brings it to the desiderated size and consistency. Thus prepared, he swings it in the air like a pendulum, or twirls it round and round rapidly, according to the elongated or circular form he requires, the molten particles obeying the tendency of the force and motion employed.
Having advanced to this stage, and the mass being ready for fashioning, a new instrument is brought to bear upon it. This is a small, solid, round iron rod, called the pontil, upon one end of which a lesser portion of material has been collected by another workman, and this portion being applied to the extremity of the globe already formed rapidly adheres to it. The whole is now detached from the tube, or blowpipe, by simply damping the point of contact, which causes the glass to crack, so that a stroke upon the tube separates it safely, leaving a small hole in the globe where the tube had originally entered.
By this time the temperature of the mass has cooled down, and it becomes necessary to reheat it, which is done as before. The artificer next seats himself on a stool with elevated arms, upon which he rests the pontil, which he grasps and twirls with his left hand, having thus a command over the red-hot glass with his right hand, in which he holds a small iron instrument called a procello, consisting of two blades with an elastic bow, similar to a sugar-tongs. With this little instrument the whole work of fashioning is performed, and as it must be completed while the glass is yet ductile (having always, however, the power of reheating it when necessary), the process is effected with wondrous celerity. By the aid of the procello he enlarges or contracts the mass, which he adapts to its motions with his left hand, and where any shapeless excrescences appear he instantly cuts them off with a pair of scissors as easily as if they were so much lace or cotton. And thus, almost in less time than it has occupied us in the description, articles of the most exquisite form and delicacy are created by the art-magic of these Vulcans of the glass-furnace.
That which chiefly excites astonishment and admiration in the spectator is the ease and security with which a material so fragile is cut, joined, twirled, pressed out and contracted, by the hands of the workmen. Long practice alone can insure the requisite certainty and quickness of manipulation, and the eye must be highly educated to its work before it can achieve off-hand, and, by a sort of accomplished instinct, the beautiful shapes which are thus rapidly produced.
The moment the article is finished it is detached from the pontil and dropped into a bed of ashes, from whence it is removed while it is yet hot, by a pronged stick or wooden shovel, to the tray to be deposited in the annealing oven where it is gradually cooled.
In making crown-glass, which is used for windows, a slight alteration in the process is observed. When the globe is prepared as before at the end of the tube, it is flattened at its extremity by pressure against a plain surface; the new material at the end of the pontil is then attached to the flattened side, and the whole mass detached from the tube, leaving a circular hole at the point of separation. The mass is now twirled round and round, at first slowly, then more quickly, till its diameter, obeying the centrifugal force, becomes wider and wider, the hole expanding in proportion. At last, as the motion increases in velocity, the double portion suddenly bursts open, the whole forming a plain disc of uniform density throughout, except at the spot in the centre where the pontil is attached to it, and where there is accumulated that small lump which is vulgarly called a bull's eye. The most surprising incident in this process is the bursting open of the flattened globe, a circumstance which would shiver the entire mass if it were not kept up at a certain heat.
The mode of casting plate-glass presents a remarkable illustration of the skillful adaptation of means to ends. When the glass is melted in the crucible, a portion of it is transferred to a smaller crucible, called a cuvette, which contains the exact quantity requisite for the size of the plate about to be formed. The cuvette is then raised by means of a crane, and lifted over the casting table. These tables have smooth metallic surfaces which are carefully ground and polished, and wiped perfectly clean, and heated before they are used. Formerly they were made of copper, but the British Plate Glass Company have found that iron slabs answer the purpose better. The table used by them is fifteen feet long, nine feet wide, and six inches thick, and weighs fourteen tons. For the convenience of moving it to the annealing ovens it is placed upon castors. The cuvette being swung over the casting table, is gradually turned over, and a flood of molten glass is poured out upon the surface, and prevented from running off by ribs of metal. As soon as it is entirely discharged, a large hollow copper cylinder is rolled over the fluid, spreading it into a sheet of equal breadth and thickness. When the glass is sufficiently cool to bear removal it is slipped into the annealing oven, where it is placed in a horizontal position,14 great care having been taken to exclude the external air, it being indispensable to the beauty of these plates that the process of cooling should be regular and gradual.
No less than twenty workmen are engaged in these operations, and during the whole time the apartment is kept perfectly still, lest a motion of any kind should set the air in motion, the slightest disturbance of the surface of the plate being calculated to impair its value. "The spectacle of such a vast body of melted glass," observes Mr. Parks, "poured at once from an immense crucible, on a metallic table of great magnitude, is truly grand; and the variety of colors which the plate exhibits immediately after the roller has passed over it, renders this an operation more splendid and interesting that can possibly be described."15
To attempt the briefest outline of the vast number of objects that are composed of glass, and the variety of processes to which the material is subjected in their production, would carry us far beyond the limits within which we are unavoidably confined. Even the most trifling articles of daily use, apparently very simple in their formation, involve many elaborate details. Take a watch, for example. The history from the furnace to the workshop, of those parts of a watch which are composed of glass, is full of curious particulars. The watch-glass maker exercises a function distinct from any one of those we have hitherto been considering. He receives from the blower an accurate hollow globe of glass, measuring eight inches in diameter, and weighing exactly twelve ounces, which is the guarantee at once of the regularity and thinness of the material. Upon the surface of this globe the watch-glass maker traces with a piece of heated wire, sometimes with a tobacco pipe, as many circles of the size he requires as the globe will yield, and wetting the lines while they are yet warm, they instantly crack, and the circles are at once separated. He finds the edges rough, but that is got rid of by trimming them with a pair of scissors. The circles thus obtained are deficient, however, in the necessary convexity; he accordingly reheats them, and, with an instrument in each hand, beats or moulds them into the precise form desired, much in the same manner as a dairy-maid, with her wooden spoons beats a pat of butter into shape. The edges are now ground off, and the watch-glass is complete. The preparation of the dial, which is composed of opaque white glass, ordinarily known as enamel, is a much more complicated work, involving several minute processes and a larger expenditure of time. Upon both sides of a thin plate of slightly convex copper, bored with holes for the key, and the hour and minute hands, is spread with a spatula a coat of pounded glass which has gone through several stages of solution and purification before it is ready for application. In the management of this operation, and the absorption of any moisture that may linger in the enamel, considerable care and delicacy of hand are necessary. As soon as the dial-plate is perfectly dried it is put into the furnace to be heated gradually. These processes of firing and enameling must be repeated altogether three times before the work is finished; after which the lines and divisions for the hours and minutes are marked upon the surface by a totally different process. We have here merely touched the principal points in the formation of dial-plates; the details are too complex for enumeration.
If we find in such articles as these the employment of numerous chemical agencies, special tools, and peculiar manipulation, we may easily give credit to the greater wonders that remain to be developed in more costly processes; such as the composition of artificial gems, of the pastes that are made to resemble diamonds and pearls, amethysts, emeralds, and precious stones of all colors and degrees of brilliancy, beads, bulbs, striped tubes, and a hundred other fanciful toys and ornaments; the formation of lenses and eye-glasses; the coloring of glass for various purposes; and
In this respect plate-glass is treated differently from crown and broad glass, which is always placed on its edge in the annealing furnace.
Lard. Cyclo.