Scientific American 26, 15.3.1851
As the formation of glass is produced by the simple operation of fusing certain minerals together, it follows that the furnaces for fusing, &c., must form prominent features in the manufacture. There are two kinds of furnaces, namely, one called the "Calcar," the other "Working Furnace." There is connectted with the furnaces an annealing or tempering oven for the last operation.
The calcar, built in the form of an oven, is used for the calcination of the materials preliminary to their fusion and vitrification. This process is of the utmost importance:—it expels all moisture and carbonic acid gas, the presence of which would hazard the destruction of the glass pots in the subsequent stages of the manufacture, while it effects a chemical union between the salt, sand, and metallic oxides, which is to prevent the alkali from fusing and volatilizing, and to ensure the vitrification of the sand in the heat of the working furnace to which the whole of the materials are to be afterwards submitted.
The working furnace, which is round and generally built in the proportion of three yards in diameter to two in height, is divided into three parts, each of which is vaulted. The lower part, made in the form of a crown, contains the fire which is never put out. Ranged round the circumference inside are the glass pots or crucibles, in which the calcined materiel is placed to be melted; and from several holes in the arch of the crown below issues a constant flame which, enveloping the crucibles, accomplishes the process of melting. There are a number of mouths round the outside, through which the calcined materials are served into the crucibles inside. The heat is here so intense that the mouths are provided with movable collars or covers, generally composed of lute and brick, to screen the eyes of the workmen who stand outside in recesses formed for the purpose in the projections of the masonry. The severest part of the work arises when any of the pots or crucibles happen to become cracked or worn out, in which case the mouth must be entirely uncovered, the defective pot taken out with iron hooks and forks, and a new one substituted in its place, through the flames, by the hands of the workman. In order to enable him thus literally to work in the fire, he is protected by a garment made of skins in the shape of a pantaloon, and heavily saturated with water. This strange garment completely covers him from head to foot, all except his eyes, which are defended by glasses.
The material being now melted, is fashioned into the desired forms by the hands of the workmen while it is yet hot, and then placed to cool gradually in the annealing oven. This oven is a long low chamber heated at one end, and furnished with movable iron trays or pans, called fraiches (from the French), upon which the various articles are set down, and finally removed, when they are sufficiently cold, through an opening which communicates with the room where the finished articles are kept.
The intensity of the fire requires that the furnaces and crucibles should be constructed of materials the least fusible in their nature, and the best calculated to resist the violent and incessant action of heat; or the manufacturer would incur the most serious losses and delays from casualties which, even after the most careful and costly outlay, cannot be always averted. The crucibles especially demand attention in this respect, in consequence of the solvent property of some of the materials which are incited in them. These crucibles are deep pots varying in size according to the extent of the objects of manufacture; and some notion may be formed of the importance attached them from the fact that they are not unfrequently made large enough to contain individually no less than a ton weight of glass.
Great skill and care are requisite in their structure so as to adapt them to the temperature in which their qualities are to be tested; and even with the utmost attention that can be bestowed upon them they are often found to break soon after they are exposed to the furnace, by which heavy losses are entailed upon the manufacturer. Nor is this the only point which must be considered. The size of the crucible should bear a proportionate relation to that of the furnace, or one of two consequences, equally to be avoided, will ensue; either that there will be a waste of fuel, if the crucibles are too small, or an inadequate heat, if they are too large.
The initial movement of the glass-blower is to dip a hollow iron rod or tube, about 5 feet long, through the mouth into one of the crucibles containing the meltedglass. Having collected at the end of the tube a sufficient quantity of material for the article he is about to fashion—a drinking glass, finger-glass, jug, or whatever it may be (which requires, perhaps, two or three dips, according to the quantity he wants), he withdraws the tube and holds it i perpendicularly for a few seconds with the heated mass downwards, till the fluid drops and lengthens by its own momentum beyond I the end of the tube. He then quickly raises it, and rolls it on a smooth horizontal plate till it acquires a cylindrical form. When he has got it into this shape, he applies his mouth to the opposite end of the tube, and blows into the heated mass which swiftly becomes distended into a, sphere. But as the globe thus obtained is not rendered sufficiently 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. This is a small solid round iron rod, called the pontil, upon one end of which a lesser portion of material is collected by another work wan, 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 blow pipe, 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 seats himself on a stool with elevated arms, upon which lie 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 re-heating it when necessary,) the process in 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 workman. Long practice alone can ensure the requisite certainty and quickness of manipulation, and the eye must be highly educated to its work before it can achive off-hand, and by a sort of acomplished 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 he deposited in the annealing oven where it is gradually cooled, after which it comes out tough and strong. Without the annealing process glass would be very brittle.
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