Dismissing history and theory, we will now step into the glass-house itself, where the practical work of converting sand into goblets, vases, mirrors, and window-panes is going forward with a celerity and accuracy of hand and head that can not fail to excite wonder and admiration. As the whole agency employed is that of heat, the interior of the manufactory consists of furnaces specially constructed for the progressive processes to which the material is subjected before it is sent out perfected for use. Look round this extensive area, where you see numbers of men in their shirt-sleeves, with aprons before them, and various implements in their hands, which they exercise with extraordinary rapidity, and you will soon understand how the glittering wonders of glass are produced. Of these furnaces there are three kinds, the first called the calcar, the second the working furnace, and the third the annealing oven, or lier.
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 essential to prevent the alkali from fusing and volatilizing, and to insure 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 frit, or calcined material, 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. Round the exterior of the furnace, you perceive a series of holes or mouths; these are called boccas, from the Italian, and it is through them the frit is served into the crucibles and taken out when melted. The volume of heat is here so intense, that the boccas 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 bocca 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 third furnace, or annealing oven, called the lier. 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 sarosel, or 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 will incur the most serious losses and delays from casualties which, even after the most careful and costly outlay, can not be always averted. The crucibles especially demand attention in this respect, in consequence of the solvent property of some of the materials which are melted in them. These crucibles are deep pots, varying in size according to the extent or objects of the manufacture; and some notion may be formed of the importance attached to them from the fact, that they are not unfrequently made large enough to contain individually not 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.[13]
We have now before us the three principal processes—the calcination, by which the materials are prepared in the first instance—the melting down of these materials into glass in the great working furnace, and the annealing of the finished article after it has been fashioned by the workmen. These processes are broad and simple; but that part of the manufacture which is, probably, most calculated to surprise the uninitiated, is the manner in which the red-hot mass of glass, as it is taken out of the crucible, is instantly, so to speak, shaped into form by the dextrous hands and practiced eyes of those men whom you see standing about at tables and stools, twisting long iron rods called pontils, blowing through pipes, and performing mysterious evolutions with scissors, pronged sticks, compasses, and other instruments, with a rapidity that baffles the most vigilant observer. From the infinite diversity of objects into which glass is thus moulded, it must be obvious that the operations of these artificers embrace a variety of curious details which it is impossible to enter upon here; but a glance at some of them will enable the reader to form a general notion of the curious manipulations upon which they are so actively employed.
The initial movement of the glass-blower is to dip a hollow iron rod or tube, about five feet long, through the bocca, into one of the crucibles containing the melted glass. 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 perpendicularly for a few seconds with the heated mass downward, till the fluid drops and lengthens by its own momentum beyond 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 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.