Harper's New Monthly Magazine, No. IX.—February, 1851.—Vol. II.. Various
course of heroic enthusiasts.
Many attempts have been made to produce a material resembling glass that should possess the quality of malleability, and respectable evidence is not wanting of authorities who believed in its possibility, and who are said to have gone very near to its accomplishment. An Arabian writer12 tells us that malleable glass was known to the Egyptians; but we must come closer to our own times for more explicit and satisfactory testimony. Descartes thought it was possible to impart malleability to glass, and Boyle is reported to have held the same opinion. But these are only speculative notions, of no further value than to justify the prosecution of experiments. Borrichius, a Danish physician of the seventeenth century, details an experiment by which he obtained a malleable salt, which led him to conclude that as glass is for the most part only a mixture of salt and sand, he saw no reason why it should not be rendered pliant. The defect of his logic is obvious; but, setting that aside, the fallacy is practically demonstrated by his inability to get beyond the salt. Borrichius also thought that the Roman who made the vase for Tiberius, may have successfully used antimony as his principal ingredient. Such suppositions, however, are idle in an experimental science which furnishes you at once with the means of putting their truth or falsehood to the test. There is a substance known to modern chemistry, luna cornea, a solution of silver, which resembles horn or glass, is transparent, easily put into fusion, and is capable of bearing the hammer. Kunkel thought it was possible to produce a composition with a glassy exterior that should possess the ductile quality; but neither of these help us toward an answer to Aristotle's question. Upon a review of the whole problem, and of every thing that has been said and done in the way of experiment and conjecture, we are afraid we must leave it where we found it. The malleability of glass is still a secret.
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
Ibn Abd Alhakim.
For details see Loysel "Sur l'Art de la Verrerie;" and Lard. Cyclo.