Glass-making E-Book

Charles H. Henderson

• Herausgeber: EHS
• Kategorie: Religion und Spiritualität
• Sprache: Französisch

Seitenzahl: 176

Veröffentlichungsjahr: 2022

Beschreibung

"O Carlyle!" exclaimed Emerson, in his diary, at the time 'Sartor Resartus' was being republished in America, 'the merit of glass is not to be seen, but to be seen through; but every crystal and lamina of the Carlyle glass shows.' With admirable precision this defines the proper function of a pane of glass. Decorative art, in casting about for new fields of conquest, has too frequently induced a contrary feeling; but, after all, a window-pane at its best is something to be seen through and not to be seen. It is our means of looking out upon the world and letting the sun look in upon us. The more perfectly, then, it fulfills its function, the less evidence will it bear of its evolution from such dull things as sand and lime and soda-cake. Our window-pane is transparent in all things save its own history.

Leseprobe

Glass-Making

Glass-Making

Chapter I

I. A PANE OF GLASS.

"O Carlyle!" exclaimed Emerson, in his diary, at the time "Sartor Resartus" was being republished in America, "the merit of glass is not to be seen, but to be seen through; but every crystal and lamina of the Carlyle glass shows."

With admirable precision this defines the proper function of a pane of glass. Decorative art, in casting about for new fields of conquest, has too frequently induced a contrary feeling; but, after all, a window-pane at its best is something to be seen through and not to be seen. It is our means of looking out upon the world and letting the sun look in upon us. The more perfectly, then, it fulfills its function, the less evidence will it bear of its evolution from such dull things as sand and lime and soda-cake. Our window-pane is transparent in all things save its own history. It gives no hint of what it is made of, or how it is made. It is, indeed, easier to look through it than it is to look into it. If one look in the right direction, however—and in America this means toward Pittsburgh—he will see, in the cluster of glass-factories which have gravitated toward the natural gas of that neighborhood, a side of industrial activity possessing much interest. The brilliant pane of glass itself tells no stories, but the white-hot furnaces and pots of molten metal, the active, hurried figures, and the movements of rare dexterity that one sees at these places, are far more communicative.

They can well afford to publish their achievements, for about few of its material products can the nineteenth century boast with so much show of justice as about its window-glass. It is true that past ages have produced quite as remarkable technical results in other departments of industry, but in this one product, at least, the present decade appears to be unique. Not even China and Egypt, which have a standing claim of priority on all the arts and sciences, dispute with the modern glass-maker. His triumphs are without rival.

Contrary legends are afloat, but they can be chased into no fact: There is, for instance, a story current about the Queen of Sheba and the wise King Solomon that quite puts into the shade even the deceitfulness of riches. It is related by some gossipy chronicler that, at the time of the famous visit, the royal audience was so arranged that the queen and her suite in approaching were obliged to pass over a floor of glass under which were flowing water and fishes swimming. For the legend has it that the wisest of men was decidedly curious. The Paul Prys of Jerusalem declared that the queenly visitor labored under the disadvantage of a deformed foot. The ingenuity of the monarch, it is said, suggested the device of the simulated stream, thinking that the lady's anxiety for her draperies would disclose to the court of Israel whether rumor had rightly reported her. But this performance is probably attributable to the imagination of one somewhat later than Solomon. It hardly sounds like the author of the "Proverbs," nor have we any record that the lady ever repaid him for his discourtesy—a bit of negative evidence that is almost conclusive. But at that time so inhospitable a scheme, even if seriously entertained, could scarcely have been carried out. The hundred and twenty talents of gold and the very great store of spices and precious stones, which to the queen's presence in Jerusalem added their tribute of admiration for wisdom, could not have purchased in any of the marts of the ancient world a plate of glass sufficiently large and sufficiently clear to have made such a deception possible.

SUCCESSIVE STAGESINTHE MANUFACTUREOF SHEET GLASS.

The glass-blowers of the olden times undoubtedly produced some fine results in color, which can scarcely be equaled in the present. They had already attained, in the fabrication of rare and curious forms, a considerable fame before the days of the Roman supremacy; but the magnificent sheet of glass through which we of a morning study the signs of the weather, or glance at the too tempting displays in the shop-windows, is a luxury which we must admit to be peculiar to our own times. It might gain for us the title of the "age of glass," had not the age already been devoted successively to the genius of iron, of steel, and of electricity.

There is also observable a marked difference in the spirit in which the earlier and the later artisans worked. While glass was still a product of some rarity, its manufacture naturally occupied a place among the fine arts rather than among the more common industries. The early glass-makers, in consequence, busied themselves more with the production of the costly and the beautiful than in any attempt to bring glass in its manifold applications within the reach of the poor. Later workers, on the other hand, have shown the influence of democratic institutions. They have found their greatest pleasure, as well as their greatest profit, in the production of wares of such utility and cheapness that their market includes even the very poorest. As a result of this enlarged production, the history of glass shows a marked increase in quality and a marked decrease in price. The interest aroused by this progress is not only technical and commercial, but, in the case of window-glass, in a still wider sense social and economic. In filling the windows of our houses with transparency, the glassmaker has been a public benefactor. His benefaction is the greater, since the material he supplies is now at the disposal of even those of limited means. We hardly appreciate the full significance of cheap window-glass. It lengthens the day to the dimensions assigned by Nature, and permits one to enjoy the sunshine of out-door life without exposure to the inclemencies of the weather. These are substantial contributions to the public health and well-being. At no previous time, we believe, could the dwellers in northern climes introduce into their homes so many square feet of sunlight for so little money.

It is the purpose of the present article to offer a brief glimpse of some of the processes involved in the metamorphosis of the crude materials into a serviceable pane of glass. As the operations are actually carried out in the arts, the attention of the onlooker is constantly distracted by the flame and glare of the furnaces, the passing and repassing of red-hot glass, the clouds of steam, and puffs of dust and smoke. He comes away from the factory with an impression limited to the more spectacular features of the process. Of necessity he is quite oblivious of a hundred details of which it is very necessary that the glass-maker should be distinctly sensible. In making this visit by deputy, it is proposed that little more shall be seen than falls to the lot of the flesh-and-blood visitor. It is, perhaps, wiser that the uncertain light and the steam and smoke shall be permitted to cover with their convenient mantle those technical details that would fail to attract general interest. In such matters it is uncomfortable to have your guide too knowing if he insists on sharing all his knowledge with you.

To define glass physically would be a superfluous task. Everyone is informed of its hardness and solidity. A series of annoying accidents has demonstrated beyond doubt its exceeding brittleness. The ragged-edged splinters that result from such occasions suggest that the solid is amorphous, or without regular crystalline form.

To define the material chemically may be less unnecessary. It is a mixture of different silicates—that is to say, of mixtures of silicic acid with the bases soda, potash, lime, magnesia, alumina, iron, and lead. Considering that we are to be non-technical, this is rather a formidable list, but it must not be thought that any one glass contains all of these ingredients. Every true glass consists of at least two metallic bases united with the silicic acid, and generally, by virtue of the impurities of the crude material, traces of several more. So we have grown into the habit of designating the different kinds of glass by the names of the two predominant bases. Window glass, for instance, is known as a lime-soda glass; table crystal as a lead-potassium glass, and so on through the list. This system of nomenclature is open to the objection that the name of the product and its composition do not correspond in all the glass-producing countries, but these technical discrepancies seem unavoidable. The physical properties of the glass follow very closely its chemical constitution. Many of the silicates employed in glass-making are entirely infusible alone, but, when given suitable associates, are quite manageable. The weight of the glass is also directly dependent upon the metallic bases with which the silica is combined. Crystal is made heavy by the lead present, while window glass, having only light bases in its make-up, has a correspondingly small weight. It is little more than two and a half times as heavy as water. Each chemical change has its physical counterpart.

In spite, however, of the relative cheapness of glassware, we have still a pane of glass for the rich and another for the poor. Both products, the plate and the sheet glass, have essentially the same composition, but they differ very much in the purity of the crude materials used, and in the method of fabrication. Of recent years the improvements in the manufacture of sheet glass have been so marked that it is now frequently introduced into buildings of the better class in place of the more expensive plate. On the other hand, the processes of manufacture by which the latter glass is produced have been so far perfected that its use is now permitted to many who would hitherto have thought themselves unable to afford it.

THE INTERIOROFA GLASS-SHEETFACTORY.

A third form of window glass, the so-called crown glass, must also be mentioned for the sake of completeness, though it has little commercial importance, and less in America than in England. Both sheet and crown glass owe their origin to the blower's breath. Though they are less brilliant than the plate, their methods of fabrication are much more interesting, since they involve a far greater amount of manual dexterity on the part of the artisans. It is, indeed, difficult to know which to admire the more, the chemistry or the physics of the operation; the nicety with which the glass-maker regulates the proportions of his charge so as to produce this beautifully clear substance, or the skill with which he subsequently handles the finished glass and adapts it to our uses.

Sheet glass forms the window-pane of the multitude. The possibility of making it of excellent quality and in large sizes is due almost entirely to the substitution of gaseous for solid fuel. No other among our numerous American industries has been so benefited by the utilization of natural gas. European sheet glass was up to this time unquestionably superior to our own. A larger experience and more approved furnaces made it possible for the foreign manufacturers, and particularly those of France and Belgium, to solve with greater success the knotty problems connected with glass-making. In many places they had already substituted gas for coal, and obtained the happiest results. With the advent of natural gas the position of the American producer was suddenly changed. He had at his command the most desirable of fuels, and one that was at the same time very cheap and almost totally free from sulphur. As a result, he soon equaled and now surpasses his transatlantic rivals.

But the manufacture of window glass is essentially difficult. Even when the troublesome question of fuel has been satisfactorily settled, there remain many other substantial difficulties which must be met and conquered. From the mixing of the crude materials to the annealing of the finished product, the glass-maker must be alert and intelligent. It is a very easy matter simply to make glass. Sand, metallic bases, and heat are the only elements needed. But to make good glass—glass that is clear, transparent, colorless; that simulates the purest water of a mountain-stream—this requires skill and patience. From beginning to end the process is one of painstaking and delicate manipulation.

In the genesis of a pane of glass, the first step is naturally the provision of such stuff as it is made of. While glass is theoretically a definite chemical compound, the proportions in which the ingredients of the "batch" are mixed vary in every establishment. Sand is the basis of the operation. It is the commercial representative of silicic acid. With this are mixed lime and alkali (either carbonate or sulphate of soda, or both) in sufficient quantity to furnish an easily fusible mass that, on solidifying, shall be both clear and transparent. There is very wide range in the choice of materials. The purest grades have the disadvantage of costliness, while the inferior glass has the equal disadvantage of commanding but an indifferent price. Between these two considerations swings the balance of expediency.

When the batch has been made up, it is melted in large clay crucibles, or glass pots, as they are commonly called. The manufacture of the pots is the most tedious and exacting process connected with glass-making. It is one of the few industrial operations in which machinery has not been able to supersede man. A mixture of raw and burned fire-clay is employed. It is necessary that this should be prepared with the greatest care.

THE OPERATIONOF "MARVERING."

Once a day for at least four weeks the mass must be turned and worked, m order to get it free from air and give it the proper toughness. For this kneading process no tool has been found equal to the bare foot. There are a warmth and an elasticity about it that better than anything else develop the required plasticity in the clay. Bare-footed men, pacing up and down in lead-lined troughs, present a very primitive industrial picture. The impression is not removed when one goes up-stairs and watches the transformation of this much-worked material into crucibles. The hand here occupies the place that the foot does below-stairs. By equally slow stages the crucible is built up. First the bottom is formed, a circular slab about four inches thick and some forty inches in diameter; then the sides are gradually raised, a little addition being made each day, until at the end of about six weeks the work is completed, and a heavy, tub-shaped crucible is the result. Meanwhile the incompleted walls are kept constantly covered with damp cloths to prevent premature hardening. The temperature and humidity of the work-room are also objects of unremitting attention. But, though the crucible has now taken form, and its material been under treatment for more than ten weeks, it is not yet ready for the trial by fire. Several months must pass before it is considered sufficiently dried to withstand even a preliminary heating. When the time comes, this is done very cautiously in a little furnace specially constructed for the purpose. Here the temperature is gradually raised to that of the melting furnace. The transfer from the one to the other is accomplished as rapidly as possible. The interior of the crucible is then glazed with a little molten glass, and the vessel is ready to lend itself to the transformation of the opaque into the transparent. After a variable term of servitude, whose length is totally unpredictable, the crucible finally succumbs to the combined attacks of heat and chemical action, and must be replaced by a fresh one.

When gas is used as the fuel, the melting furnace is a very simple affair. It consists of a plain rectangular floor or hearth, which supports from eight to ten crucibles, two abreast. On each side of the furnace there is a series of round openings giving access to each pot. Arches at the end permit the admission of the fresh crucibles and the removal of the exhausted ones. The chimney is placed in the center, the gas being admitted at each end. The air necessary for combustion is first heated by passing through chambers in the base of the furnace. It will not be necessary to go into any further details of construction, for if one will simply imagine a white-hot apartment, perhaps forty feet long, eight feet wide, and six feet high, with ten crucibles of molten glass standing two abreast on the floor, and half as many openings on each side, he will have a sufficiently vivid picture of the melting furnace of a glass-factory. The batch is introduced into the crucibles in small quantities at a time, and then patiently coaxed into a proper degree of fluidity. When the last portion is added, a decolorizing agent goes with it, for, however pure the crude materials may be, there is always sufficient iron present to give the glass a greenish cast. Arsenic is a favorite bleaching agent. It acts by converting the iron into a higher oxide. In some establishments the peroxide of manganese is used for this purpose, but the least excess gives the glass a pinkish color, and it is also thought to make its transparency less durable.

Style dominates even so apparently an unmodifiable thing as window glass. Some years ago a slight excess of manganese was employed intentionally. It was thought that the mistress of the house—or her daughters—looked the prettier when seen through rose-colored window-panes. This decidedly pink glass may still be seen in not a few of the older houses in our Eastern cities. Its use is occasionally revived by some emergency.

This completes the chemistry of the process; the remaining operations are purely physical.

After the contents of the crucibles have become thoroughly fused, the temperature of the melting furnace is gradually reduced, so that the molten glass shall become less liquid, and thus ready for the process of gathering. The impurities floating on the surface of the "metal" are first removed by skimming, much as the housewife does with her preserves. When the crucible was originally put into the furnace a fire-clay ring was placed in the bottom of it, and now floats on the bath. By removing all the scum from the interior of this ring the gatherer always has a clear surface from which to draw. The blow-pipe which he uses is simply a wrought-iron pipe about five feet long. It is provided at one end with a mouth-piece and wooden handle; the other end is thickened and somewhat flared, after the manner of a trumpet. This is dipped into the molten metal, and when withdrawn brings with it a small lump of glass. By a dexterous turn of the pipe this plastic mass is formed into the shape of a symmetrical oval. The dipping process is several times repeated until a considerable mass of glass adheres to the end of the pipe. When window glass of double thickness is to be made, at least four or five gatherings are necessary. It is at the final dip that the gatherer's greatest skill is called into requisition. It is a pleasure to watch him as he seemingly toys with his blow-pipe. But each little movement is done with a purpose. The mass of glass on the end of his pipe is the result of successive gatherings, and must now be brought into a state of perfect homogeneity. To accomplish this, the last glass added is made to completely overlap the whole mass. The ball is then brought almost to a liquid condition, and seems ready to fall from the pipe. In less skillful hands, it would certainly come to grief. By a quick turn of the implement, however, the gatherer throws the fold of glass into a spiral form, and so works it to the end of the mass. This leaves a perfectly clear and semi-plastic ball. The pipe is now withdrawn from the furnace and taken to an open wooden mold, or trough, where the glass is formed into a pear-shaped mass. The mold is kept constantly wet, to prevent its burning. The water, in contact with the red hot glass, assumes the spheroidal condition, and looks like so many globules of mercury. The gatherer's duty is now at an end, and he returns to the melting furnace to repeat the operations of gathering until the crucibles are emptied of their contents. The blow-pipe and its red-hot burden, meanwhile, have been taken in charge by the blower.

CROWN GLASSIN DECORATIVE WORK.