Popular Science Monthly/Volume 34/March 1889/Glass-Making I
By C. HANFORD HENDERSON,
PROFESSOR OF PHYSICS AND CHEMISTRY IN THE PHILADELPHIA MANUAL TRAINING SCHOOL.
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
Successive Stages in the Manufacture of Sheet Glass.
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.
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 6oo THE POPULAR SCIENCE MONTHLY.
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 them- selves 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 en- larged 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 glass- maker 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 sig- nificance 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 opera- tions are actually carried out in the arts, the attention of the on- looker 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 feat- ures 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
�� � GLASS-MAKING. 601
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 sui^erfluous task. Every one 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 corre- spond 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 chemi- cal 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 build- ings of the better class in place of the more expensive plate. On
The Interior of a Glass-sheet factory.
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.
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 Eng- land. 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.
|The Operation of "Marvering."|
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.
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 conies, 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 lit- tle molten glass, and the vessel is ready to lend itself to the trans- formation of the oj^aque into the transparent. After a variable term of servitude, whose length is totally unpredictable, the cruci- ble 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 ac- cess to each pot. Arches at the end permit the admission of the fresh crucibles and the removal of the exhausted ones. The chim- ney 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 stand- ing two abreast on the floor, and half as many openings on each side, he will have a sufficiently vivid picture of the melting fur- nace of a glass-factory. The batch is introduced into the cruci- bles 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 ma- terials 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 estab- lishments 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 re- duced, 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
Crown Glass in Decorative Work.
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 GLASS-MAKING. 607
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 opera- tions of gathering until the crucibles are emptied of their con- tents. The blow-pipe and its red-hot burden, meanwhile, have been taken in charge by the blower.
���AtTACIUNU the '■ PONTT.'
��On the continent of Europe the same furnace is generally used for both melting and blowing, but in England and America it has been found more advantageous to employ separate furnaces. They are very similar in construction. The blowing furnaces have, however, somewhat larger side-openings, and the gas, in- stead of being introduced at the ends, is burned directly under the openings, or blow-holes. The furnace simply provides an in- tensely hot chamber for controlling the temperature of the glass under manipulation. On each side of the furnace, and directly in front of the blow-holes, there is a wide platform built over a cel- lar, or pit, perhaps ten feet deep. Long openings in this platform run at right angles to the furnace, and permit the blower, when occasion demands, to swing his pipe and its burden in the pit beneath.
The sheet-glass factories of Pittsburgh are equipped as thor- oughly as any in the world. The division of labor is everywhere carried to the extreme. Each man knows how to do a particular thing, and does it. The blower, for instance, ' " into whose hands the red-hot ball of
T T . , T . , . Forming the "Nose."
glass has just been consigned, is sup- posed to know little or nothing about the other operations involved in glass-making. He begins at a certain point, and leaves off at a certain point. The skill with which he effects his part in the many transformations required in the genesis of a pane of glass is, how- ever, the most attractive in a process nowhere devoid of interest.
His first act is to grasp the pipe, and, with the ball of glass still resting in the wooden mold, blow through the mouth-piece
�� � until a large bubble of air is formed in the midst of the mass; then, with alternate blowing and manipulating, he so increases the size of the bubble that the ball of glass assumes a shape not unlike that of the large carboys used in the transportation of
Casting Plate Glass.
acids. He now transfers himself and his work to the platform in front of the furnace. Sometimes blowing, sometimes swinging his pipe in the pit beneath him, and sometimes reheating the glass in the blowing furnace in front of him, the blower gradually ex-tends the dimensions of the bubble of air until the ungainly carboy, with its disproportionately thick bottom, has been replaced by a beautifully symmetrical figure, the shape of an enormous test-tube. But it occasionally happens that the glass flows a little too freely, and there is danger of the sides of the tube becoming too thin. To avoid this result, the blower throws his tube into the air whenever he finds that the glass is too liquid, and thus permits it to settle back upon itself. As the tube by this time is about five feet long, in addition to the length of the pipe, itself, one can readily fancy that this apparently playful toss requires both skill and muscle. It is a fine sight to watch the graceful ease with which these brawny fellows accomplish it.
When the tube has been formed to the satisfaction of the blower — and it requires a surprisingly short time for the whole operation — he allows it to become comparatively cool. He then thrusts the rounded end into the furnace, blows into the mouth- piece of his pipe, and quickly covers the opening with his hand. Presently, a slight report is heard, like a mild explosion. The confined air, expanding with the increasing temperature, has blown a hole in the end of the softened tube. Resting his pipe on a convenient support, and still keeping the glass in the furnace, the blower gradually rotates the tube. Under the influence of centrifugal force, the hole grows larger and larger, until the tube becomes an open cylinder. It is then quickly withdrawn from the furnace and permitted to depend into the pit beneath the platform. When the plastic edge passes to a cherry heat, the cyl- inder may be taken away without danger of getting out of shape.
The blower's part is now finished. After a moment's rest, he has another pipe in his hand, and is repeating his heavy labor. His wages are considerably larger than the salary of many a learned professor or divine, but the service for which they are paid is of such a kind that it makes a man grow old very early. The severe muscular exertion, and the high temperature at which it is performed, are not conducive to either health or longevity. There may be exceptions, but as a rule there are few constitutions that can withstand for any length of time the quickly alternating heat and chills to which the glass-blower is daily subject.
The cylinder just laid aside is now cold. As soon as the neck and its attached blow-pipe are separated from it, a red-hot iron is passed along the interior surface from end to end. A piece of cold iron applied to any part of the heated line makes a complete longitudinal crack. We have now before us a perfect cylinder, open at both ends, and cracked from end to end. It has only to be ironed out into a flat sheet to be ready for service as a pane of glass.
The ironing process is carried out in a separate building, in what is known as the laying-in furnace. The ease with which it 6io 'THE POPULAR SCIENCE MONTHLY.
is accomplished is one of the triumphs of modern glass-making. In this department particularly, the merits of natural gas are each day more apparent. The hearth of the laying-in furnace is circu- lar, and is divided into a number of sectors by fire-clay bridges. It is made movable about a vertical axis. As it rotates, the dif- ferent sectors of the hearth pass through as many separate com- partments of the furnace, the temperature of which may be varied at pleasure. The first compartment, the laying-in oven, is only moderately warm, and permits the glass cylinder to become grad- ually heated. It is then carried by a partial rotation of the hearth into the next compartment, the laying-out oven, where the heat is sufficient to make the glass plastic. A large, flat stone occupies each hearth-section and forms the ironing-board upon which the cylinders of glass are to be smoothed. In the laying- out oven, the crack in the cylinder is brought uppermost. Under the influence of the heat the glass gradually unfolds, until it lies open on the stone like a sheet of rumpled paper. In the succeed- ing compartment — the flattening oven — the plastic sheet is made perfectly smooth and flat by means of a moist block of wood on the end of a long iron rod. The cylinder has now disappeared, and in its place there is a pane of red-hot glass. One more turn of the hearth carries the glass into the compartment known as the dumb oven, where it gradually cools. It is then brought, by a final movement of the hearth, to the entrance of the annealing leer. One whole rotation has now been accomplished, and the cir- cuit is complete. Meanwhile other cylinders have been put into the furnace and are in different stages of the flattening process.
The operation of the furnace is continuous, and speedily trans- forms the cylinder into a smooth sheet. But still it is not ready for use. Were the glass removed to the air immediately, it would be much too brittle for service. It must first go through the pro- cess of annealing, or gradual cooling, before it can possess any durability. In the improved " rod leer " the hot glass is received at one end of a long brick chamber, and in thirty or forty min- utes it is automatically discharged at the other end, nearly or quite cold. Where gas is used, the glass, just as it comes from the leer, is beautifully clear and brilliant. It could scarcely be more so had it been washed with hot water and dried with linen — the process, we believe, by which madam, our hostess, secures such a glittering display on her table. The sheets are at once cut into proper sizes and stored away in suitable wooden frames. The process of manufacture is completed, and only the service of the glazier is needed to put the pane in place, and so inaugurate its luminous mission.
Strange fancies attend the visitor as he wanders through the silent warehouse. He loses himself amid the possible pictures
�� � which, these window-panes are destined to confront. Perhaps the old fable of the wind and the sun comes back to him. The cloak that the one in all its fury could not tear off is gladly thrown aside when the other exerts its power. The same with our pane of glass. It is to repel the rougher storms and winds, yielding passage only to the gentler elements. Sunshine and moonlight are to filter through it, and back of it pleasant fireside pictures are to group themselves. But the imagination is not always so obliging; darker possibilities obtrude themselves as well. The shuddering forms of want and wretchedness are also there, and the solitary dreamer is glad to turn away from them all, and pass out of the open door back again into the world of reality.
Rotary Grinding Machine for Plate Glass.
The manufacture of crown glass possesses considerable historical interest, but little beyond that. Within the past few years it has been brought into some prominence again from its growing use in decorative windows. It possesses, it is true, a brilliancy much superior to that of sheet glass, but the small size and unequal thickness of the pane obtainable do not permit it to compete successfully with the generous dimensions and remarkable uniformity of the glass now dominant. In mode of fabrication the crown glass proceeds precisely as the sheet up to the time of blowing; at this point the two processes diverge. The ball of semi-plastic glass on the end of the blow-pipe is fashioned into the shape of a cone, as the result of successive rollings on a table of metal or stone, known as the "marver." The process itself goes under the name of "marvering." The apex of the cone forms the so-called "bullion-point." By blowing into the mouth-piece of his pipe, the blower expands the glass into a small globe. This is subsequently enlarged, care being taken that the bullion-point is always kept in line with the pipe. The globe of glass is then 6i2 THE POPULAR SCIENCE MONTHLY.
flattened into something the shape of an enormous decanter. The bottom is very flat, and has the bullion-point exactly in the cen- ter. The pipe and its ungainly burden are now permitted to rest horizontally on two iron supports. In the mean time a second workman has gathered a small lump of glass on the end of his iron rod, or " ponty," and by pressing it against an iron point forms it into the shape of a tiny cup. This is fitted over the bull- ion-point of the glass, and, as they are both hot, soon becomes firmly attached to it. The glass has now two handles, but one of these, the blow-pipe, is speedily separated from it by means of a sharp blow. The open neck which is thus exposed is known, in the glass-maker's parlance, as the " nose." It gives its name to the furnace where it is subsequently reheated. During this oper- ation the ponty is constantly and rapidly revolved. The nose gradually expands under the combined action of heat and centrif- ugal force. The oj)ening grows larger and larger until the glass assumes the shape of a typical crown. This appearance, however, remains but an instant. One sees in its place a brilliant circular plate of glass whose shape is only maintained by continuing the rotation of the ponty until the plate, or table, as it is now called, can be placed upon a flat support. The ponty is then detached from the bullion-point by means of shears. The mark that is left is known as the bull's-eye. The tables vary in size from a few inches in diameter up to six feet, but this latter dimension is ex- treme. After annealing they are cut into panes by means of a diamond. The loss involved in the operation more than counter- balances the admirable brilliancy of the material. At the present time, the circular tables, just as they come from the annealing oven, are being used in decorative windows with the most excel- lent effect. Frequently the glass is tinted, or else it is left color- less itself, and the bull's-eye is either shaded or opalescent.
The window-pane of the rich is commonly plate glass. Of the three varieties, this is by far the most desirable in every- thing except, it must be added, the price. Though similar in com- position to the sheet and the crown glass, its fabrication is carried out upon a totally different principle. Instead of being, like them, the result of the blower's breath, the plate glass is cast into a flat sheet and then ground and polished, a process of manufacture which at once accounts for its expensiveness. The best practice to be seen anywhere in America, if not in the world, is at Creigh- ton, some twenty miles north of Pittsburgh. It is near the well- known natural-gas district of Tarentum. There are a number of large establishments in this country where plate glass is manu- factured, but the Creighton plant enjoys the reputation of pos- sessing the most favorable economic conditions as well as the best equipment.
�� � GLASS-MA KING, 6 1 3
In the selection of the crude materials great care is taken to secure purity. Tlie melting is carried out in large, open pots, the furnaces differing in their construction from those already de- scribed only in their greater size, and in the substitution of iron doors lined with tiles for the customary gathering-holes. When the melting is completed, the door nearest the pot to be emptied is opened, and a two-pronged fork, mounted on wheels, is inserted into the furnace. The prongs fit into depressions in the sides of the melting-pot, and thus secure it in a firm grasp. The pot of molten metal is then removed from the furnace and carried on a low truck to the casting table. At Creighton, the casting house, containing furnaces, tables, and annealing ovens, is 650 by 160 feet, about four times as large as the famous halle of Saint- Gobain in France, and nearly double the size of the British casting house at Ravenshead. The capacity of the American works has re- cently been greatly increased, and several new plants established in different sections of the natural-gas territory. The casting tables, the most important pieces of apparatus in a plate-glass works, are nineteen feet long, fourteen feet wide, and seven inches thick. Each is provided with an iron roller, thirty inches in diameter and fifteen feet long. Strips of iron on each side of the table afford a bearing for the rollers and determine the thickness of the plate of glass to be cast. The rough plate is commonly nine sixteenths of an inch in thickness ; after polishing, it is reduced to six or seven sixteenths. The casting tables are mounted on wheels and run on a track that reaches every furnace and annealing oven in the building. The table having been wheeled as near as possible to the melting furnace, the pot of molten glass is lifted by means of a crane, and its contents quickly poured on the table. The heavy iron roller is then passed from end to end, spreading the glass into a layer of uniform thickness. The whole operation of casting scarcely occupies more time than it takes to describe it. Each movement is made with almost nervous rapidity. Few industries offer such fine scenic display as the pouring of the molten glass. One feels like crying Encore ! it is so very brilliant.
In contact with the cold metal of the table, the glass cools rap- idly. As soon as possible, the door of the annealing oven is opened, and the plate of glass introduced. The floor of the oven is on the same level as the casting table, so that the transfer can be conveniently and quickly made. When, after several days, the glass is taken out of the oven, its surface is found to be decidedly rough and uneven. A small quantity is used in this condition for sky-lights and other purposes where strength is required without transparency. It is known in the market as rough plate. The greater part of the glass, however, is ground, smoothed, and pol- ished before it leaves the establishment. The work of the " hall
�� � 6i4 THE POPULAR SCIENCE MONTHLY.
men"' ends with the production of the rough plate. Its next guardians bear the name of " grinders." These confer upon the glass the property of transparency. The grinding is accomplished by means of rotary grinding machines, the abrading material being common river-sand, dredged from the Alleghany. Several million bushels are annually required for this purpose. The plates are firmly fixed on rotary platforms by means of plaster of Paris, and rotating disks are so arranged that they cover the entire surface of the plate at each rotation of the platform. Small jets of water keep the grinding-sand constantly wet. But such treat- ment only removes the rough exterior ; the smoothing is accom- plished by means of emery, finer and finer grades being used as the process proceeds. The final polish is given by means of rouge (carefully calcined sulphate of iron), which leaves the glass per- fectly smooth and ready for use.
Many doubtless remember the time — not so very long distant — when such a thing as American plate glass was totally un- known. It all came from France. But we have discovered — much to our satisfaction — that quite as good plate glass can be made at home as can be brought from across the water. Some, not as cau- tious as ourselves, say that the home product is the superior. Certainly the demand for it increases about as rapidly as new factories can be built to supply it. The joint product of the two Creighton plants is nearly two hundred and fifty thousand square feet per month, or about seventy acres of plate glass a year ! It takes some eleven hundred hands to turn out such a product as this, and its value is reckoned in the hundred thousands. Nat- ural gas is used everywhere throughout both works, displacing, perhaps six thousand bushels of coal daily. Among other duties, it supplies steam for engines of probably not less than three thousand aggregate horse-power. The new factory, some miles to the east of Creighton, will have a capacity, when completed, of three hundred thousand square feet of glass a month. We should hesitate to introduce so many figures, remembering the general aversion to statistics ; but they will present, better than anything else, a just conception of the magnitude of the operations con- nected with a large factory, and will perhaps dispel the notion — if such exist — that we are still largely dependent upon French dexterity for our supply of plate glass.
Such, in brief, is an outline of the three processes by which a pane of glass may be produced. Each day it becomes more per- fect, until now there seems little further to hope for, unless it be that the glass might lose some of its readiness to break into pieces on the least provocation. Our windows are already as large as we care to have them, and so clear that, every once in a while, some unlucky soul ignores the fact that the window has any glass in it.
�� � Such flattery as this encourages the glass-maker, in a double sense, to renew his efforts. It assures him of a continued demand, and also that his window-pane has fulfilled its highest function — in having been seen through, without being seen.