Manufacturing Pyrex glass at Corning

MANUFACTURING PYREX GLASS AT CORNING GLASS WORKS *

Glass is one of the most widely used substances of man's making. Even so, it is undeniably true that the public at large knows less about the composition and manufacture of glass than it does about any other product employed as variously and as extensively.

It is not our purpose to tell the story of the antiquity of glass, but evidence there is that the Egyptians knew how to make it 7,000 years ago. Indeed, there were depicted on the walls of the tomb of an Egyptian King in the fifth dynasty — that is, 4,000 years before the Christian era — glass workers in the act of blowing a bottle. However, notwithstanding this ancient lineage of glass, it is safe to say that more has been done towards developing glass for a multiplicity of purposes in the last half century than was achieved during the whole previous history of the product. This advance has been mainly in direct response to the greater demand for glasses capable of meeting extremely diversified services.

In this important department of industry, no plant has done more in advancing the art of glass making than the Coming Glass Works of Corning. N. Y. This enterprising and well-known establishment has to its credit a record of unique accomplishments covering a period of more than half a century. The Corning Glass Works has long specialized in producing glass of one sort or another capable of meeting exacting requirements. In short, the company has distinguished itself by producing and by manufacturing glasses that would stand up under service conditions under which ordinary glass would fail.

In the course of its growth, the Corning Glass Works has expanded continually and has added department after department until the main plant has become an aggregation of buildings boasting a floor space of more than 612,000 square feet. We have not at our disposal space available to describe in any detail the many different activities that are carried on in the several associated departments, so we shall have to confine the present story to work centering about the production of one class of glass - that is, glass distinguished by an exceptionally low coefficient of expansion.

It is painfully familiar to most of us how promptly a hot tumbler cracks when filled with cold water; and we can easily recall numerous other examples of the splitting and shattering of glass containers when suddenly heated or chilled. This fracturing was due mainly to unequal stresses abruptly developed in the glass walls by opposing movements of expansion and contraction. This sensitiveness of most glasses has been a feature of them for many centuries, and it has imposed great care in the handling of them and it has also made it impossible to utilized glass for certain purposes for which it was suitable because of its transparency.

The physical property which is technically termed the coefficient of expansion is the measure of linear expansion or contraction that takes place when a substance is heated or cooled 1º C - that is, 1.8º F. It was the desire of the experts of the Corning Glass Works to find ways to greatly reduce this coefficient that led them to undertake a line of research to this end more than 20 years ago. The object of this investigation was to produce, if possible, a type of glass that would meet the rigorous conditions imposed in railroad signal work in which lantern globes and semaphore bull's eyes were exposed to sharp and rapid changes. In passing, it should be said, that the Corning Glass Works has supplied glass for this purpose to the railroads of the United States for quite half a century.

When the Corning Glass works began experimenting, it was known that there was being manufactured at Jena, Germany, a glass having an expansion coefficient of 0.0000064. This was a borosilicate glass, designated by its makers as thermometer glass 59"'. The facts that it was a borosilicate glass, and that it had the lowest coefficient of linear expansion among the then existing glasses, were indications that assisted the early investigators.

The first of the low-expansion glasses developed by the Corning Glass Works to be marketed in large quantities was given the trade name of "Nonex." It was the outcome of patient and persistent labors, and was capable of meeting every condition imposed by railway signal service. It was not enough merely to produce a glass of notably low expansion, but it was equally essential that the glass should have durability and that it should faithfully transmit its true colors under all atmospheric conditions so that there could be no mistaking a semaphore's message of "clear," "caution," or "danger." It is easy to realize that security of life and property may hinge upon glass standing up despite any state of the weather.

In consequence of its success in evolving low-expansion glasses for the uses mentioned, the Corning Glass Works permanently organized a research laboratory in the plant, and in doing this the company set a precedent for the glass industry in this country. At the present time, this laboratory is large and splendidly equipped and administered by a corps of experts continually engaged in testing the plant's regular products and in bettering them whenever and wherever that may be possible. Furthermore, these highly trained technicists carry on investigations looking to the development of other commercial products and to the solution of glass problems submitted to them from many directions. It is thus that the company is able to turn out the exceptional in the art and to do this while adhering rigidly to a very exacting standard of excellence.

Besides satisfying the needs in the field of railroad signaling, Nonex was soon found admirably suited to such uses as lamp chimney's, inner arcs, battery jars, etc. This was merely another instance of how a material developed in the first place for one service could be adapted to other services. Only a person familiar with the technology of glass could appreciate the various obstacles that were overcome in evolving Nonex. This was especially true when it came to introducing the coloring chemicals required to give the lantern globes and bull's eyes their prescribed hues, because the colors at first had decidedly different effects when combined with the heat-resisting glass.

And now we come to a still more amazing achievement; the evolution of a borosilicate glass possessing greater heat-resisting properties than Nonex - a glass that could be employed successfully in the making of cooking utensils that could be used in an oven without fear of breaking. Never before had a glass dish been fashioned that could be subjected to those severe conditions and survive unharmed. It was necessary, of course, to give the baking dish fairly thick walls. This, in itself, presented a difficulty, because any lag in the transfer of heat through the walls would induce unequal expansion stresses unless the coefficient of expansion of the glass mass as a whole could be kept so low and the transfer of heat made so rapid that any stresses set up would be a very brief duration and of well nigh negligible strength. What was finally accomplished can best be grasped when it is known that "Pyrex" baking ware - for so the new glass was called - remains uninjured even when removed from an ice box and placed in an oven.

It was not enough to produce a glass of low expansion in order to make the glass suitable for oven use; the men in the laboratory had also to give the glass exceptional sturdiness so that the cooking utensil would be strong enough to withstand the rough handling normally expected in the average kitchen. Furthermore, the technicists had to see to it that the glass would be capable of resisting the corrosive action of acids developed in the liquids of cooking foodstuffs. After several years of experimenting and a wide test use of Pyrex oven ware, the Corning Glass Works was in a position to offer with confidence, in 1915, glass cooking utensils of several sorts that could be sold to the housewife and others with ample guaranty of satisfaction. Since then, these unique commodities have won for themselves an enviable position and a measure of recognition that has been widening apace.

Some of the reasons why Pyrex oven ware has won popularity are: it can easily be kept clean - in fact, promptly shows when it is soiled and, therefore, is especially sanitary; it is exceptionally durable for glass, and will give years of service if handled with reasonable care; it is unusually efficient in the economical use of oven heat, and, accordingly, Pyrex utensils bake foods more quickly than do similar utensils of metal or earthenware; and, finally, a Pyrex cake pan, for instance, will bake a cake evenly on the bottom, the sides, and the top. The last two advantages of Pyrex are due to a physical characteristic which causes the glass to transmit to the cooking food stuffs much of the radiant heat which is generally reflected and not transmitted by utensils made of other materials.

In a series of tests made in the laboratory of the Corning Glass Works, it was brought out that tin dish took up in a gas oven only 50 per cent. and in a coal oven only 31 per cent. as much heat as a glass dish. Other experiments revealed that there would be a fuel saving of substantially 50 per cent. in a gas oven when glass was substituted for a tin dish. Strange as it may seem, the glass dish, even though transmitting heat more rapidly than a tin dish, cools off more slowly than the metal dish when the oven door is opened. This enables a housewife to watch her baking from time to time without running the risk of her bread or her cake falling.

As might have been expected, the special properties possessed by Pyres invited the adaptation of the glass to other uses; and also in 1915 the Corning Glass Works placed upon the market a full line of flasks, beakers, test tubes, and fabricated apparatus for laboratory use. Today, there is hardly a laboratory in the United States that has not a fairly complete outfit of Pyrex vessels, etc. This is understandable when we keep in mind the fact that this low-expansion glass is peculiarly resistant to the solvent action of water and the corrosive action of acids. As an insulator Pyrex is especially effective, and for this purpose it has latterly found a field of service in radio. Last, but by no means least, Pyrex is now being worked into nursing bottles for babies, to the abundant satisfaction of many thousands of mothers.

The task of experts at Corning was not by any means finished when the composition of Pyrex was finally determined. The development of that borosilicate glass brought in its train manufacturing problems of a troublesome nature. To begin with, Pyrex is much more refractory — that is, resistant to heat — than ordinary glass, and requires a temperature of about 200º F. higher than the usual run of glasses. In other words, the melting temperature is 2,800º F., and the glass must be worked into shape before the temperature drops to any marked extent below 2,400º F. This meant that the furnaces in which the glass was melted would have to stand up under unusually severe conditions, and that machinery would have to be modified or devised and men trained to deal with the glass with much greater speed than previously employed in the processes of manufacture.

Manifestly, we are not in a position to tell just what enters into the composition of a Pyrex batch, other than to say that the batch is composed of 50 per cent. Pyrex cullet or scrap and 50 per cent. fresh raw materials in which virtually pure silica sand and borax in one or more forms figure conspicuously. Owing to certain chemical changes that take place during the melting, when a considerable percentage of the batch is volatilized and carried off up the chimney, experience has shown that a definite proportion of the mix must be made up of broken bits of Pyrex. The melting is carried on in continuous tanks of many tons capacity. Each tank is divided into a melting compartment and a working compartment. As the glass passes from the melting chamber to the working end, it settles, clarifies itself, and its heat is gradually diminished until the proper working temperature is reached. The intensity of this working heat is vividly indicated by the dazzling white incandescence of the the glass when withdrawn by the gatherer preparatory to blowing or pressing the gob of Pyrex into any desired form.

If the object is to produce a cylinder for an X-ray shield or cylinder for a visible gasoline filling pump, the gatherer passes the hollow iron rod, to which the glowing gob of Pyrex is attached, to a gaffer who first blows the glass into a large elongated bubble and then lowers it into a wet 2-piece iron mold which is closed about the bubble by a mold boy. The gaffer then distends the bubble within the mold so that it shall conform to the interior walls of the mold. With this operation completed, the mold boy opens the mold, and the gaffer hands his rod to a lehr boy who carries the article to a nearby annealing oven in which the still hot product is gradually cooled as it travels from one end to the other of the heated oven. It is understood, of course, that the newly made commodity is detached from the blow iron, after which that pipe is turned over to a cracker-off boy who removes the remaining glass so that the iron is ready for use again.

If the work in hand is that of automatically blowing Pyrex baby bottles, their manufacture is accomplished by a somewhat complex and decidedly ingenious machine equipped with two revolving tables and provided with a series of 2-piece molds. A gob of glass is picked up mechanically from an adjacent tank, quickly given a pear-like form by a rotary motion, and then penetrated at the upper end by a plunger that produces an initial cavity. In this state, the incipient bottle is passed to the second table where it is caught up by one of the molds, enclosed and subjected to an air pressure of 18 pounds that distends the punched cavity and causes the plastic glass to take the shape of the confining mold. This is all done faster than it takes to tell. Both the blowing of the bottle and the operating of the machine are accomplished by compressed air. The manipulation of the glass and the wrist-like movements of the mold spindles are amazing examples of what might be termed mechanical sleight-of-hand. After leaving the blowing machine, the bottles travel by conveyor to a lehr through which they progress slowly during a prescribed annealing period.

Many of the Pyrex commodities are made in hand-operated presses in which the mold or die parts are kept properly cooled by streams of compressed air. Compressed air has various other helpful services to perform in the production of Pyrex articles of one sort or another; and this very adaptable motive medium is utilized to great advantage in speeding up the work of dismantling damaged furnaces so that they may be quickly repaired. Air-driven drills are able to break the concrete in short order so that the task of rehabilitation can be taken in hand with the least delay.

It is necessary to say that every Pyrex article is examined carefully by trained employees whose sharp eyes are ever on the watch for defects which might mar either the appearance or the serviceability of the commodity. Only the very best pass to the shipping department, while the rejects go to the cullet pile for melting and reworking. This not only reduces wastage to a practicable minimum but it insures that high market standard which characterizes the output of one of the world's most interesting and most famous glass plants wherein are employed no fewer than 1,900 persons.

* Copyrighted article in COMPRESSED AIR MAGAZINE for November, 1925, by S. G. Roberts. Used by permission.