Popular Science Monthly/Volume 7/September 1875/Tempered Glass
By PERRY F. NURSEY, C. E.
A CONSIDERABLE degree of well-merited attention has of late been directed toward an invention which may be justly termed remarkable, even in these days of startling discoveries, inasmuch as it is one which promises to effect a complete change in the physical character of glass. This invention is the toughening process of M. François Royer de la Bastie, by which the natural brittleness of ordinary glass is exchanged for a condition of extreme toughness and durability. And this invention is perhaps the more remarkable in that it does not emanate from one engaged in, or practically conversant with, the manufacture of glass; nor is the discovery due to one of the great lights of science of our day; neither was it the result of a happy momentary inspiration. On the contrary, M. de la Bastie is a French private gentleman of fortune, residing in his native country—who, however, is given to the study of scientific matters. He was educated as an engineer, but his position and means rendered it unnecessary for him to follow the profession into which he had been initiated. He, however, is fond of experimenting in matters relating to engineering, and among other things he, some years since, conceived the idea of rendering glass less susceptible to fracture, either from blows or from rapid alternations of heat and cold. The early training of his mind naturally led him to look to mechanical means for the accomplishment of this end; and he, in the first place, set himself a purely mechanical problem to solve. He thought—as did Sir Joseph Whitworth with regard to steel—that by submitting glass when in a soft or fluid condition to great compressive power, he should force its molecules closer together, and, by thus rendering the mass more compact, the strength and solidity of the material would be greatly increased. This was not an unreasonable line of argument, inasmuch as the fragility of glass results from the weakness of the cohesion of its molecules. Success, however, did not follow experiment, and the mechanical problem was laid aside unsolved.
M. de la Bastie, however, continued to regard the question from an engineering point of view, and turned his attention to another method of treatment. Aware that the tenacity of steel was increased and that a considerable degree of toughness was imparted to it by dipping it, while hot, into heated oil, he experimented with glass in a similar manner. The results were sufficiently successful to encourage him to persevere in this direction, and, by degrees, to add other fatty constituents to the oil-bath. Improved results were the consequence; and they continued to improve until at length, after several years of patient research and experiment, De la Bastie succeeded—with a bath consisting of a mixture of oils, wax, tallow, resin, and other similar ingredients—in producing a number of samples of glass which were practically unbreakable. As may be supposed, there were other conditions upon which success depended besides the character and proportions of the ingredients constituting the bath. M. de la Bastie, not being a glass-manufacturer, purchased sheets of glass, as well as glass articles, which he heated in a furnace or oven, to a certain temperature, and transferred to the oleaginous bath, which was also heated to a given temperature. These questions of relative temperature, therefore, had to be worked out; and De la Bastie had further to determine, very precisely, the condition of the glass most favorable for the proper action of the bath upon it. This he found to be that point at which softness or malleability commences, the molecules being then capable of closing suddenly together, thus condensing the material when plunged into a liquid at a somewhat lower temperature than itself, and inclosing some portion of the constituents of the bath in its opened and susceptible pores. Having determined all these conditions, and constructed apparatus, M. de la Bastie was enabled to take ordinary glass articles, and pieces of sheet-glass, and to toughen them so that they bore an incredible amount of throwing about and hammering without breaking. Just, however, as De la Bastie had perfected his invention, he lost the clew to success, and for two years he was foiled in every attempt to regain it. There was the hard fact staring him in the face, that he had succeeded in depriving glass of its brittleness, as shown by specimens around him; but there was the harder fact before him, that he had lost the key of his success. Nevertheless he labored on, and at the end of the period above mentioned he had the satisfaction of finding all his anxieties at an end; his toils were requited by the rediscovery of his secret. He has since worked at it most assiduously, and has now brought it into practical working order, rendering the process as certain of success as any in use in the arts and manufactures in the present day.
As already observed, M. de la Bastie is not a glass-manufacturer; he therefore had to reheat glass articles when toughening them. It, however, by no means follows that the toughening process cannot be applied in the course of manufacture, thus avoiding reheating. On the contrary, it not only can be, but has been, applied at glass-works to glass just made, and so saves the costly and time-absorbing process of annealing. But, for reasons stated, M. de la Bastie had to apply the process to the manufactured article; and the method adopted, and the apparatus used in its application, next merit attention. In the first place, the glass to be toughened had to be raised to a very high
temperature—the higher the temperature the better—the risk of breaking the glass being thereby reduced, and the shrinkage or condensation being increased. It was therefore advantageous, and often necessary, to heat the glass to the point of softening; but in that condition glass articles readily lost their shape, and had to be plunged into the bath almost without being touched. Then came another difficulty—that of preventing an already highly-heated combustible liquid taking fire upon the entrance of the still more highly-heated glass. The latter difficulty was met by placing the tempering bath in direct communication with the heating oven, and inclosing it so as to prevent access of air; and the former by allowing the heated glass articles to descend quickly by gravitation, from the oven to the bath.
The apparatus used by M. de la Bastie is shown in the accompanying illustrations, in which Fig. 1 is a front view, and Fig. 2 a vertical section, of furnace for annealing glass objects; Fig. 3 a sectional plan of the oven for annealing flat plates. The working oven, a, is heated by a furnace, b. The bottom of the oven, c, and the slope to the bath, are made in one piece of refractory material, and are very smooth on the surface. At the side of the oven is a preparatory oven, communicating by a passage in the separating wall. In this oven the glass is partially heated before being placed in the main oven, a. The products of combustion are carried away in the direction of the arrows through the chimney. When the oven, a, is sufficiently heated, the ash-pit and fire-doors are closed, and rendered air-tight by luting,
and the fire is maintained by small pieces of fuel introduced by a hole in the fire-door. The draught is then stopped by lowering the chimney-cap, or closing the damper. The vertical damper, f, is then raised, so that the flame passes by the flue, g, to a second chimney, passing thus along the slope and heating it, and also opening communication from the oven, a, to the bath, h, which is filled with the oleaginous compound. It is covered from the external air by a lid, and within it is a basket of fine wire gauze, k, hung from brackets. A tube, l, contains a thermometer, m, to indicate the temperature; and by this tube the contents of the bath may be added to, or any excess may overflow by the discharge-pipe, n. A plug, o, on the cover may be removed to observe the interior, without entirely uncovering the bath. A fire-truck, p is charged with live fuel, heats the bath to the desired temperature. The glass is introduced into the preparatory oven by an opening in the outer wall, and thence it is moved through a second open-in on to the floor of the oven, a. The workman who watches the glass through a spy-hole, when he finds it at the proper heat, pushes it by an iron rod to the slope, d, whence it slides into the bath and is received on the basket, k. When the glass has cooled to the temperature of the bath, the lid is removed, and the basket, k, is raised out of the bath with the tempered glass.
In tempering sheet-glass the arrangements of both oven and bath are slightly modified, as shown in Fig. 3. In place of the sloping exit
for articles from the oven to the bath, M. de la Bastie has a rocking table, E, which is hinged underneath to the mouth of the oven, and which also forms the floor of the oven. When the glass has been sufficiently heated, the workman, by means of a lever, tilts the table, and the glass slides gently down an easy incline on to a table set at a corresponding incline in the bath. If it is not of importance that the transparency of the glass should be preserved, no special precautions are taken to prevent the dust from the furnace settling on its face. Where, however, clearness is required, the glass is heated in a muffle, perfect transparency being obtained. The process of tempering or toughening, exclusive of the time required for heating the glass, occupies but a minute or so, the glass being immersed in the bath and at once withdrawn and set aside to cool. The cost per article, as may be supposed, is merely nominal.
Glass which has been treated in this manner undergoes a physical transformation as complete as it is remarkable. Its appearance is in no way altered, either as regards transparency or color—if colored glass be so treated—and its ring or sound is not in any way affected. It has, however, exchanged its distinguishing characteristic of extreme brittleness for a degree of toughness and elasticity which enables it to bear the impact of heavy falling weights and smart blows without the least injury. A great number of experiments have been made, the results of which fully corroborate this fact. From these it will suffice to select a few by way of illustration. Watch-glasses, which perfectly retain their transparency, have resisted every attempt to break them by crushing between the fingers, or by throwing them about indiscriminately on the bare floor. Glass plates, dishes, colored lantern-glasses, and the like, have been similarly thrown about by the handful, stood upon, and otherwise maltreated, but without the slightest injury accruing to them, except, perhaps, when a solitary specimen which had been imperfectly tempered got in with the rest. Experiments have also been carried out to ascertain the comparative strength of toughened and untoughened glass when submitted to bending stress. Here a number of pieces of glass, each measuring six inches in length, by five inches in breadth, and having a thickness of about one-fourth of an inch, were tried. Each sample in its turn was supported at the ends, and a stirrup-piece was hung upon the centre of the glass, a weight-rod hanging vertically from the under-side of the stirrup. With this arrangement applied to a piece of ordinary glass, the weight-rod was gradually loaded until a weight of 279 pounds was reached, when the glass broke. A piece of toughened glass of similar dimensions, similarly treated, did not give way until a strain of 1,348 pounds had been reached, and before it yielded a considerable deflection was produced in it, showing its elasticity. Had its strength been due to rigidity or inflexibility alone, it would not have assumed a curve before yielding to the pressure brought upon it.
Satisfactory as the above results may appear at the first glance, they will be seen upon reflection most inadequately to represent the relative strength of toughened and untoughened glass. It will be observed that the test applied was that of long-sustained and gradually-increasing pressure, which could rarely occur to glass articles in everyday use. Glass is subject to sudden, sharp blows, either from articles falling down on other substances or from extraneous bodies falling upon or being brought in contact with them. Hence it is clear that to obtain a true estimate of the new process, glass must be subjected to tests which fairly represent the conditions of the accidents to which it is ordinarily exposed. This estimate has been arrived at repeatedly by placing pieces of plate-glass in a frame and allowing weights to fall on them from given heights. One experiment from a number—and which was made publicly—will illustrate this test: A piece of ordinary glass, six inches long by five inches wide, and one-fourth of an inch thick, was placed in a small frame which supported the glass around its edges, and kept its under-side about half an inch from the floor. A four-ounce weight was dropped on it from a height of one foot, and the glass was broken. A piece of toughened glass of corresponding dimensions was then placed in the frame and the same weight dropped upon it several times from a height of ten feet, but without fracturing the glass. An eight-ounce weight was then substituted, and repeatedly dropped upon the glass from the same height as before, and with the same result, no impression whatever being made upon it. The eight-ounce weight was then thrown violently upon it several times, but without damaging it. Its destruction, however, was finally accomplished by means of a hammer. Perhaps the most crucial test to which toughened glass could be put would be to let it fall on iron. This has been done, and in public too. A thin glass plate was dropped from a height of four feet on to an iron grating, from which it rebounded about one foot, sustaining no injury whatever.
As singular as any other feature presented by toughened glass are the results of its destruction. Ordinary glass, upon being fractured, gives long, needle-shaped, and angular fragments. Not so toughened glass, which is instantaneously resolved into mere atoms. The whole mass is at once disintegrated into innumerable pieces, ranging in size from a pin's-point to an eighth of an inch in diameter. It sometimes occurs that pieces measuring half an inch or an inch across may remain whole, but these pieces are traversed in all directions by a net-work of fine lines of fracture, and with the fingers are easily reduced to fragments. Microscopical examination shows the fragments of toughened glass—large and small—to follow the same law as regards the form and character of the crystals, and on some of the larger crystals being broken up they have been found to separate into smaller ones of the same character. The edges of these fragments, too, are more or less smooth instead of being jagged and serrated as are those of fragments of ordinary glass. Hence a diminished tendency in the former to cause incised flesh-wounds when handled.
When glass has been imperfectly treated, as has sometimes happened in M. de la Bastie's experiments, it will not stand the same amount of rough usage as will perfectly-toughened specimens. The fact of the toughening process having been incomplete is made manifest upon the destruction of a sample in three different ways chiefly. Independently of its yielding at an early stage either to blows or pressure, it will show upon destruction either needle-fractures approaching in appearance those of ordinary glass, or pieces varying from the size of a sixpence to that of a half-crown will remain unbroken and untraversed by lines of fracture. Again, the mass may be wholly fractured, but on looking at the fragments edgewise a narrow, milky streak will be apparent midway between the upper and under sides of the glass, indicating that the influence of the bath has not extended through the glass. Where the process has been perfectly applied, no such phenomena are exhibited, the crystals being of uniform transparency throughout the whole mass.
Such, then, is De la Bastie's toughened glass, which possesses enormous cohesive power, and offers great resistance to the force of impact. There is, however, one peculiarity which, for the present, tells against it in a slight degree—it cannot be cut through with a diamond. Scratched its surface can be, but there the action of the diamond ceases. This drawback only applies in the case of window-glass in odd-sized frames; for the practice of the present day, with builders, is to make window-sashes of certain fixed dimensions, and glass-manufacturers work to these dimensions. It is not at all improbable, however, that ere long a means will be for cutting toughened glass to any size or shape; experiments are, in fact, now being conducted with this view, and so far as they have gone they give promise of success. But if toughened glass cannot be cut by the diamond, it can be readily cut and polished by the wheel, as for lustres and the like, so that wine-glasses and articles of cut glass-ware can be toughened directly they are made, and cut and polished subsequently.
Superficial observers have affected to detect in the toughening process a similar condition of matter to that which obtains in Prince Rupert's drops. The error of such a conclusion, however, becomes evident upon a little consideration. Prince Rupert's drops are made by allowing melted glass to fall into cold water; the result of which is a small pear-shaped drop, which will stand smart blows upon the thick end without injury; but the moment the thin end, or tail, is broken, the drop flies into fragments. Now, glass and water, and—as far as present knowledge goes—no other substances besides, expand while passing from the fluid into the solid condition. The theory of the Rupert drops is, that the glass being cooled suddenly, by being dropped into cold water, expansion is checked by reason of a hard skin being formed on the outer surface. This exterior coating prevents the interior atoms from expanding and arranging themselves in such a way as to give the glass a fibrous nature, as they would if the glass was allowed to cool very gradually. An examination of the Rupert's drop shows the inner substance to be fissured and divided into a number of small particles. They exist, in fact, in a state of compression, with but little mutual cohesion, and are only held together by the external skin. So long as the skin remains intact the tendency of the inner particles to expand and fill their proper space is checked and resisted by the superior compressive strain of the skin. Nor is the balance of the opposing forces disturbed by blows on the thick end of the drop, which vibrates as a whole, the vibrations not being transmitted from the exterior to the interior. But by breaking off the tail of the drop a vibratory movement is communicated along the crystalline surface, admitting of internal expansion, by which the cohesion of the particles composing the external skin is overcome, and the glass is at once reduced to fragments. As the skin of toughened glass can be cut through with the diamond, and as, moreover, its surface can be removed by polishing and cutting with the wheel, without injury to the mass, it is evident that it must exist under conditions very dissimilar from those of a Rupert's drop. Moreover, melted glass, on being dropped into De la Bastie's bath, gives a similarly shaped body, from which the tail can be broken off, piece by piece, without injury to the body, which can be scratched, knocked and thrown about, without exhibiting any signs of deterioration. Bearing upon this point, too, comes the fact that toughened glass can be elegantly engraved, either by Tilghman's sand-blast process, or by means of hydrofluoric acid, in the ordinary way, the surface or outer skin being thus removed.
M. de la Bastie's invention marks a distinct era in the history of one of our most important industries. Never during the history of glass-manufacture, which extends over some 3,500 years, has any radical change been effected in its character. The glass-blowers of Egypt, who practised their art before the exodus of the children of Israel, and representations of whom have been found on monuments as ancient as that event, produced a similar glass to that of our own times. This has been proved by an examination of glass ornaments which have been discovered in tombs as ancient as the days of Moses. It has been proved, too, by a large bead of glass, found at Thebes, upon which was inscribed the name of a monarch who lived 1,500 years b. c., and which glass was of the same specific gravity as our own crown-glass. It is true Pliny mentions that a combination was devised in the reign of Tiberius, which produced a flexible glass; but both the inventor and apparatus were destroyed, in order, it is said, to prevent the value of copper, silver, and gold, from becoming depreciated. There is, however, no evidence whatever that this was the toughening process of De la Bastie, nor does the record in any way detract from the merits of that gentleman as the inventor of an important economic process. The fact remains that the world has now given to it for the first time, in a practical form, an invention by which the brittleness of glass is superseded by an attribute of the most valuable nature—toughness. It is by no means improbable that the old adage, "as brittle as glass," will soon be superseded by a new one—"as tough as glass."
What may be the ultimate result of the introduction of this invention in practice it is difficult to foresee, so wide-spread, so universal does its application seem. Not only is it desirable to render durable such articles as are at present made from glass, but to satisfy a want long felt in every department of art, science, and manufacture, of such a material as toughened glass; and this want can now be satisfied. So numerous are the opportunities which present for its application, and so well adapted does it appear to be where cleanliness, transparency, resistance to heat and chemical action, and comparative indestructibility are desiderata, that it would be idle to attempt to categorize them.
The invention is being taken up practically on the Continent, and no less in England. Messrs. Powell, of Whitefriars, are introducing it in their glass-works, and two other firms in the north of England are doing the same. It is by no means improbable that its first introduction in practice in this country will be at the aquarium now in course of erection at Westminster, where it is intended to use it for the tanks.
There still remain some questions to be answered with regard to the phenomena exhibited by toughened glass—questions, however, which in no way affect the practical value of the material. Its peculiarities continue to form the subject of investigation, and, as soon as any conclusions of value to science have been arrived at, they will be made known, so that the physical aspect of toughened glass may again be reverted to in these pages. It only remains to observe that the remarkable character and unique nature of M. de la Bastie's invention are such as to render it probable that lie will not only materially benefit those of his own time, but will bequeath to posterity an invaluable legacy.—Popular Science Review.