TOUGHENED GLASS.

New Zealand Herald, Volume XII, Issue 4298, 23 August 1875, Page 3

TOUGHENED GLASS.

Although the manufacture of glass has been carried on for about 2000 years, it does not appear that any attempts to overcome its inherent brittleness and liability to fracture, and at the same time to preserve its transparency, have proved successful—if, indeed, they have ever Deen made, which is doubtful. It is true that the French philosopher Reaumur many years since hardened glass somewhat by exposing it to a high temperature for a considerable time. But this process, which is technically termed devitrification, while it hardens, at the same time crystallizes the glass and renders it opaque, the product being known as Reaumur's porcelain. Seven years since, however, M. Francois de la Bastie, a French engineer, after long and patient investigation into the subject, discovered a simple means of rendering glass practically unbrittle, and at the same time of preserving its transparency. There were many delicate conditions involved in the process by which he obtained this result, his success being achieved much in the same way as Pallissy's, but on endeavouring to repeat the successful experiment ho failed signally. For two years more M. de la Bastie, who possesses ample means, strove without avail to rediscover the secret of his success. At length, however, he succeeded in so doing, and has since been engaged in perfecting his invention and in developing a laboratory experiment into practical working. The process of conversion in the main is a very simple one, so simplo that it seems singular it was never thought of before. Broadly stated, it consists in heating the glass at a certain temperature and plunging it while hot into a bath consisting of a heated oleaginous compound. There are, however, many conditions in connection with the details of the process upon which a satisfactory result depends, and the neglect of any, even in a slight degree, constitutes the difference between success and failure. Thus, the glass may be uuderheated and will not be susceptible to the effect of the bath, or it may ue overheated and it will then lose its shape, or, again, it may be rightly heated and yet be spoilt in the course of transference to the bath. Moreover, the oleaginous constituents of the bath and their temperature have an important bearing upon the ultinate result. These and numerous other points of detail have all been satisfactorily settled by M. de la Bastie, who has designed furnaces and baths by means of which his toughening process can be carried out practically without fear of mischance. The time occupied in the actual process of tempering is merely nominal, for directly the articles are brought to tho required temperature they aro plunged into the bath and instantly withdrawn. The cost of tempering, too, is stated to be very small. We have observed that M. do la Bastie went through a long course of experimental research before he attained success. He first worked, as an engineer naturally would, upon mechanical principles. Knowing that the fragility of glass results from the weakness of the cohesion of its molecules, he not unreasouably expected that, by forcing those molecules more closely together, and thus rendering tho mass more compact, the strength and solidity of the material would be increased. But this doctrine, which holds good with iron and steel, as Sir Joseph Whitworth has practically demonstrated, does not apply to glass : compression failing to toughen it, even if applied to it when in a fluid or soft condition. By applying heat, however, which is only force in another form, the desired end is attained, and the physical properties of tho material become altered in a very remarkable manner. To this singular fact we can testify from the inspection of a number of toughened glass articles at the offices of Messrs. Abel Key and Brothers, 29, Mincing-lane, the representatives of M. dela Bastie in England. In these articles, which consisted of|watch-glasses, plates, dishes, and shset-glass, both coloured and plain, neither transparency nor colour is affected at all, and the ring or sound only slightly. These articles, some of them being exceedingly thin, were thrown indiscriminately across a room and against a wall, and fell spinning on the deal floor. Water was boiled in a saucer over a fire and the saucer was quickly removed to a comparatively cold place, and was unaffected by tho sudden change of temperature. One corner of a piece of glass was held by the hand iu a gas flame until the corner became exceedingly hot, but the heat was not communicated to the other portion of tho glass, neither was it cracked from unequal expansion. A comparative experiment was then made with a piece of ordinary plate-glass and a similar piece of toughened glass, in order to shew their respective powers of resistance to fracture, from the force of impact by a falling weight. In each caso the glass was about 0 inches square, and was placed iu a frame, the weight being dropped upon its centre. With the ordinary glass, a 2-oz. weight falling on it from a height of 12 and IS inches respectively did no damage, but at 24 inches the glass was broken into several fragments. With a thinner piece of toughened glass no impression was made by the same weight falling from heights ranging from 2 to 10 feet, the weight simply rebounding from off the glass. An S-oz. iron weight tried at 2 and •! feet respectively gave similar results. Upon the height being increased to G feet, however, the glass broke. But here another singular result was produced ; instead of breaking into about a dozen pieces, as did the ordinary glass, it was literally smashed to atoms. The largest fragments measured about half an inch in length and breadth, and these were easily reduced by the fingers to atoms varying in size from that of a pin's point to that of a largo pin's head. The lines of fractures in the fragments presented to the eye the appearance of irregular lace work, and these lines were, moreover, apparent to the touch, but more palpably so on one side the glass than the other. Which of the two sides was thcoue that received thefirst impact of the blow wo were not able to determine. Another peculiarity is that the edges of the fractures are by no means so sharp, and therefore not so capable of causing incised wounds as are those of ordinary glas. It would seem that the toughened glass possesses enormous cohesive power, but that if the equilibrium of the mass is disturbed at any one point the disturbance or disintegration instantly extends throughout the whole piece, the atoms no longer possessing the power of cohesion. Of the practical nature of M. de la Bastie's unique discovery there can be no question whatever, nor can there bo any doubt of its yalue in tho arts, sciences, and manufactures. The applications which suggest themselves are inuumcrable, and above and beyond tho usefulness of the process with regard to articles of domestic use come important considerations affecting the applied sciences, especially iu coutiexion with chemical manufactures and similar industries, where a material, alike unintluenced by the action of heat or acids, has been so long and so vainly sought for—notably in connexion with vitriol chambers in the manufacture of sulphuric acid, and for piping iu chemical works. For the present there remains one purpose to which toughened "glass cannot bo so easily applied, and that is to window glazing in odd sizes, inasmuch as it cannot be cut by a diamond or other ordinary means. Our glaziers will therefore have a respite, but we cannot give them much hope that it will prove a long ono, as experiments of considerable promise are being conducted with the view of solving this problem. Moreover, the glass can be cut to the proper sizes before toughening, if desirable. The glass, however, is readily engraved either by tlourie acid in the usual way, or by Mr. Tilghman's elegant sandblast process. It can be easily polished, and it can also be cut by the wheel as for lustre wmk and the like. It only remains to congratulate M. de la Bastie on the useful and valua'ole discovery he has made, and to adil that ho la now erecting works in France to cany out in practice his ingenious process. London Tim?*.