294 



SCIENCE 



[Vol. VI., No. 139. 



addition to known methods of glass-making. The 

 new invention was based upon a very complicated 

 and costly process. The objects were likely to have 

 their shapes spoiled or their surfaces injured, and, in 

 addition to these objections, the toughened glass was 

 liable to burst spontaneously or by a sudden shock. 

 The cooling influence, which acts from the surface in- 

 wards, is not in proportion to the bulk of the glass, 

 but to its surface, and must always act more quickly 

 where the surface is large in comparison with the 

 volume : in a sheet, for instance, the edges cool more 

 quickly than the middle, and the sheet is liable to 

 explode. 



Mr. Siemens has invented a process by which hard- 

 ened glass may be manufactured and thoroughly 

 toughened without this objection. His method con- 

 sists of placing the parts of the object which have 

 the least surface between slabs of a cold material 

 suitable for the purpose. The edges are not exposed 

 to this rapid-cooling process, and hence a uniform 

 cooling of the whole mass is secured. The plan em- 

 ployed for various articles varies with their shapes, 

 but each is based upon the same principle. He has 

 three processes, — press-hardening, casting, and a 

 third, theoretically less perfect than the other, viz., 

 semi-hardening, or hard-tempering, which may be 

 employed advantageously in cases where the others 

 cannot be used, as in the case of lamp-chimneys, 

 bottles, etc. 



Press-hardened glass has now been manufactured 

 for six years, and is constantly increasing in impor- 

 tance. Besides plain work, decorated sheets, in- 

 scriptions, and ornaments form an important part 

 of the goods produced : the process is therefore one of 

 manufacture, not simply one of hardening and tough- 

 ening. The glass is so hard that the diamond will not 

 touch it; and it cannot, therefore, be cut or bent after 

 manufacture. It may, however, be polished, etched, 

 and slightly ground. Its strength is at least eight 

 times that of ordinary glass. 



The process of manufacture is as follows: The 

 glass is cut and shaped in the ordinary way, and is 

 then exposed to the radiant heat of a peculiarly con- 

 structed furnace until quite soft. As soon as it has 

 attained the required temperature, it is placed between 

 cold metal plates to be cooled down with a rapidity 

 which varies with the thickness of the glass, but is 

 always very great. The heating of a sheet of glass of 

 ordinary thickness requires one minute ; the cooling, 

 half a minute. 



It is a remarkable circumstance that glass can be 

 thus quickly heated and cooled without causing it 

 to break. This is altogether due, in heating, to the 

 fact that all the heat comes by radiation in a uniform 

 manner. The success of the cooling is entirely due 

 to the uniform temperature of the glass and metal 

 plates during the cooling process. It is owing to the 

 very high temperature of the glass during the process 

 that it can be moulded, enamelled, and decorated at 

 the same time it is being hardened. By this method 

 an enamel can be produced that is as hard and refrac- 

 tory as the glass itself. 



The hardness depends upon the rapidity of cooling; 



so that, if the glass is to be hardened to a very high 

 degree, a very good conductor of heat, as copper, is 

 used; if to a lower degree, iron is substituted; and, 

 for a still lower degree, the iron press may be lined 

 with asbestos or clay-stone. Much care must be used 

 in the entire process; the furnace floor must be 

 smooth, and be dusted with talc-powder; the whole 

 process must be by radiation, and must be conducted 

 uniformly. 



Semi-hardened glass is made by the same process, 

 except that, instead of using a press, the article is 

 placed in a casing of iron provided with projecting 

 internal ribs, which prevent contact except at a few 

 points. The object is then allowed to cool in air. 

 By this means a glass three times as strong as ordi- 

 nary glass is produced. It is absolutely necessary 

 that there shall be no draught at any time during the 

 process, and the method can only be used for objects 

 nearly uniform in thickness; thick-bottomed bot- 

 tles, for instance, being liable to break. 



The third process, that of casting, has not yet been 

 introduced on a manufacturing scale; but Mr. Sie- 

 mens considers it the most valuable of the three. 

 The experimental castings have been very successful, 

 and the objects consist of floor-plates, grindstones, 

 pulleys, tramway sleepers, and various ornamental 

 work. Mr. Siemens is of the opinion that castings 

 might be produced with advantage for other purposes, 

 especially to be used in connection with the building- 

 trades. 



Glass made by this process is four times as strong 

 as ordinary glass, can be made much more cheaply, 

 and cast into a variety of forms which it would be 

 impossible to produce with ordinary glass without its 

 cracking. It is manufactured in the following man- 

 ner: Glass is melted in an ordinary tank-furnace, and 

 run into moulds, as with iron castings. The process 

 differs from that of casting iron in that a special sub- 

 stance is used in place of sand, and the mould and 

 the glass inside of it are heated and cooled together. 

 The material to be used in place of sand must be 

 selected so as to have, as nearly as possible, the same 

 conductivity and capacity for heat as glass. In such 

 a case — the glass and mould forming, as it were, one 

 homogeneous body — the glass will cool without crack- 

 ing, even if the cooling process is comparatively quick, 

 which is quite necessary. When fully heated, the 

 glass and mould are taken out, and allowed to cool in 

 the open air. This process differs from the other 

 two, in that glass of any thickness maybe cast, where- 

 as in the others only glass of a uniform thickness can 

 be made. 



It will be seen from the description that these 

 three methods, although very different from each 

 other, are but different ways of treating differently 

 shaped articles, and that all three are based upon 

 the principle of keeping the whole body of the glass 

 at a uniform temperature during the operations of 

 heating and cooling. These processes are likely to 

 cause an important revolution in glass-making; and 

 it seems, that, in the future, tempered glass will bear 

 the same relation to ordinary glass that steel does to 

 iron. 



