Walter Rosenhain

“ We now turn to consider, very briefly, the general design and arrangement of some typical glass-melting furnaces. The oldest and simplest form of furnace is, in effect, simply a box built of fire-brick, in the centre of which stands the crucible, while a fire of wood or coal is placed upon either side. To attain any great degree of heat by such means, however, the size of the box or chamber and especially of the grates in which the fires are maintained must be properly proportioned both to the dimensions of the crucible and to each other. ”

“ A perfect piece of glass, having smooth surfaces and no internal regularities, allows the rays of light falling upon it to pass through undeflected in direction, and merely changed in colour, according to the tint of the glass in question. On looking at the glass, external objects can be quite clearly seen, and much of the interest and mystery of the glass itself is lost. ”

“ If a tube be drawn out of glass consisting of two layers, one considerably more expansible than the other, and the cooling process be rightly conducted, it is possible to produce a tube in which both the inner and outer layers of glass are under a considerable compressive stress. Not only is glass, as we have seen above, enormously stronger as against compression than it is against tension, but glass under compressive stress behaves as though it were a much tougher material, being less liable to injury by scratches or blows. ”

“ It is indeed hard to find any use to which glass is extensively put into which the function of transmitting light does not very largely enter. Almost the only such example of use is the modern application of opal glass to the covering of walls, and the use—not as yet widely extended—of pressed glass blocks as bricks and paving stones; in these cases it is the hardness and smoothness of surface that gives to the vitreous body its superiority over other materials, but apart from these special cases, the fact remains that well over 95 per cent. ”

“ Important and valuable as these improvements of the ancient process of bottle-blowing undoubtedly are, they do not touch the main disadvantages of the process—disadvantages that seriously affect the economy of the process and the well-being of the workers employed upon it. It is consequently not surprising that a great number of inventors have laboured at the problem of the purely mechanical production of bottles. ”

“ Although the transparency of glass is so valuable and indeed so essential a property of that substance, it must be remembered that no kind of glass is perfectly transparent. Quite apart from the fact that of the light that falls upon a glass surface, however perfectly polished, a considerable proportion is turned back by reflection at the surface of entry and again by reflection at the surface of exit from the glass, a certain proportion of light is absorbed during its passage through the glass itself, and the transmitted beam is correspondingly weakened. ”

“ We must content ourselves with saying that most modern producers are of the form of a tower in which a thick bed of fuel is partially burnt and partly gasified under the action of a blast of air mixed with steam. The chemical actions that take place are complicated, but the final result is the production of a gas containing from 2 to 8 or 10 per cent. of carbonic acid, 10 to 20 per cent. of hydrogen, 8 to 25 per cent. of carbonic oxide (CO) , 1 to 3 per cent. methane (CH4) , and 45 to 60 per cent. of nitrogen, with varying quantities of moisture, tarry matter, and ammonia. ”

“ In the case of glass, when the raw materials are melted together, a mixture is formed of liquids of differing densities similar to that which was temporarily formed in the sugar-water solution. Molten glass, however, is never so mobile a liquid as ordinary water, nor is it in the ordinary course of manufacture subjected to any such thorough mixing action as that which is produced by a spoon in a glass of water. In glass as ordinarily manufactured, therefore, it is not surprising to find that the lack of homogeneity which originates during the melting persists to the end. ”

“ The thermal conductivity of glass, except in so far as it affects the thermal endurance, is not a matter of any great direct practical importance, although the fact that glass is always a comparatively poor conductor of heat is utilised in many of its applications, as, for example, the construction of conservatories and hot-houses, although even in that case the opacity of glass to thermal radiations of long wave-lengths is of more importance than its low thermal conductivity. ”

“ In general terms it may be said that the lighter elements do not as a rule tend to the production of coloured glasses, while the heavier elements, so far as they can be retained in the glass in either solution or suspension, tend to produce an intense colouring effect. The element lead appears to form a striking exception to this rule, but this is due to the fact that while the silicates of most of the other heavy elements are more or less unstable, the silicate of lead is very stable, and can only be decomposed by the action of reducing agents. ”

“ As regards chemical behaviour during manufacture, it must be borne in mind that, although glasses are of the nature of solutions rather than of compounds, yet these solutions tend towards a state of saturation; thus a glass rich in silica and deficient in bases will readily dissolve any basic materials with which it may come in contact, while, on the other hand, a glass rich in bases and poor in acid constituents such as silica, boric acid or alumina, will readily absorb acid bodies from its surroundings. During the process of melting, glass is universally contained in fire-clay vessels. ”

“ There can, however, be very little doubt that as the processes of fusion and crystallisation become better known and understood, and the chemistry of silicate minerals is developed, the artificial production of mineral crystals in, at all events, moderate sizes will become increasingly possible; it is even to be hoped that their production will be so far perfected as to place their really valuable properties at the service of man. ”

“ In making this statement the labours of many eminent scientific workers are by no means forgotten, but the entire field is so large and beset with such great experimental difficulties that even the labours of a list of investigators that includes the names of Fraunhofer and Faraday, Stokes, Hopkinson, Abbé and Schott, have resulted in little more than an accumulation of empirical data which, while they have been productive of great direct practical results, have left the science of glass still in a very elementary condition. ”

“ No refinements for regulating the temperature of various parts of the furnace in order to ensure perfect fining of the glass are required, and the furnace generally consists simply of an oblong chamber or tank, at one end of which the raw materials are fed in,124 while the glass is withdrawn by means of ladles from one or two suitable apertures at the other end. For economical working, however, the furnace must be capable of working at a high temperature, because a cheap glass mixture is necessarily somewhat infusible, at all events where colour is considered. ”

“ In practice, particularly when the glass insulator is exposed to even a moderately damp atmosphere, the nature of the glass affects the resulting insulation or absence of insulation, in another way. Almost all varieties of glass have the property of condensing upon their surfaces a decided film or layer of moisture from the atmosphere, and, as we have seen above, glasses differ very considerably in the degree to which they display this hygroscopic tendency. ”

“ It always happens that for some special wave-lengths the substance has the power of absorbing the energy of the entering waves and converting it into heat-vibrations of its own molecules or atoms. In the most transparent and colourless glasses this process, so far as the waves of ordinary light are concerned, only goes on to a negligibly slight extent; if, however, we extend our view beyond the range of ordinary visible light, and consider the region of shorter waves that lies in the spectrum beyond the violet, we find that ordinary colourless glass becomes strongly absorbent ”

“ An attempt is sometimes made to imitate the appearance of cut glass, but the rounding of the angles during the reheating process destroys the sharpness of the effect and allows of the ready detection of the imitation, while the cheapness of the decoration when applied in the mould has frequently led manufacturers to grossly over-decorate, and, therefore, destroy all claim to beauty in their wares. ”

“ Before introducing the pipe into the opening of the tank furnace, the pipe-warmer must see that the hot end of the pipe is free from scale or dirt and must test, by blowing through it, whether the pipe is free from internal obstructions. He then places the butt of the pipe in the opening of the furnace and allows it to acquire as nearly as possible the temperature of the molten glass. When this is the case the pipe is either handed on to the gatherer, or the pipe-warmer, who is usually only a youth, may take the process one step further before handing it on to the more highly skilled workman. ”

“ The first class of defects accordingly embraces those that arise from the condition of the glass as it exists in the working-end of the furnace. Chief of these are white opaque enclosures, known as “stones.” These may arise from a variety of causes within the furnace, such as an admixture of infusible impurities with the raw materials, insufficient heat or duration of melting, leading to a residue of unmelted raw material in the finished glass, or from defective condition of the interior of the furnace, leading to contamination of the glass with small particles of fire-brick. ”

“ Copper, either as metal or oxide, added to glass in the ordinary way, always produces the green colour; but when the full oxidation of the copper is prevented by the presence of a reducing body, and the glass is cooled slowly, or is exposed to repeated heating followed by slow cooling, an intense ruby coloration is produced. In practice this colour is produced by introducing tin as well as copper into the mixture, and so regulating the conditions of melting as to favour reduction rather than oxidation of the copper. ”

“ In fact, not only does all glass absorb light, but it does this to a different degree according to the colour of the light, so that in passing through the glass a beam of white light becomes weakened in one of its constituent colours more than in the others, with the result that the emergent light is slightly coloured. Thus the purest and whitest of glasses, when examined in very thick pieces, always show a decided blue or green tint, although this tint is quite invisible on looking through a few inches of the glass. ”

“ Glass is essentially a substance that cannot be satisfactorily handled in small quantities, particularly so far as all the phenomena connected with its production and manipulation while hot are concerned; the influences of containing vessels, of furnace gases and of rapid cooling are all enormously exaggerated if ounces instead of hundredweights or tons of glass are used for experimental purposes, and these influences and others of the same nature vitally affect all the results of small-scale laboratory operations. ”

“ In pot furnaces, on the other hand, the composition of the glass can be more accurately regulated, and the molten glass itself can be more effectively protected from contamination either by matter dropping into it or by the action of the furnace gases, while in pots it is also possible to effectually melt together materials which, in the open basin of a tank, could not be kept together long enough to combine. ”

“ In practice the surface of rolled glass is always more or less dimmed by contact with the minute irregularities of table or roller, and larger irregularities of the surface arise from the buckling that occurs at a great many places in the sheet. These limitations govern the varieties of glass that can be produced by processes that involve rolling, and have led to the somewhat curious result that both the cheapest and roughest, as well as the best and most expensive kinds of flat glass, are produced by rolling processes. ”

“ An effort at producing a similar brilliance of surface on moulded and pressed articles is often made by exposing them, after they have attained their final form, to the heat of a furnace to such an extent as to soften the surfaces and allow the glass to re-solidify under the undisturbed influence of surface-tension much as it would do in solidifying freely in the first place. ”

“ The electrical properties of glass are of much greater practical importance, glass being frequently used in electrical appliances as an insulating medium. The insulating properties of glass, as well as the property known as the specific inductive capacity, vary greatly according to the chemical composition of the material. Generally speaking, the harder glasses, i.e., those richest in silica and lime, are the best insulators, while soft glasses,30 rich in lead or alkali, are much poorer in this respect. ”

“ Silver is never introduced into glass mixtures, the reason being that it is so readily reduced to the metallic state from all its compounds that it cannot be retained in the glass except in a finely-divided form, causing the glass to assume a black, metallic appearance resembling the stains produced by the reduction of lead in flint glasses. On the other hand, silver yields a beautiful yellow colour when applied to glass as a surface stain, and it is widely used for that purpose. ”

“ As we have seen above, carbon, in the form of coke, charcoal or anthracite coal, is regularly introduced, as a reducing medium, into glass mixtures containing sulphate of soda. If even a slight excess of carbon be used for this purpose, the formation of sulphides and poly-sulphides of sodium and of calcium results, and these bodies, like all sulphides, impart a greenish-yellow tint to the glass, at the same time bringing other undesirable results in their train. ”

“ The phenomena just described in reality constitute the natural limit to the range of bodies which can be obtained in the vitreous state: as we approach this limit the glass requires more and more rapid cooling through the critical range of temperature, and is thus more and more liable to devitrify during the manufacturing processes, until finally the limit is set when no industrially feasible rapidity of cooling suffices to retain the mass in the vitreous state. ”

“ The state of purity in which the various substances are commercially obtainable also enters largely into the question, since the greatest possible degree of purity in the raw materials is essential to the production of glass of good colour, or rather freedom from colour. ”

“ The glass industry as a whole, supplying some of the absolute necessaries of modern life, should be capable of offering the greatest rewards to success, and the example of other industries has shown that ultimate success is bound to reward properly-conducted and perseverant scientific research. Nowhere is this more urgently needed than in the whole field of glass manufacture. ”