The Mechanical Properties of Glass are of considerable importance in many directions. Although glass is rarely used in such a manner that it is directly called upon to sustain serious mechanical stresses, the ordinary uses of glass in the glazing of large windows and skylights depend upon the strength of the material to a very considerable extent. Thus in the handling of plate-glass in the largest sheets, the mechanical strength of the plates must be relied upon to a considerable extent, and it is this factor which really limits the size of plate that can be safely handled and installed. The same limitation applies to sheet-glass also, for, although its lighter weight renders it less liable to break under its own weight, its thinner section renders it much more liable to accidental fracture. In special cases, also, the mechanical strength of glass must be relied upon to a considerable extent. Gauge tubes of high-pressure boilers, port-hole glasses in ships, the glass prisms inserted in pavement lights, and the glass bricks which have found some use in France, as well as champagne bottles and mineral water bottles and syphons, are all examples of uses in which glass is exposed to direct stresses. It is, therefore, a little surprising that while the mechanical properties of metals, timbers, and all manner of other materials have been studied in the fullest possible manner, those of glass have received very little attention, at all events so far as published data go. One reason for this state of affairs is probably to be found in the fact that it is by no means easy to determine the strength of so brittle and hard a body as glass. As a consequence even the scanty data available can only be regarded as first approximations. The following data are only intended to give an idea of the general order of strength to be looked for in glass:—

Of the above figures the experiments of Winkelmann and Schott are probably by far the most reliable, but these refer to a series of special Jena glasses, selected with a view to determining the influence of chemical composition on mechanical properties, and, unfortunately, this series does not include glasses at all closely resembling those ordinarily used for practical purposes. The attempt to connect tensile and crushing strength with chemical composition was also only very partially successful; but the results serve to show that the chemical composition has a profound influence on the mechanical strength of glass, so that by systematic research it would probably be possible to produce glasses of considerably greater mechanical strength than those at present known. It must be noted in this connection that the mechanical properties of glass depend to a very considerable extent upon the rate of cooling which the specimen in question has undergone. It is well known that by rapid cooling, or quenching, the hardness of glass can be considerably increased; such treatment also increases the strength both as against tension and compression, and numerous processes have been put forward for the purpose of utilising these effects in practice. Unfortunately the “hardened” glass thus obtained is extremely sensitive to minute scratches, and flies to pieces as soon as the surface is broken, and the great internal stress which always exists in such glass is thereby relieved. All these peculiarities are, of course, dependent as to their degree upon the rapidity with which the glass has been cooled, and the aim of inventors in this field has been to devise a rapid cooling process which should strike the happy mean between the increased strength and the induced brittleness resulting from quenching. Thus processes for “tempering” glass by cooling it in a blast of steam or in a bath of hot oil or grease have been brought forward; but, although some such glass is manufactured, no very extensive practical application has resulted.

Elasticity and Ductility of Glass.—In a series of glasses investigated by Winkelmann and Schott, the modulus of elasticity (Young’s Modulus) varied from 3,500 to 5,100 tons per sq. in., the value being largely dependent upon the chemical composition of the glass. Measurable ductility has not been observed in glass under ordinary conditions except in the case of champagne bottles under test by internal hydraulic pressure; in these tests it was found that a permanent increase of volume of a few tenths of a cubic centimetre could be obtained by the application of an internal pressure just short of that required to burst the bottle—pressure of the order of 18 to 30 atmospheres being involved. This small permanent set has been ascribed to incipient fissuring of the glass, and this explanation is probably correct. On the other hand, it is in the writer’s opinion very probable that glass is capable of decided flow under the prolonged action of relatively small forces; the behaviour of large discs of worked optical glass suggests some such action, but the view as yet lacks full experimental confirmation.

The Hardness of glass is a property of some importance in most of the applications of glass. The durability of glass objects which are exposed to handling or to periodical cleaning must largely depend upon the power of the glass to resist scratching; this applies to such objects as plate-glass windows and mirrors, spectacle and other lenses, and in a minor degree to table-ware. On the other hand, the exact definition and means of measuring hardness are not yet satisfactorily settled. Experimenters have found it very difficult to measure the direct resistance to scratching, since it is found, for example, that two glasses of very different hardness are yet capable of decidedly scratching each other under suitable conditions. Resort has therefore been had to other methods of measuring hardness; the method which, from the experimental point of view, is, perhaps, the most satisfactory, depends upon principles laid down by Hertz and elaborated experimentally by Auerbach. This depends upon measuring the size of the circular area of contact produced when a spherical lens is pressed against a flat plate of the same glass with a known pressure. Auerbach himself found some difficulty in deciding the exact connection between the “indentation modulus” thus determined and the actual hardness of the glass. This method is, therefore, of theoretical interest rather than of use in testing glasses for hardness. A test of a more practical kind consists in exposing specimens of the glasses to be tested to abrasion against a revolving disc of cast-iron fed with emery or other abrasive, and to measure the loss of weight which results from a given amount of abrading action under a known contact pressure. If a number of specimens of different glasses are exposed to this test at one time, a very good comparison of their power of resisting abrasion can be obtained. It is not quite certain that this test measures the actual “hardness” of the glass, but it affords some information as to its power of resisting abrasion, and for many purposes this power is the important factor.

Hardness being, as indicated above, a somewhat indefinite term, it is not possible to give any precise statement as to the influence of chemical composition upon the hardness of glass. In general terms it may be said that glasses rich in silica and lime will be found to be hard, while glasses rich in alkali, lead or barium, are likely to be soft. It must, however, be borne in mind that rapid cooling, or even the lack of careful annealing, will produce a very great increase of hardness in even the softest glasses. The actual behaviour of a given specimen of glass will, therefore, depend at least as much upon the nature of the processes which it has undergone as upon its chemical composition.

The Thermal Properties of Glass, although not of such general importance as the mechanical properties, are yet of considerable interest in a large number of the practical uses to which glass is constantly applied. Perhaps the most important of these properties is that known as thermal endurance, which measures the amount of sudden heating or cooling to which glass may be exposed without risk of fracture; the chimneys employed in connection with incandescent gas burners, boiler gauge glasses, laboratory vessels, and even table and domestic utensils are all exposed at times to sudden changes of temperature, and in many cases the value of the glass in question depends principally upon its power of undergoing such treatment without breakage. The property of “thermal endurance” itself depends upon a considerable number of more or less independent factors, and their influence will be readily understood if we follow the manner in which sudden change of temperature produces stress and, sometimes, fracture in glass objects. If we suppose a hot liquid to be poured into a cold vessel, the first effect upon the material of the vessel will be to raise the temperature of the inner surface. Under the influence of this rise of temperature the material of this inner layer expands, or endeavours to expand, being restrained by the resistance of the central and outer layers of material which are still cold; the result of this contest is, that while the inner layer is thrown into a state of compression, the outer and central layers are thrown into a state of tension. Accordingly, if the tension so produced is sufficiently great, the outer layers fracture under tension and the whole vessel is shattered by the propagation of the crack thus initiated. From this description of the process it will be seen that a high coefficient of expansion and a low modulus of elasticity will both favour fracture, while high tensile strength will tend to prevent it. The thermal conductivity of the glass will also affect the result, because the intensity of the tensile stress set up in the colder layers of glass will depend upon the temperature gradient which exists in the glass; thus if glass were a good conductor of heat it would never be possible to set up a sufficient difference of temperature between adjacent layers to produce fracture; for the same reason, vessels of very thin glass are less apt to break under temperature changes than those having thick walls, since the greatest difference of temperature that can be set up between the inner and outer layers of a thin-walled vessel can never be very considerable. It also follows from these considerations, that if a cold glass vessel be simultaneously heated or cooled from both sides, it can be safely exposed to a much more sudden change of temperature than it could withstand if heated from one side alone; on the other hand, when very thick masses of glass have to be heated, this must be done very gradually, as a considerable time will necessarily elapse before an increment of temperature applied to the outside will penetrate to the centre of the mass. It should also be noted here, that in addition to the thermal conductivity of the glass, its heat capacity or specific heat also enters into this question, since heat will obviously penetrate more slowly through a glass whose own rise of temperature absorbs a greater quantity of heat. It will thus be seen that “thermal endurance” is a somewhat complicated property, depending upon the factors named above, viz.: coefficient of expansion, thermal conductivity, specific heat, Young’s modulus of elasticity, and tensile strength.

The coefficient of thermal expansion varies considerably in different glasses, and we can here only state the limiting values between which these coefficients usually lie; these are 37 × 10^-7 as the lower, and 122 × 10^-7 as the upper limit. These figures express the cubical expansion of the glass per degree Centigrade, the corresponding figures for steel and brass respectively being about 360 × 10^-7 and 648 × 10^-7 respectively. It should be noted that vitreous bodies of extremely low expansibility are obtainable by the suitable choice of ingredients, but in some cases these “glasses” are white opaque bodies, and in all cases they present great difficulty in manufacture, owing to the fact that alkalies and lime must be avoided in their composition.

Quite apart from the question of thermal endurance, the expansive properties of glass are of some importance. Thus when several kinds of glass have to be united, as, for example, in the process of producing “flashed” coloured glass, it is essential that their coefficients of expansion should be as nearly as possible the same; otherwise considerable stresses will be set up when the glasses, which have been joined at a red heat, are allowed to cool. On the other hand, this mutual stressing of two glasses owing to differences in their thermal expansion has been utilised for the production of tubes and other glass objects possessing special strength. 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. Moreover, if a tube in this condition be heated and then exposed to sudden cooling, the first effect of the application of cold will be a contraction of the surface layers, resulting in a relief of the initial condition of compression. These tubes are, therefore, remarkably indifferent to sudden cooling, although they are naturally more sensitive to sudden heating. In this respect they differ entirely from ordinary glass, which is considerably more sensitive to sudden cooling than to sudden heating, particularly when the heat or cold is applied to all the surfaces of the object at the same time. The special tubes made of two layers of glass above referred to are manufactured by the Jena Glass Works for special purposes, among which boiler gauge glasses are the most important. It should be also mentioned here that the remarkable thermal endurance of vitrified silica, which can be raised to a red heat and then immersed in cold water without risk of breakage, is chiefly due to its very low coefficient of expansion.

In another direction the expansive properties of glass are of importance wherever glass is rigidly attached to metal. At the present time this is done in several industrial products, such as incandescent electric lamps and “wired” plate glass. In certain varieties of incandescent lamps, metallic wires are sealed into the glass bulbs, and the only metal available for this purpose, at all events until recently, has been platinum, whose coefficient of expansion is low as compared with most metals, and whose freedom from oxidation when heated to the necessary temperature makes it easy to produce a clean joint between glass and metal. More recently the use of certain varieties of nickel steel has been patented for this purpose, since it is possible to obtain nickel steel alloys of almost any desired coefficient of expansion from that of the alloy known as “invar,” having a negligibly small expansion compared with that of ordinary steel. By choosing a suitable member of this series a metal could be obtained whose coefficient of expansion corresponds exactly with that of the glass to which it is to be united. The oxidation of the nickel steel when heated to the temperature necessary for effecting its union with the glass presented serious difficulties to the production of a tight joint, and several devices for avoiding this oxidation have been patented. In the incandescent electric lamp, although the joint between glass and metal is required to be perfectly air-tight, the two bodies are only attached to one another over a very short length. In wired plate glass, however, an entire layer of wire netting is interposed between two layers of glass, the wire being inserted during the process of rolling. Here a certain amount of oxidation of the wire is not of any serious importance, as it only appears to give rise to a few bubbles, whose presence does not interfere with the strength and usefulness of the glass; but any considerable difference of coefficient of expansion will produce the most serious results on account of the great lengths of glass and metal that are attached to each other. This factor has been neglected by some manufacturers, with the result that much of the wired glass of commerce is liable to crack spontaneously some time after it has left the manufacturer’s hands, while there is also much loss by breakage during the process of manufacture.

Thermal expansion is a vital factor in yet another of the uses of glass. Our ordinary instrument for measuring temperature—the mercury thermometer—is very considerably affected by the expansive behaviour of glass. When a mercury thermometer is warmed the mercury column rises in the stem because the mercury expands upon warming to a greater extent than the glass vessel, bulb and stem, in which it is contained. The subject of the graduations and corrections of the mercury glass thermometer is a very large one and somewhat outside the scope of the present volume; but attention should be drawn in this place to the peculiarities of the behaviour of glass that have been discovered in this connection. One of these is that when first blown the bulb of a thermometer takes a very considerable time to acquire its final volume, the result being, that if a freshly made thermometer is graduated, after some time the zero of the instrument will be found considerably changed, generally in a direction which indicates that the volume of the bulb has slightly increased. By a special annealing or “ageing” process this change can be completed in a comparatively short time before the instrument is graduated. There is, however, a further peculiarity which is prominent in some thermometers, although very greatly reduced in the best modern glasses. This becomes apparent in a decided change of zero whenever the thermometer has been exposed for any length of time to a high temperature, the zero gradually returning more or less to its original position in the course of time. With thermometers made of glasses liable to these aberrations, the reading for a given temperature depended largely upon the immediate past history of the instrument; but, thanks to the Jena Works, thermometer glasses are now available which are almost entirely free from this defect. In this connection the curious fact has been observed that glass containing both the alkalies (potash and soda) shows these thermal effects much more markedly than a glass containing one of the alkalies only.

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. Similar statements apply, in a still more marked degree, to the subject of the specific heat of glass.

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, rich in lead or alkali, are much poorer in this respect. 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. The softer glasses are much more hygroscopic than the hard ones, and the resulting film of surface moisture serves to lessen or even to break down the insulating power of the glass, the electricity leaking away along the film of moisture. In the case of appliances for static electricity, where very high voltages have to be dealt with, an endeavour is sometimes made to avoid this leakage by varnishing the surface of the glass with shellac or other similar substance, and this proves a satisfactory remedy up to a certain point. Quite recently a variety of glass has been brought forward which is peculiar in having a comparatively low electrical resistance, so that for certain purposes it can be used as an electric conductor. Although interesting in itself, this glass is not very likely to prove useful even for the limited number of applications that could be found for an electrically conducting glass, since it is very rich in alkali, and is, therefore, likely to be unstable chemically, even under the action of atmospheric agencies alone.

The most valuable and in many ways the most interesting of the properties of glass—its transparency—has not been dealt with as yet, and all mention of this subject has been postponed to the end of the present chapter, because the whole subject of the optical properties of glass will be dealt with more fully in the chapter on optical glass (Chap. XII.), so that a very brief reference only need be made to the matter here.

There can be no doubt that, in most of its practical applications, transparency is the fundamental and essential property which leads to the employment of glass in the place of either stronger or cheaper materials. By transparency, in this sense, we wish to include mere translucence also, since very frequently it is as necessary to avoid undisturbed visibility as it is to secure the admission of light. 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. of the glass used in the world is employed for purposes where transmission of light is essential to the attainment of the desired result, either from the point of view of utility or from that of beauty. It is interesting to note that the power of transmitting light is not shared by many solid bodies. Some colloidal organic bodies, such as gelatine and celluloid, possess the property to a degree comparable with glass, while certain mineral crystals, such as quartz and fluor-spar, may even surpass the finest glass in this respect; while some of the other optical properties of glass are greatly exceeded by such natural substances as the diamond and the ruby. But the very brevity of this list is in itself striking, because it must be borne in mind that transparency by no means constitutes the only common characteristic of vitreous bodies.

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. In the purest and best glasses this absorption is so small that in any moderate thickness very delicate instruments are required to show that there has been any loss of light at all; but even the best glass, when examined through a thickness of 20 in. or more, always shows the effects of the absorption of light quite unmistakably. 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. The ordinary glass of commerce, however, is far removed from even this approach to perfect transparency. The best plate glass shows a slight greenish-blue tint, which is just perceptible to the trained eye when a single sheet of moderate thickness is laid down upon a piece of white paper. When a sheet of this glass is viewed edgewise, in such a way that the light reaching the eye has traversed a considerable thickness, the greenish-blue tint of the glass becomes more apparent. By holding strips of various kinds of glass, cut to an equal length, close together and comparing the colour exhibited by their ends, a means of comparing the colours of apparently “white” glasses is readily obtained. It will be found that different specimens of glass differ most markedly in this respect. Sheet glass is, as a rule, decidedly deeper in colour than polished plate, but rolled plate is as a rule much greener—the colour of this glass can, in fact, in most cases be seen quite plainly in looking through or at the sheets in the ordinary way.

The question of how far the colour of glass affects the value of the light which it transmits depends for its answer upon the purpose to which the lighted space is to be put. Where delicate comparisons of colour are to be made, or other delicate work involving the use of the colour sense is to be carried on, it is essential that all colouration of the entering daylight should be avoided, and the use of the most colourless glass obtainable will be desirable. Again, in photographic studios it is important to secure a glass which shall absorb as small a proportion of the chemically active rays contained in daylight as possible, and special glasses for this purpose are available. Although for the present the price of these special glasses may prove prohibitive for the glazing of studio lights, their use is found highly advantageous where artificial light is to be used to the best advantage. On the other hand, for every-day purposes, the slight tinge of colour introduced into the light by the colour of ordinary sheet and plate glass, or even of greenish rolled plate glass, has no deleterious effect whatever, the majority of persons being entirely unconscious of its presence. The transmission of light by glass, its absorption, refraction, dispersion, etc., are, however, best grouped together as the “optical” properties of glass, and under that heading they will receive a fuller treatment in connection with the subject of the manufacture of glass for optical purposes.