The field of glass-manufacture is so wide and the number and variety of its products so great, that in the limited compass of this volume it is impossible to fully enumerate them all; there are, however, a certain number of these products which, while of considerable importance in themselves, yet do not fall readily under any of the headings of the preceding chapters. A short space will therefore be devoted to some of these in this place.

Glass Tubing.—A widely-useful form of glass is that of tubes of all sizes and shapes, ranging from the fine capillary tubes used in the construction of thermometers to the heavy drawn or pressed pipes that have been employed for drainage and other purposes. The process of manufacture employed varies according to the size and nature of the tube that is required. Thus lamp-chimneys are really a variety of tube, used in short lengths and made of relatively wide diameter and thin walls. These are not, however, ordinarily made in the form of long tubes cut into short sections, but—as has already been mentioned—they are blown into moulds in the form of a thin-walled cylindrical bottle, whose neck and bottom are subsequently removed. By this process the various forms of chimneys for oil-lamps, having contractions at certain parts of their length, can be readily produced.

The articles more strictly described as glass tubes are, however, produced by a process in which actual blowing plays only a very minor part. A gathering of suitable size is taken up on a pipe, a very small interior hollow space is produced by blowing into the pipe, and then the gathering is elongated by swinging the pipe in a suitable manner. The end of the elongated gathering furthest from the pipe is then attached to a rod or “pontil” held by a second workman, and the two men then proceed to move apart, drawing out the gathering of glass between them. According to the bore and thickness of wall required in the tube, the men regulate the speed at which they move apart; the thinner the tube is to be the more rapidly they move, in order to draw the glass out to a sufficient extent before it hardens too much. The rate of drawing must, of course, also be adapted to the nature of the glass in question, and this will vary very widely. For the production of the smaller bored tubes the men find it necessary to separate at a smart trot, while heavy tubes such as are used for gauge-glasses, are drawn of hard glass by a very gradual movement. In some cases, the setting of the glass, when the tube has attained the desired thickness, is hastened by the aid of an air-blast, or—in more primitive fashion—by boys waving fans over the hot glass. In any case, suitable troughs are provided for receiving the tube when drawn, and from these the tube is taken to an annealing kiln to undergo this necessary operation.

The glass used for the production of tubing varies very widely according to the purpose for which the product is intended. Almost any of the more usual varieties of glass can be readily drawn out into tubes, and the choice of the kind of glass to be employed is therefore left to other considerations. Tubing required for the use of the lamp-worker, i.e., for the production of instruments or other articles by the aid of the glass-blower’s blow-pipe, must have the capacity of undergoing repeated cooling and heating without showing signs of crystallisation (devitrification), while reasonable softness in the flame is also required. For this purpose, also, glass containing lead is not admissible, since this would blacken under the influence of the blow-pipe flame. Soda-lime glasses rather rich in alkali are most frequently used for these purposes; one consequence of their chemical composition, however, is that such glass tends to undergo decomposition when stored for any length of time, more especially in damp places. Frequently this decomposition only manifests itself on heating the glass in a flame, when it either flies to pieces or turns dull and rough on the surface. Such glass is sometimes said to have “devitrified,” but this is not really the case; what has actually happened is that the atmospheric moisture has penetrated for some little distance into the thickness of the glass, probably hydrating some of the silica; on heating, this moisture is driven off, with the result that either a few large cracks, or innumerable fine ones, are formed. In the latter case these do not readily disappear when the glass is softened and the dull, rough surface is left at the end of the operation.

For purposes where the glass is to be exposed to high temperatures, tubing made of so-called “hard glass” is employed. This is practically a form of Bohemian crystal glass, the chemical composition being that of a potash-lime glass rather rich in lime. To some extent this Bohemian hard glass has been superseded by the special “combustion tube” glass manufactured by Schott, of Jena. This is a very refractory borosilicate glass containing some magnesia; it certainly withstands higher temperatures than hard Bohemian glass, and is rather less sensitive to changes of temperature; on the other hand, it has the inconvenient property of showing a white opalescence when it has once been heated, and this, after a time, renders the glass completely opaque.

For many purposes, where heat-resisting qualities are chiefly required, ordinary glass has now a formidable rival in the shape of vitrified silica, which is now available as a satisfactory commercial product. This substance offers the great advantage that for most ordinary purposes it may be regarded as entirely infusible, since the intense heat of an oxygen-fed flame is required to soften or melt the silica. Further, vitreous silica has an extremely low coefficient of expansion, and appears also to have a rather high coefficient of thermal conductivity. The result is that tubes and other articles made of this material possess an astonishing amount of thermal endurance (see Chapter II.).

A white-hot tube or rod of this material can be plunged into cold water with impunity, and no special care need be exercised in heating or cooling articles made of this substance, unless articles of great size and thickness are involved, and even with these only little caution is needed. The only disadvantages which must be balanced against the great advantages just named lie in the relatively high cost of the articles and in their somewhat sensitive behaviour to certain chemical influences. As regards cost, vitreous silica is at present available in two different forms; in the first form it resembles ordinary glass very closely in appearance, the shape and finish of the tubes and vessels of this kind having undergone very great improvements quite recently. This silica glass has, in fact, been worked from molten silica in a way more or less analogous to that in which ordinary glass is worked, the great extra cost of the silica ware being due, in part, at all events, to the extremely high temperature required for melting and working this material; ordinarily, in the production of the class of silica ware now referred to, this heat is generated by the liberal—and therefore expensive—use of oxygen gas. In great contrast to this glass-like, transparent silica ware is the other form in which this material is available. This is a series of products obtained from the fusion of silica in special forms of electric furnace; in this ware the minute bubbles so readily formed in the fusion of all forms of quartz are not even partially eliminated, and by their presence—often in the form of long-drawn-out, capillary hollows—they impart to this ware its very characteristic milky appearance. The price of this product, which is mostly used in the form of tubes, although such articles as basins, crucibles, and even muffles of considerable size are available, is much lower than that of the transparent variety, being in fact decidedly lower than that of the best porcelain; on the other hand, even this price is considerably above that of the best glass tubing.

Apart from the question of cost, the use of silica ware is further limited by its sensitiveness to all forms of basic materials. Thus alkaline solutions cannot be allowed to come into contact with this substance, since they attack it vigorously, especially when warm. At high temperatures all basic materials produce a rapid attack on silica ware, the silica, in fact, behaving as a strongly acid body at and above a red heat. The attack which occurs when such a substance as iron or copper oxide is allowed to come into contact with heated vitrified silica is, in fact, so rapid that a tube is completely destroyed in a few minutes, the formation of silicates resulting in the cracking and disintegration of the whole piece. While, therefore, silica ware, especially in its cheaper forms, undoubtedly possesses great advantages and possibilities, its use must be carried on with careful reference to its chemical nature.

Vitreous silica, in addition to the uses and advantages just named, has also an interest from the optical point of view; this arises from the fact that it is transparent to short (ultra-violet) light waves to which all ordinary varieties of glass are completely opaque. Quite recently, the Jena works have produced special glasses which are more transparent to these ultra-violet rays than ordinary glass, but even these fall far short of silica in this respect. This property of transparence to ultra-violet light is utilised in two widely different directions. One of these is in the production of ultra-violet light when required for medical or other special purposes; a most energetic source of such rays is available by the use of tubes of vitrified silica within which the mercury-vapour arc is produced. In another direction the employment of quartz lenses makes it possible to take advantage of the optical properties of ultra-violet light in connection with microscopy; for the purpose of constructing a perfect optical system, crystalline quartz would be useless, since its property of double refraction would interfere hopelessly with the performance of the lenses. This is now overcome by the use of vitreous silica lenses, in the case of the “ultra-violet microscope,” as made by Carl Zeiss, of Jena. So far, however, it has only been possible to produce quite small pieces of vitreous silica sufficiently free from bubbles to be used for optical purposes. The great difficulty lies not so much in merely melting the quartz down as in freeing it from the air-bubbles enclosed within it; the course usually adopted with glass, of raising the temperature and allowing the bubbles to rise to the surface, becomes impossible in this case, because the silica itself begins to vapourise and even to boil vigorously at temperatures not very far above its melting point. Quite recently, however, two American workers have claimed to be able to overcome this difficulty by the use of both vacuum and high pressure applied at the earlier and later stages of the fusion process respectively, so that it may shortly be possible to produce vitreous silica in large and perfectly clear blocks.

We have already indicated that glass tubing and rod form the basis upon which the glass-worker, with the aid of the blow-pipe or “lamp,” fashions his productions, which, of course, include a great number of scientific instruments and appliances used more especially in the field of chemistry. In another direction also glass tubing serves as a basis for a branch of the glass industry; this is the manufacture of certain classes of glass beads, which are formed by cutting up a heated glass tube of suitable diameter and colour into short, more or less spherical sections. In some cases the colour of the beads is secured by using glass of the desired tint, but in other cases the beads are made of colourless glass, and a colouring substance is placed in the interior of the bead.

Solid glass rods are also employed for a variety of purposes; their mode of manufacture is exactly analogous to that of tubing, except that the gathering is drawn out without having first had a hollow space produced at its centre by the blower. In its most attenuated form glass rod becomes glass thread or fibre; this is produced by drawing hot glass very rapidly, the resulting thread being wound on a large wheel. At one time this material found considerable use, since it was found possible to spin and weave the thinnest glass fibres into fabrics which could be used for dress purposes. It is not, however, to be regretted that this fashion has neither extended nor survived, since it was certainly liable to produce serious injury to health. It is a well-known fact that there are few more injurious or even dangerous substances to be inhaled into the human throat and lungs than finely-divided glass; glass fibre, moreover, when subjected to constant bending and wear, is bound to undergo frequent fracture, and the atmosphere of a ball-room, for example, in which several such dresses were worn would soon be contaminated with innumerable fine, sharp particles of glass which would produce an injurious effect on those inhaling them. At the present time glass fibre is used for little else than the “glass wool” required for certain special purposes in chemical laboratories.

Fused quartz or silica fibres, of extreme tenuity, but of relatively very great strength, are employed in many scientific instruments, where their extreme lightness and perfect elasticity and freedom from what is known as “elastic fatigue” renders them of very great value. These fibres are not drawn from a mass of molten silica, as is done with glass, but are produced by attaching a nail or bolt to a small bead of fused silica produced by the aid of an oxygen-fed blowpipe; the nail or bolt is then suddenly shot away down a long passage or similar space by means of a cross-bow, drawing a very fine fibre of silica with it; the most difficult part of this operation, however, consists in finding and handling the fibres thus produced.

Artificial Gems.—The fact that pieces of suitably-coloured glass can be made to show a superficial, but sometimes more or less deceptive, resemblance to precious stones, has led to the manufacture of imitation jewels of all descriptions. The glass used for this purpose is usually a very dense flint-glass whose high refractive index facilitates the imitation which is aimed at. The external shapes of gems are, of course, readily imitated by cutting and grinding the glass, while the requisite colours are attainable by means of the colouring materials described in Chapter XI. To a casual observer the difference in sparkle and brilliance which arises from the difference between the refractive index of the heavy flint-glass (about 1·8) and that of minerals (which ranges from 1·7 to 2·2) is not readily apparent, but closer examination will at once reveal the difference. The determination of the optical constants by means of a refractometer would at once reveal the true character of the imitation, but an even readier test is that of hardness. The dense flint-glass is naturally soft, and is readily scratched by most of the harder minerals, while the precious stones, more particularly garnets, rubies and diamonds, are very hard. If an attempt is made to scratch an ordinary sheet of window-glass, it will be found that most real precious stones will do so readily, while flint-glass imitations will fail to make more than a slight mark, which is more smear than scratch. The test by determining the specific gravity is also obviously applicable, since the flint-glass will readily betray its presence by its high density (over 4).

In quite a different class from the imitation gems made of cut flint-glass are the artificial gems, which in nature and composition are exact reproductions of natural gems, but which have been produced by artificial processes. As far as the writer is aware these are only found in any large numbers in the case of the ruby, but in that case, at all events, it is said that the production of the artificial crystals is at least as costly as the purchase of the natural stones. 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.

Chilled Glass.—In all the processes of glass manufacture described in the present book, annealing has always played an important part. The glass, after it has undergone its last treatment under the influence of heat, is subjected to a gradual cooling process with the object of freeing it from the internal strains which it would otherwise retain, and which would, ordinarily, endanger its existence and interfere with its use. It is, however, well known that surfaces of glass subjected to such internal strains as result in a compressive stress on the glass near the surface, are less liable to injury, and are apparently stronger than when the glass is annealed and the stresses are removed. On the other hand, glass surfaces under tension are extremely delicate and fragile. In some respects, therefore, glass which has not been annealed may appear to be stronger than the annealed product. The well-known case of the Rupert’s drop is an example of this kind. Rupert’s drops are produced by dropping molten glass into water; they generally take the form of a more or less spherical body having a long tail, tapering off into a thread, attached to it. Such a Rupert’s drop may be struck with a heavy hammer, and will safely resist a blow that would splinter a similar body made of annealed glass. If, however, the surface be scratched, or the tip of the tail be broken off, the entire “drop” breaks up, sometimes with a violent explosion, into minute fragments. Numerous inventors, among whom De la Bastie and Siemens figure most conspicuously, have endeavoured to utilise these properties of chilled glass, not exactly by endeavouring to produce that extreme degree of internal strain which is characteristic of the Rupert’s drop, but by producing what they describe as “tempered” glass, in which the internal strains have been reduced by less violent cooling to such an extent as to retain some of the advantages of the hardened, internally strained condition while approximating more or less to the safer state of annealed glass. At one time articles of this kind were frequently seen as curiosities, such as tumblers that could be dropped on the floor without breaking, etc., but these articles generally ended by receiving a slight scratch or chip and promptly falling into fragments. As a matter of fact, however, some tempered glass is actually manufactured by the firm of Siemens at the present time for special purposes. De la Bastie’s process was tried in England, and some success was claimed for it; but it is not in commercial operation at the present time, and never appears to have attained any great importance.

Massive Glass.—Enthusiasts for the extension of the use of glass have endeavoured to apply it to a great variety of purposes, including the construction of buildings and the paving of streets. In the former case, which was exemplified at the Paris Exhibition of 1900, advantage was taken of the light-transmitting power of the material, but although the buildings erected with large blocks of cast glass were not displeasing in effect, this use has not found any considerable extension. For paving purposes, the hardness and durability of glass are the only useful qualities, and here also—although several trials have been made in France—no signs of any considerable application of the new products are as yet visible. What has been said above with reference to the injurious character of glass dust applies, further, to glass pavements, since their natural wear would result in the formation of considerable quantities of this dust. The advocates of glass paving, however, suggest that the hardness of glass would greatly reduce the actual amount of wear, and that consequently the dust would be reduced considerably. This is a matter which prolonged experience alone can decide, but it does not seem obvious that glass blocks should wear more slowly than stone setts made of good granite, for example. On the other hand, the glass blocks could probably be produced more cheaply, since the labour of cutting to size would be obviated by casting the blocks to the desired dimensions.

Water-glass, or silicate of soda or potash is perhaps scarcely to be classed under the heading of “Glass Manufacture” at all, but it bears a certain relationship to glass in several ways. Thus one of the modes of manufacturing water-glass is by the fusion of sand and alkali in tank furnaces somewhat resembling those used for glass production; the fused silicate, moreover, solidifies as a vitreous mass, in which respect it also resembles such substances as borax, etc. The uses of silicate of soda and potash are, however, so far removed from the field of glass-manufacture that we cannot enter into them here.

In concluding this chapter, we wish to describe one more product of the glassworks, and this includes some of the most impressive and splendid examples of the glass-maker’s art. These are the great mirrors and lenses by whose aid our lighthouses and searchlights send forth their powerful beams of light. Although these objects are called “mirrors” and “lenses,” since they fulfil the functions of such optical organs, yet in their nature and mode of manufacture they are so far removed from the glass used for the production of other kinds of lenses that they could not be included under the heading of “optical glass.”

The characteristic feature in the manufacture of optical glass is the manner in which each separate pot or melting is allowed to cool down and to break up into irregular fragments which are subsequently moulded to the desired shape. Were it attempted to manufacture the large glass bodies required for lighthouse purposes in this manner, the cost would approximate to that of the large discs used for telescope objectives, and this would of course be entirely prohibitive. The requirements as regards colour, homogeneity and freedom from other defects, which must be met in lighthouse lenses, are further not nearly so stringent as those which are essential in ordinary optical work of good quality. The reason for this difference arises from the fact that lighthouse lenses and searchlight mirrors are used merely to impart a desired direction to a beam of light, and not for the purpose of producing sharply-defined images; slight irregularities in the glass are therefore not of such serious importance.

Lighthouse glass can therefore be produced by rather less elaborate means; although every care is taken to make the glass as perfect as possible, it is brought into approximately the desired form by casting the molten glass in iron moulds of the proper shape. When removed from these moulds and annealed, the glass is fixed on large revolving tables and ground and polished to the final shape of lenses and annular lens-segments as required for the various types of Fresnel lighthouse lenses. In this way complete rings, forming annular lenses, are produced up to 48 inches diameter. Rings of larger size are usually built up of a number of segments, and these built-up rings sometimes have a radius as large as 7 feet. For the majority of lighthouse lenses, it should be added, a hard soda-lime glass having a refractive index of 1·50 to 1·52 is used, but for special purposes a dense flint-glass having a refractive index of 1·63 is employed.

Mirrors for searchlight purposes are of very varied forms and sizes, the shape depending largely upon the particular form of beam which they are designed to project. For many purposes a parabolic form is required, while in others, where a flat, fan-shaped beam is to be produced, a form having an elliptical section in a horizontal plane and a parabolic section in the vertical plane is required. In most cases these mirrors are produced by bending plates of glass, previously raised to the necessary degree of heat, over suitably shaped moulds, the surface being subsequently re-polished to remove any roughness resulting from the bending process. Another type of mirrors is that known as “Mangin,” which has two spherical surfaces placed eccentrically in such a way that the centre of the mirror is considerably thinner than the periphery; in this type of mirror the reflecting action of the back surface is modified by the refracting action of the front surface, but both are spherical, and can therefore be accurately ground and polished by the usual mechanical means. Such mirrors are manufactured of single pieces of glass up to 6 feet in diameter.