Glass Manufacture

by Walter Rosenhain

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.