Glass Manufacture

by Walter Rosenhain

As already indicated, the first step in the production of such an object as a tumbler consists in gathering a suitable quantity of glass on the pipe and blowing it into a small bulb. This bulb is blown out to the proper size and is then elongated by gently swinging the pipe. The next step is the flattening of the lower end of the bulb by gently pressing it on the “marver” or flat plate provided for such purposes; in this way the flat bottom of the glass is formed, and the bulb now has the shape of the finished glass, but remains attached to the pipe by a shoulder and neck. The earliest practice was to separate the tumbler from the pipe at such a point as to leave the tumbler of the correct length, the remaining operation consisting in holding the glass, first fixed to a pontil for the purpose, into the furnace so as to heat the broken edge; this edge was thereby rounded off, and the brim of the glass could be widened or otherwise shaped by rotating the glass or pressing it in or out by the aid of pieces of wood. In modern practice, however, this is not usual, the glass being separated from the pipe well above the shoulder and annealed in this shape. Subsequently the glass is finished in a trimming room or workshop by being cut off at the desired point and having the rough edge rounded off by the aid of a blowpipe flame. The cutting-off operation is carried out in a great variety of ways, the most usual being by the action of heat applied locally and suddenly, either by the aid of specially-shaped flat blowpipe flames or by an electrically-heated wire. Machines for carrying out this operation, as well as the subsequent rounding of the edge automatically, are in use, but the latter process is sometimes replaced by slightly grinding and polishing the edges.

Fig. 8.—Sectional diagram of the evolution of a tumbler.

The evolution of an ordinary tumbler, as just described, and as illustrated diagrammatically in [Fig. 8], is typical of the whole process of hollow-glass blowing, but of course the number of operations, as well as the care and skill involved in each step, increases rapidly as the form of the vessel becomes more complex; in the highest class of work a very considerable element of artistic taste and judgment on the part of the operative also becomes essential, for, although the form of the object as well as the choice of colour and ornamentation are chosen by the designer, the blower has to translate the drawing of the designer into glass, and although his skill enables him to attain a considerable degree of fidelity in his rendering, many details remain at his own option, and the proper management of these is no small factor in the success of the whole work.

In this connection mention should perhaps be made of the application of colour and other decorations to this kind of glass. A very considerable range of effects of this kind is now available to the glass-worker. In the first place the body of the glass used for the production of the articles in question may be coloured by the addition of suitable colouring materials to the molten glass or raw materials, as explained in [Chapter XI.], but this procedure has very obvious limitations; where the article is built up of glass from several gatherings—as, for example, is the case in an ordinary wine-glass, where the bowl, leg and foot are each made of separate gatherings—it is possible to use glass of different colours for these different parts, and this is commonly done in the production of wine glasses having ruby or green bowls and white legs and feet. A further modification in the application of colour is obtainable by taking up two or more gatherings on the same pipe and superposing a large gathering of white glass on a smaller one of coloured glass; this is analogous to the process of “flashing” sheet glass, described in [Chapter X.] and this process lends itself to a variety of manipulations resulting in the distribution of the coloured layer of glass in almost any desired manner over the object in hand. The principal objection to this process, however, lies in the fact that pots of molten glass of all the colours desired must be kept available to the blower at the same time, and this is not easily arranged for in any reasonably economical manner. For this reason, and also because the manipulations are simpler, coloured glass intended for application to blown glass-ware is generally used in the form of short rods previously prepared; these rods are suitably heated, and the coloured glass can then be applied to the article in hand at any desired place and in as small or large a quantity as required. If the two glasses thus brought into contact are properly related to one another as regards chemical composition and physical properties, they blend very readily and perfectly, and the result is quite as good as could be obtained by using the coloured glass in the molten condition. Other decorations, such as gilding or other metallic lustres and also various kinds of iridescence, are produced upon the finished glass. Metallic lustres are obtained by placing upon the surface of the glass, and slightly fusing into it a layer of particles of the actual metal. In some cases this is done by rolling the glass vessel, while still hot, in a mass of metallic foil of the kind desired, when a sufficient quantity readily adheres; in other cases the metal is applied in the form of a flux or glaze containing a large proportion of an easily-reduced compound of the metal, and this is afterwards reduced to the metallic state by the action of heat, sometimes aided by that of smoke or other reducing gases. An iridescent surface is produced upon certain varieties of glass by the corrosive action of acid vapours; in fact, in localities where the atmosphere is tainted with sulphur fumes it is quite usual to see an iridescent lustre on the surface of ordinary window glass. There are, of course, numerous other means of decorating blown and other glass, such as cutting, engraving, etching, silvering, etc., but it would lie beyond the scope of the present volume to deal with these, since they are outside the field of actual glass manufacture.

In the production of hollow glass-ware by hand, the glass-blower avails himself to the full of the property so characteristic of glass of assuming a pasty or viscous condition when suitably heated; by raising or lowering the temperature of his material, the blower can at will render it stiffer or more fluid; by blowing he can distend it, draw it out by the aid of gravity or centrifugal action, or he can mould it with the aid of rods and tongs of suitable shape, while at times he allows it to fall or festoon under its own weight while held aloft. With all these manipulations at his disposal, the skilful operative is able to work the glass to his will and to fashion objects of great variety and beauty, but it should be noted that objects produced by hand in this way will bear the mark of the processes employed in their production in the fact that they do not possess the extreme regularity of size and shape which are associated with machine-made articles; there is a certain natural variability in the exact shape of curves and festoons that is foreign to the products of mechanical processes. For some purposes this variability is a disadvantage, while to some minds it appears as a defect, and methods have been devised for facilitating the production of strictly uniform glass-ware by the use of moulds as an aid to the work of the glass-blower. While undoubtedly reducing the value and beauty of the ware from the purely artistic standpoint, these aids to hand-work have rendered possible an immense expansion of the entire industry, since, with the use of moulds, presentable glass-ware can be produced by hands far less skilled than those required for pure hand-work.

In the description given above of bottle-blowing by hand we have already seen an example of the use of moulds in aiding the blower to form his object to the desired size and shape. Much more complicated and decorative objects can, however, be produced by the use of moulds. Such objects as globes and shades for gas, oil and electric lamps, when of a light substance and suitable shape, are usually produced by blowing bulbs of glass into moulds, where they acquire the general shape as well as the detailed decorated surface configuration which they afterwards present. Here again the body remains a closed vessel, and is only opened and trimmed to the final shape at the end of the operation when all the blowing and moulding have been done. Articles blown in this way very frequently show “mould marks,” since the contact of the hot glass with the relatively cold surface of the mould results in a certain crinkling or roughening of the glass, much as in the process of rolling. This effect can be minimised by dressing the interior surfaces of the moulds with suitable greasy dressings, whose chief property should be that they do not stick to the hot glass and leave little or no residue when gradually burnt away in the mould; the proper care of the moulds and their maintenance is in fact the first essential to successful manufacture in this as well as in the pressed-glass industry. Even under the most favourable conditions, however, the surface of glass blown into moulds is not so good as that of hand-blown articles which have never come into contact with cold materials, and therefore retain undiminished the natural “fire polish” which glass possesses when allowed to cool freely from the molten state. 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. Unfortunately this process cannot be carried out without more or less softening the entire article, so that skilful manipulation is required to prevent serious deformation of the object, while a certain amount of rounding off in all sharp corners and angles cannot be avoided.

The air-pressure required to bring the whole of the surfaces of a large and possibly complicated piece of glass into contact with the surfaces of the mould is sometimes very considerable, and the lung-power of the blower is often insufficient for the purpose; in many works, therefore, compressed air is supplied for the purpose, arrangements being employed whereby the operative can quickly connect the mouthpiece of his pipe with the air-main, while he can accurately control the pressure by means of a suitable valve. The Sievert process of moulding by the aid of steam pressure has already been described.

Although the evolution of the industry scarcely followed this path, it is not a large step to pass from a process in which air-pressure is used to drive viscous glass into contact with a mould to a process in which the pressure of the air is replaced by the pressure of a suitably-shaped solid plunger, and this is essentially the widely-used process of glass pressing. In the first instance this mode of manufacture is obviously applicable to solid or flat and shallow articles which could not be conveniently evolved from the spherical bulb which stands as embryo of all blown glass; at first sight it would seem in fact as though the process must be limited to articles of such a shape that a plunger can readily enter and leave the concave portions. By the ingenious device, however, of pressing two halves of a closed or nearly closed vessel simultaneously in two adjacent moulds and then pressing the two halves together while still hot enough to unite, it has been made possible to produce by the press alone such objects as water-jugs, for example, into which a plunger could not possibly be introduced when finished. The process of pressing being a purely mechanical one and requiring no very elaborate plant and little skilled labour, has placed upon the market a host of cheap and extremely useful articles, thus serving to widen very considerably the useful applications of glass. On the other hand, the process has been and is still used to some extent for the production of articles intended to imitate the products of other processes such as hand-blown and cut glass, with the result that a great deal of glass has been produced which cannot possibly be classed as beautiful and much of which can lay as little claim to utility.

The essential feature of the process of glass pressing consists, as already indicated, in forcing a layer of glass into contact with a mould by the pressure of a mechanically actuated plunger. For this purpose a suitable mould and plunger as well as a press for holding the former and actuating the latter are required. The moulds are generally made of a special quality of close-grained cast-iron, and they are kept trimmed and dressed in much the same manner as the moulds used for blowing (except that the latter are sometimes made of wood). For the purpose of facilitating the removal of the finished article, the moulds are generally made in several pieces which fit into one another and can be separated by means of hinges. A very important point about these moulds is that the various pieces should fit accurately into one another, since otherwise a minute “fin” of glass will be forced into every interstice, and the traces of these fins will always remain visible on the finished article; the very perfect fit required to entirely prevent the formation of such fins is, of course, scarcely attainable in practice except in the case of new moulds, so that the traces of fins are generally to be found on all pressed articles, and serve as a ready means of identifying these products when an attempt is made to imitate better classes of glass-ware by their means. The presses used in this process are generally of the hand-lever type; power presses could no doubt be used, but it is contended that the hand-press has a very great advantage in allowing the operator to judge by touch when sufficient pressure has been exerted, and this is an important consideration, since an excessive pressure would either force the glass out of the mould altogether or would be liable to burst or injure the mould seriously. The actual presses consist of vertical guides and levers for controlling the movement of the plunger and a table for holding the moulds, and in some cases a system of cranks and levers for opening and closing the moulds. The process of pressing is exceedingly simple. The proper quantity of glass is gathered from the pot on a solid rod and dropped into the mould. The thread of glass which remains between the glass in the mould and that remaining on the iron is cut off with a pair of shears, and then the plunger is lowered into the mould and allowed to remain there until the glass has stiffened sufficiently to retain its shape, when the plunger is withdrawn. In this proceeding it will be seen that the glass is forced into intimate contact with the relatively cold surfaces of mould and plunger, and while undergoing this treatment the glass must remain sufficiently plastic to readily adapt itself to the configuration of the mould. It is therefore not surprising to find that the pressing process can only be used successfully with glass of a kind specially adapted for it. Certain varieties of flint glass and some barium glasses are used for this purpose, but the greater quantity of pressed glass, particularly as produced on the Continent, is made of a lime-alkali silicate containing considerable quantities of both soda and potash and relatively little lime; while sufficiently resistant for most purposes, this glass is particularly soft and adaptable while in the viscous condition.