A process of producing hollow glass vessels of very large size by purely mechanical means has, however, been introduced during recent years by P. Sievert, of Dresden. By the methods of this inventor, glass vessels of quite unprecedented size—such as bath-tubs freely accommodating full-grown men—can be produced. For this purpose the glass is spread out on the surface of a large cast-iron plate, provided with numerous small holes through which steam or compressed air may be blown when desired. The slab of viscous glass, when properly spread over this plate, is clamped down against it all around the outside edge by means of a suitably-shaped iron collar, which holds the glass in air-tight contact against the plate beneath. The whole iron plate, with the slab of glass clamped to it, is now turned over, so that the glass hangs down under the plate. The glass immediately begins to sag under its own weight, and is assisted in this tendency by a suitable blowing of steam or air into the space between the plate and the glass. In blowing bath-tubs in this way the glass is allowed to distend downwards until the desired depth is attained, when further distension is arrested by bringing a flat supporting plate under the glass, which is pressed against this flat plate by the pressure of the air, thus forming the flat bottom of the tub. In this process the outline of the object is determined by the shape of the clamping bars or plate that fix the edges of the hot glass against the iron plate described above, and by this means almost any desired shape can be given to objects of simple form.
It is obvious that this process can also be employed for blowing a hollow body into contact with a mould of any desired form and forcing the hot glass to take the exact shape of the mould; for smaller bodies, however, the blowing in of separately generated steam is not required, the heat of the molten glass itself being used to generate the necessary steam. For this purpose the requisite quantity of glass is dropped on the surface of a wet slab of asbestos. On this surface the glass remains floating upon a layer of steam, which is constantly renewed by the intense heating action of the hot glass on the water contained in the asbestos below. The moulds used in this process are provided with a sharp edge or lip, and as soon as the glass has spread into a slab of sufficient size, the inverted mould is brought down upon the glass and pressed against it. The sharp lip or edge of the mould forces the glass into close contact with the asbestos under it all around the edge of the mould, thereby enclosing the space existing between the rest of the glass and the wet asbestos. The heat of the glass continues to generate steam at a rapid rate, but now the steam can no longer escape from under the glass around the edges, and therefore blows the glass upwards into the mould, ultimately forcing the glass into intimate contact with the surface of the mould; when this is accomplished, the pressure of the steam rises rapidly, and ultimately lifts the entire mould and glass sufficiently to allow the excess steam to escape—and this is the sign that the blowing is complete. The whole process takes only a very few seconds, and is very successful when applied to suitable glass and used with moulds of proper shape. It is, of course, obvious that ordinary narrow-mouthed bottles could not be produced in this way, but wide-mouthed bottles and jars are made in this manner, although the chief utility of the process lies in the production of comparatively shallow articles, which are not of a shape that lends itself to pressing.
CHAPTER VIII.
BLOWN AND PRESSED GLASS.
In many ways very similar to the processes employed in the production of bottles are those used in the manufacture of all hollow glass vessels that are produced by blowing, either with or without the aid of moulds. Apart from the actual shapes of the articles themselves, however, the principal difference between bottles and the better classes of hollow glass-ware lies in the composition and quality of the glass itself. In this respect all grades of manufacture are to be met with, from the light-coloured greenish or bluish glass used for medicine bottles to the most perfectly colourless and brilliant “crystal” or flint glass. This gradation in the perfection of the glass represents a corresponding gradation in the care bestowed upon the choice of raw materials and the various manipulations of melting the glass. As we have seen, for the commonest kinds of bottles, where colour and quality are immaterial, all kinds of fusible materials can be utilised, loamy or ferruginous sands and refuse glass of all kinds being employed. Where somewhat higher requirements have to be met, rather purer sands have to be used as sources of silica, while lime and alkali must be introduced in purer forms, the alkali in the shape of the cheapest qualities of salt-cake and the lime in that of lime-stones reasonably free from iron and magnesia. Finally, for the best qualities of glass the purest sand obtainable is used, being often specially washed to remove all loamy matter, while the alkali is introduced in the form of carbonate, a chemical product which in its better qualities is practically free from injurious impurities. In these high-class products two very distinct kinds of glass are met with. One class, of which the Bohemian “crystal” is the highest example, is chemically of the nature of an alkali-lime silicate, the alkali in the case of the Bohemian glass being potash; the other variety of glass contains no lime, its place being taken by lead, typical of this class being English flint glass. In some varieties of glass, lead is also replaced, partially or entirely, by barium, but this material is chiefly used for the manufacture of pressed glass.
The higher grades of quality in glass, which thus require increased refinement in the raw materials, also demand increased refinement in the furnaces and appliances employed in their melting. The tank-furnace, which holds the field in bottle manufacture, is scarcely met with in the production of the better qualities of hollow glass-ware; medicine bottles and other articles of moderate quality might be produced in tanks, but the quantity of glass required for such purposes is seldom large enough to justify such large plant. For the best qualities of colourless glass-ware, however, the tank-furnace could not be used on account of the fact that both as regards colour and freedom from defects, the product of a tank-furnace is never equal to the best product of pot-furnaces. For flint-glass, indeed, covered pots or crucibles must be used in order to adequately protect the molten glass from the reducing action of the furnace gases and from contamination by dust. The materials of which the pots are constructed are also chosen with a view to avoiding all risk of introducing colouring or otherwise injurious impurities from that source.
In all processes for the production of hollow glass-ware, the glass or “metal” is taken from the pot by the process of gathering which has already been described; where blown articles are to be produced, as distinct from pressed goods, the initial stage is always the formation of a small hollow globe or bulb at the end of the glass-blower’s pipe. The subsequent manipulations depend upon the nature of the article to be produced. The article may either be made entirely by hand work, or rather “chair” work, as it is usually called, or the manipulations may be facilitated and the product cheapened—while its character is, of course, also modified—by the aid of moulds, which are used to bring the object to its proper shape and to impress upon it certain decorative mouldings or markings. As we have already seen, ordinary bottles are now always blown with the aid of moulds, and the same applies to medicine bottles, lamp chimneys, and the bulbs for electric light; in connection with lamp-chimneys it should be noted that they are blown in moulds in the form of cylindrical bottles with a flat bottom and a domed top, the ends being subsequently cut off.
Many of the cheaper varieties of tumblers and glasses are also blown in moulds, but they can be, and sometimes are, produced by hand, and as their manufacture is typical of that of all hand-blown hollow ware, we shall now describe it in some detail as an example of this class of work.
The implements used by the glass-blower and his assistants for this work are few and simple. The largest item is the glass-blower’s bench or chair, which is simply a rough wooden bench provided with two projecting side-rails or arms. When finishing a piece of work the blower sits on this bench, and the pipe lies across the two rails in front of him in such a position that by rolling it backwards and forwards along the rails he can readily keep the pipe in gentle rotation. In addition to the ordinary blower’s pipe and a “pontil” or rod for attaching small quantities of glass whereby the piece in hand can be held, the only other tools used by the blower are a number of shears and pincers of various shapes which serve for cutting off, pressing in, and distending the glass as required, a flat board and a stone or metal plat or “marver” being also used for the purpose of moulding the glass.