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.