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.