The deleterious effect produced upon glass surfaces when brought into contact with relatively cold metal has already been referred to above, and it only remains to add that this is the principal difficulty with which the glass-pressing process has to contend. It is overcome to some extent by the aid of the reheating process described above; but this is only a partial remedy, and in the majority of pressed glass products the surface is “covered” as far as possible by the application of relief decorations such as grooves, spirals, and ribbings. An attempt is sometimes made to imitate the appearance of cut glass, but the rounding of the angles during the reheating process destroys the sharpness of the effect and allows of the ready detection of the imitation, while the cheapness of the decoration when applied in the mould has frequently led manufacturers to grossly over-decorate, and, therefore, destroy all claim to beauty in their wares.
CHAPTER IX.
ROLLED OR PLATE-GLASS.
In the present chapter we propose to deal with all those processes of glass manufacture in which the first stage consists in converting the glass into a slab or plate by some process of rolling. We have already considered the general character of the rolling process, and have seen that, although hot, viscous glass lends itself readily to being rolled into sheets or slabs, these cannot be turned out with a smooth, flat surface. In practice the surface of rolled glass is always more or less dimmed by contact with the minute irregularities of table or roller, and larger irregularities of the surface arise from the buckling that occurs at a great many places in the sheet. These limitations govern the varieties of glass that can be produced by processes that involve rolling, and have led to the somewhat curious result that both the cheapest and roughest, as well as the best and most expensive kinds of flat glass, are produced by rolling processes. Ordinary rough “rolled plate,” such as that used in the skylights of workshops and of railway stations, is the extreme on the one hand, while polished plate-glass represents the other end of the scale. The apparent paradox is, however, solved when it is noted that in the production of polished plate-glass the character of the surface of the glass as it leaves the rollers is of very minor importance, since it is entirely obliterated by the subsequent processes of grinding, smoothing, and polishing. Intermediate between the rough “rolled” and the “polished” plate-glass we have a variety of glasses in which the appearance of the rolled surface is hidden or disguised to a greater or lesser extent by the application of a pattern that is impressed upon the glass during the rolling process; thus we have rolled plate having a ribbed or lozenge-patterned surface, or the well-known variety of “figured rolled” plate, sometimes known as “Muranese,” whose elaborate and deeply-imprinted patterns give a very brilliant effect.
Rolled plate-glass being practically the roughest and cheapest form of glazing, is principally employed where appearance is not considered, and its chief requirement is, therefore, cheapness, although both the colour and quality of the glass are of importance as affecting the quantity and character of the light which it admits to the building where the glass is used. On the ground of cheapness it will be obvious from what we have said above ([Chapter IV.]), that such glass can only be produced economically in large tank furnaces, and these are universally used for this purpose. The requirements as regards freedom from enclosed foreign bodies of small size and of enclosed air-bells are not very high in such glass, and, therefore, tanks of very simple form are generally used. No refinements for regulating the temperature of various parts of the furnace in order to ensure perfect fining of the glass are required, and the furnace generally consists simply of an oblong chamber or tank, at one end of which the raw materials are fed in, while the glass is withdrawn by means of ladles from one or two suitable apertures at the other end. For economical working, however, the furnace must be capable of working at a high temperature, because a cheap glass mixture is necessarily somewhat infusible, at all events where colour is considered. This will be obvious if we remember that the fusibility of a glass depends upon its alkali contents, and alkali is the most expensive constituent of such glasses.
The actual raw materials used in the production of rolled plate-glass are sand, limestone and salt-cake, with the requisite addition of carbon and of fluxing and purifying materials. The selection of these materials is made with a view to the greatest purity and constancy of composition which is available within the strictly-set limits of price which the low value of the finished product entails. These materials are handled in very large quantities, outputs of from 60 to 150 tons of finished glass per week from a single furnace being by no means uncommon; mechanical means of handling the raw materials and of charging them into the furnace are therefore adopted wherever possible.
The glass is withdrawn from the furnace by means of large iron ladles. These ladles are used of varying sizes in such a way as to contain the proper amount of glass to roll to the various sizes of sheets required. The sizes used are sometimes very large, and ladles holding as much as 180 to 200 lbs. of glass are used. These ladles, when filled with glass, are not carried by hand, but are suspended from slings attached to trolleys that run on an overhead rail. The ladler, whose body is protected by a felt apron and his face by a mask having view-holes glazed with green glass, takes the empty ladle from a water-trough, in which it has been cooled, carries it to the slightly inclined gangway that leads up to the opening in the front of the furnace, and there introduces the ladle into the molten glass, giving it a half-turn so as to fill it with a “solid” mass of glass. By giving the ladle two or three rapid upward jerks, the operator then detaches the glass in the ladle as far as possible from the sheets and threads of glass which would otherwise follow its withdrawal; then the part of the handle of the ladle near the bowl is placed in the hook attached to the overhead trolley, and by bearing his weight on the other end of the handle, the workman draws the whole ladle up from the molten bath in the furnace and out through the working aperture. This operation only takes a few seconds to perform, but during this time the ladler is exposed to great heat, as a more or less intense flame generally issues from the working aperture, whence it is drawn upward under the hood of the furnace. From the furnace opening, the ladler, generally aided by a boy, runs the full ladle to the rolling table and there empties the ladle upon the table just in front of the roller. In doing this, two distinctly different methods are employed. In one, only the perfectly fluid portion of the glass is poured out of the ladle by gradually tilting it, the chilled glass next to the walls of the ladle being retained there and ultimately returned to the furnace while still hot. In the other method, the chilling of the glass is minimised as far as possible, and the entire contents of the ladle are emptied upon the rolling table by the ladler, who turns the entire ladle over with a rapid jerk which is so arranged as to throw the coldest part of the glass well away from the rest. When the sheet is subsequently rolled this chilled portion is readily recognised by its darker colour, and since it lies entirely at one end of the sheet it is detached before the sheet goes any further. Neither method appears to present any preponderating advantage.
Fig. 9.—Rolling table for rolled plate-glass.