Glass Manufacture

by Walter Rosenhain

The method of transferring the glass from the melting-pot to the rolling table differs somewhat in different works. In many cases the melting-pots themselves are taken bodily from the furnace and emptied upon the bed-plate of the rolling machine, while in other cases the glass is first transferred to smaller “casting” pots, where it has to be heated again until it has freed itself from the bubbles enclosed during the transference, and then these smaller pots are used for pouring the glass upon the rolling slab. The advantage of the latter more complicated method lies, no doubt, in the fact that the large melting-pots, which have to bear the brunt of the heat and chemical action during the early stages of melting, are not exposed to the great additional strain of being taken from the hot furnace and exposed for some time to the cold outside air. Apart from the mechanical risks of fracture, this treatment exposes the pots to grave risks of breakage from unequal expansion and contraction on account of the great differences of temperature involved. Where smaller special casting-pots are used, these are not exposed to such prolonged heat in the furnace, and are never exposed to the chemical action of the raw materials, so that these subsidiary pots may perhaps be made of a material better adapted to withstand sudden changes of temperature than the high-class fire-clay which must be used in the construction of melting pots. On the other hand, the transference of the glass from the melting to the casting-pots involves a laborious operation of ladling and the refining of the glass, with its attendant expenditure of time and fuel. Finally, the production of plate-glass in tank furnaces could only be attempted by the aid of such casting-pots in which the glass would have to undergo a second fining after being ladled from the tank, and this would materially lessen the economy of the tank for this purpose, while it is by no means an easy matter to produce in tank furnaces qualities of glass equal as regards colour and purity to the best products of the pot furnace.

The withdrawal of the pots containing the molten glass from the furnace is now universally carried out by powerful machinery. The pots are provided on their outer surface with projections by which they can be held in suitably-shaped tongs or cradles. A part of the furnace wall, which is constructed each time in a temporary manner, is broken down; the pot is raised from the bed or “siege” of the furnace by the aid of levers, and is then bodily lifted out by means of a powerful fork. The pot is then lifted and carried by means of cranes until it is in position above the rolling table; there the pot is tilted and the glass poured out in a steady stream upon the table, care being taken to avoid the inclusion of air-bells in the mass during the process of pouring. When empty, the pot is returned to the furnace as rapidly as possible, the glass being meanwhile rolled out into a slab by the machine. Except for the greater size and weight of both table and roller, the plate-glass rolling table is similar to that already described in connection with rolled plate. Of course, since the glass is poured direct from the pot, there is no chilled glass to be removed. Further, owing to the large size of sheets frequently required, the bed of the rolling table cannot be made of a single slab of cast-iron, a number of carefully jointed plates being, in fact, preferable, as they are less liable to warp under the action of the hot glass.

In arranging the whole of the rolling plant, the chief consideration to be kept in mind is that it is necessary to produce a flat sheet of glass of as nearly as possible equal thickness all over. The final thickness of the whole slab when ground and polished into a sheet of plate-glass must necessarily be slightly less than that of the thinnest part of the rough rolled sheet. If, therefore, there are any considerable variations of thickness, the result will be that in some parts of the sheet a considerable thickness of glass will have to be removed during the grinding process. This will arise to a still more serious extent if the sheet as a whole should be bent or warped so as to depart materially from flatness. The two cases are illustrated diagrammatically in [Fig. 11], which shows sectional views of the sheets before and after grinding on an exaggerated scale.

Fig. 11.—Sectional diagram illustrating waste of glass in grinding curved or irregular plate.

While it is evident that careful design of the rolling table will avoid all tendency to the formation of sheets of such undesirable form, it is a much more difficult matter to avoid all distortion of the sheet during the annealing process and while the sheet is being moved from the rolling table to the annealing kiln. Owing to the great size of the slabs of glass to be dealt with, and still more to the stringent requirement of flatness, the continuous annealing kiln, in which the glass travels slowly down a tunnel from the hot to the cold end, has not been adopted for the annealing of plate-glass, and a form of annealing kiln is still used for that glass which is similar in its mode of operation to the old-fashioned kilns that were used for other kinds of glass before the continuous kiln was introduced. These kilns simply consist of chambers in which the hot glass is sealed up and allowed to cool slowly and uniformly during a more or less protracted period. In the case of plate-glass, the slabs are laid flat on the stone bed of the kiln. This stone bed is built up of carefully dressed stone, or blocks of fire-brick bedded in sand in such a way that they can expand freely laterally without causing any tendency for the floor to buckle upwards as it would do if the blocks were set firmly against one another. The whole chamber is previously heated to the requisite temperature at which the glass still shows a very slight plasticity. The hot glass slabs from the rolling table are laid upon the bed of this kiln, several being usually placed side by side in the one chamber, and the slabs in the course of the first few hours settle down to the contour of the bed of the kiln, from which shape and position they are never disturbed until they are removed when quite cold. In modern practice the cooling of a kiln is allowed to occupy from four to five days; even this rate of cooling is only permissible if care is taken to provide for the even cooling of all parts of the kiln, and for this purpose special air-passages are built into the walls of the chamber and beneath the bed upon which the glass rests, and air circulation is admitted to these in such a way as to allow the whole of the kiln to cool down at the same rate; in the absence of such special arrangements, the upper parts of the kiln would probably cool much more rapidly than the base, so that the glass would be much warmer on its under than on its upper surface.

When the slabs of plate-glass are removed from the annealing kilns they very closely resemble sheets of rolled plate in appearance, and they are quite sufficiently transparent to allow of examination and the rejection of the more grossly defective portions; the more minute defects, of course, can only be detected after the sheets have been polished, but this preliminary examination saves the laborious polishing of much useless glass.

The process of grinding and polishing plate-glass consists of three principal stages. In the first stage the surfaces of the glass are ground so as to be as perfectly flat and parallel as possible; in order to effect this object as rapidly as possible, a coarse abrasive is used which leaves the glass with a rough grey surface. In the second stage, that of smoothing, these rough grey surfaces are ground down with several grades of successively finer abrasive until finally an exceedingly smooth grey surface is left. In the third and final stage, the smooth grey surface is converted into the brilliant polished surface with which we are familiar by the action of a polishing medium.

Originally the various stages of the grinding and polishing processes were carried out by hand, but a whole series of ingenious machines has been produced for effecting the same purpose more rapidly and more perfectly than hand-labour could ever do. We cannot hope to give any detailed account of the various systems of grinding and polishing machines which are even now in use, but must content ourselves with a survey of some of the more important considerations governing the design and construction of such machinery.

In the first place, before vigorous mechanical work can be applied to the surface of a plate of glass, that plate must be firmly fixed in a definite position relatively to the rest of the machinery, and such firm fixing of a plate of glass is by no means readily attained, since the plate must be supported over its whole area if local fracture is to be avoided. While the surface of the plate is in the uneven condition in which it leaves the rolling table, such a firm setting of the glass can only be attained by bedding it in plaster, and this must be done in such a manner as to avoid the formation of air-bubbles between plaster and glass; if bubbles are allowed to form, they constitute places where the glass is unsupported. During the grinding and polishing processes these unsupported places yield to the heavy pressure that comes upon them, and irregularities in the finished polished surfaces result. The most perfect adhesion between glass and plaster is attained by spreading the paste of plaster on the up-turned surface of the slab of glass and lowering the iron bed-plate of the grinding table down upon it, the bed-plate with the adhering slab of glass being afterwards turned over and brought into position in the grinding machine. When one side of the glass has been polished, it is generally found sufficient to lay the slab down on a bed of damp cloth, to which it adheres very firmly, although sliding is entirely prevented by a few blocks fixed to the table in such a way as to abut against the edges of the sheet. In many works, however, the glass is set in plaster for the grinding and polishing of the second side as well as of the first.