In the previous chapter we have followed in outline the process of fusion and fining of glass, leaving the molten material ready for working up into the final shape. Up to that point the process is very similar in all kinds of glass, although the furnaces, pots and utensils employed vary considerably, as do also the temperatures to which the materials are heated at various stages. The working processes, however, differ entirely from one class of product to another, as obviously the process employed for the production of a sheet of plate-glass can have little in common with that used in the manufacture of a wine-glass. On the other hand, the modes of working hot glass are not so numerous as the products that are produced, so that we find very similar appliances and manipulation recurring in various branches of the industry. For that reason we propose to deal here with the principal methods of manipulating glass, leaving the details of each method as applied to special purposes to be discussed in connection with the special product in question.

The first stage in the working of all glass is the removal of a suitable quantity of molten glass from the furnace. Practically only three methods are available, viz., ladling, pouring and gathering. If we think of a familiar substance of physical properties somewhat resembling those of glass, we may take thick treacle and suppose it confined in a jar or bottle; there are three obvious ways of extracting it from the bottle: we may ladle it out with a spoon, or we may pour it out by tilting the whole bottle, or we may dip a spoon or fork into the thick liquid, slowly draw it out and turn it round as we do so, thus bringing out on the spoon or fork a round adherent mass or “gathering” of treacle. In the case of molten glass, the process of ladling is by far the simplest, but it has certain very decided limitations and disadvantages. These arise from the fact that a ladle cannot be introduced into molten glass without contaminating the whole mass of glass, at any rate with numerous air bubbles. The metal of the ladle carries with it a considerable amount of closely adherent air which is partially detached while in contact with the hot glass, so that both the contents of the ladle and the glass remaining in the furnace are contaminated. These bubbles might perhaps be avoided if hot ladles were used, but in that case the glass would adhere to the surface of the metal, and each ladle would require laborious cleaning after each time that it was used. In practice, therefore, ladling is only used for the production of those classes of glass where the presence of a certain number of air-bells is not injurious, and the ladles are kept cold by immersion in water after each time of use. The use of the cold ladle has, however, the further disadvantage that a certain quantity of the glass withdrawn in the ladle is very considerably chilled by contact with the cold metal, and is thus too stiff to undergo the further processes satisfactorily—this chilled glass has, therefore, to be rejected from each ladleful; this not only involves loss of glass, but also necessitates the separation of this spoilt glass from the rest.

The general process of rolling requires little treatment here. Two essentially different processes are used; in one the glass is thrown on a flat table and rolled out by a moving roller passing along the table; in the other the glass passes between two moving rollers, and the sheet so formed is received on a moving table or slab. The former mode of rolling is used for the production of the ordinary rolled plate glass; if the surface of both table and roller is smooth, the glass also has a comparatively smooth surface, but the surface is far from being level or free from irregularities. It has been found that it is quite impossible to prevent these irregularities, which appear to arise from the buckling of the glass against the iron surfaces with which it comes into contact; when rolled, the glass is too stiff to recover its true, smooth surface under the influence of surface tension, so that it retains all the marks of roller and table—nor can the roller be made perfectly smooth, since in that case it appears to slip over the glass and does not roll it out properly. All efforts, therefore, to produce a glass having a true and smooth surface by direct rolling have failed, and are likely to fail, so long as tables and rollers are made of materials similar to those now in use. The process of rolling on a stationary table is, however, used for the manufacture of plate-glass; but here the slab as rolled has still the rough, uneven surface similar to that of ordinary “rolled plate,” and this is removed and replaced by a true polished surface by the mechanical processes of grinding and polishing. The second mode of rolling, i.e., with two or more “stationary” rollers and a moving table, is used for the production of rolled plate having special surface features or patterns; the variety of rolled glass known as “figured rolled plate,” having a deeply imprinted pattern, is produced in this way. This method requires much more complicated mechanical appliances, some of which are still protected by patent rights.

Ladling being thus limited to the production of inferior kinds of glass, the better varieties are dependent upon either gathering or pouring. The former process is limited as regards the quantity of glass that can be dealt with in one piece, although surprisingly large quantities can be gathered upon a single pipe; the great masses of glass, however, that are required for the production of modern polished plate could not be handled in this way, and the method of pouring is accordingly adopted. For this purpose either the pots in which the glass has been originally melted, or others specially designed for this purpose, and into which the molten glass has been transferred, are removed bodily from the furnace by the aid of powerful mechanical appliances; they are then carried by overhead cranes to the place where the glass is to be rolled into the form of a plate, and there the pot is tilted and the molten glass is allowed to run out and to form a pool on the rolling table, the passage of the great roller ultimately rolling the pool out into a sheet much as dough is rolled out with a rolling-pin. This process is obviously only possible with pots or crucibles of a suitable size, and is, moreover, very destructive to these pots, since they are exposed to such great variations of temperature. In the case of tank furnaces, numerous devices have been patented for allowing the glass to flow out over a sill or weir of suitable size, ready to be rolled or drawn into the form of sheets or slabs; but none of these devices have, so far as the writer is aware, found their way into practice; the reason for this probably lies in the fact that it is not easy to find a material which will present a smooth face to the outflowing glass, such materials as fire-clay leading to contamination from detached fragments, while chilled metal leads to local chilling of the glass. Unless, therefore, the various processes of drawing glass into sheets direct from the furnace undergo very material improvement, the laborious process of gathering is likely to retain its importance even in the production of such large objects as sheets of window glass.

In its essence the process of gathering consists in introducing into the glass a heated iron rod or tube to which a small quantity of glass is allowed to adhere; rod and glass are removed from the furnace together, and the small adherent ball of glass is allowed to cool so far as to become stiff enough to carry its own weight. The rod with its adherent ball is then again dipped into the glass, where a fresh layer of glass attaches itself to the ball already on the rod. The whole is again withdrawn, allowed to cool down, and then dipped into the molten glass again to gather a fresh quantity. This cycle of operations is repeated until the desired quantity of glass is attached to the rod or tube. These operations, particularly when weights of thirty or forty pounds of glass have to be gathered, require the exercise of a great deal of skill and care; the introduction of the gathering into the molten glass is each time liable to produce air-bells which would spoil the whole mass of glass or would contaminate the contents of the crucible, while subsequently the mass of hot glass adhering to the rod or pipe tends to run down and even to drop off entirely if not properly checked by suitable rotation of the pipe. Further, the manual labour and exposure to heat involved for the operator all tend to increase the cost of such work. Mechanical aids have been invented, and some of these are in actual use, but they are chiefly confined to mechanism for relieving the operator of the great weight of the gathering in its later stages.

Just as ladling is nearly always preliminary to rolling, so gathering is usually the preliminary to some blowing process, although the blowing is often combined with and sometimes replaced by the mechanical pressing of the glass. Where the glass is to be blown, the gathering is always made on a glass-maker’s pipe. This is an iron tube from 4 to 6 ft. long, provided at one end with a wooden casing to serve as a handle, and with a suitably arranged mouthpiece for blowing. The shape of the lower or “butt” end of the pipe depends upon the character and size of the objects to be blown; for small articles the pipe must be narrow and light, but for heavy sheet-glass the butt of the pipe is extended into a conical mass whose base is from 2 to 3 in. in diameter. The bore of the pipe at both ends also depends upon the class of work for which it is intended. The first stage of all blowing processes consists in the formation of a hollow sphere by blowing into the pipe, the pressure of the breath being as a rule sufficient to cause the gradual distension of the hot mass of glass. From this rudimentary hollow sphere the various shapes of blown articles are then evolved by a series of manipulations which vary very widely in different branches of manufacture. They generally consist, however, in gradually changing the shape of the mass of glass by the pressure either of hand tools or of specially prepared moulds or blocks against which the glass is held or turned, either with or without simultaneous blowing into the pipe. The extent to which the aid of such moulds and blocks is invoked varies continuously from the production of the hand-made vase or glass to the moulded bottle; in the former, practically only hand tools, whose shape bears no direct resemblance to that of the finished article, are employed, while in the latter the elongated hollow mass of glass is placed inside a mould, and internal air-pressure is used to press the glass into contact with the mould from which the shape of the finished bottle is thus directly derived.

The art of the blower further takes the fullest advantages of the peculiar physical properties of glass while in the heated viscous condition, the material being made to flow under the action of gravity and centrifugal forces, as well as under the pressure of the breath, the glass being held aloft, twirled or swung about to ensure the production of the various shapes required. For the great majority of such purposes the unaided manipulations of the operator are sufficient, but various mechanical aids are used to facilitate the more laborious stages of the work, while for the simpler forms that are required in very great numbers, such as bottles, the whole of the operations are now carried out by automatic machines. Of the more usual mechanical aids at the disposal of the glass-blower, we have already mentioned hand-tools, blocks, and moulds of various kinds. Next in importance to these is the use of compressed air for blowing large or heavy articles; the pressure available by the human breath is very limited, and the volume of air that can be thus delivered is not very large, while the constant use of the lungs for such a purpose is trying for the workman. In many works, therefore, air under pressure is supplied to the benches or stages where the blowing is done, and the blowers’ pipes can be coupled to this air-supply by means of flexible connections when required. The principal difficulty lies in the correct regulation of the air-pressure for each special purpose; but this difficulty has been overcome by the use of delicate valves under the control of each blower, who can thus regulate the pressure to his own exact requirements. Such a system, of course, requires some little practice on the part of the men using it, but when they have become accustomed to the working of the plant the results achieved are decidedly better and more regular than those obtained by mouth blowing. Besides the use of compressed air supplied in the way just indicated, several other devices are in use to aid the blower in producing the requisite pressure in the interior of the hollow bodies he is producing. The simplest of all these consists in utilising the expansive force of the air enclosed in the hollow body when that body is exposed to heat. Thus, for instance, in blowing a cylinder of sheet-glass, if the blower holds his thumb over the aperture of his pipe, and brings the closed end of the cylinder near the hot “blowing hole,” the heat which softens that end of the glass will also act upon the enclosed air, and will very rapidly produce such an expansive effect as to burst open the softened end of the cylinder, and this means of opening the closed ends of the cylinder is frequently employed in practice. It is, of course, obvious that any other expansive fluid might be employed in a similar manner, and in some blowing processes it has long been the practice to introduce a small quantity of water into the interior of the hollow body, when the rapid expansion of the steam produced thereby is utilised for the purpose of generating the requisite internal pressure. This use of the expansive force of steam generated by the heat of the hot glass body has received great development at the hands of Sievert in Germany, whose process is described in Chapter VII.

Whatever mechanical aids are employed to facilitate the various stages of the process, all glass blowing involves a series of operations requiring considerable skill, while the whole manner of dealing with the glass is essentially extravagant of material, except perhaps in the production of bottles or flasks having narrow mouths. The reason for this latter statement lies in the fact that by blowing it is only possible to produce closed or nearly closed hollow bodies or vessels; thus a blown wine-glass or tumbler is formed with a hood or dome closing in the open top of the glass, and this hood or dome has subsequently to be removed by subsidiary processes, such as cutting off by the aid of strong local heat or by grinding, and the cut edge has to be provided with a smooth finish. In the case of comparatively small articles like glasses the loss involved from this cause is not so very great, but were large flat bowls or dishes to be produced by blowing, the loss in the dome or covering would be very serious. This difficulty is entirely avoided by the process of pressing glass. We have already indicated the manner in which moulds are used for the production of the desired shape in the case of bottles, etc., but in these cases, where the final object is to be a hollow vessel, the glass is readily forced into contact with the mould by means of internal air—or steam—pressure; in the process to which we are now referring, however, the hot glass is forced into contact with the external mould by means of an internal plunger which is forced downward with considerable force. By this means, flat or shallow bodies can be produced without the preliminary formation of a completely closed vessel, while it is obvious that by the use of suitable moulds, complicated and elaborate shapes can be produced. It is true, of course, that pressed articles do not show the same smooth and brilliant surface which is characteristic of the fire-polish of blown articles, while the facility with which elaborate surface ornamentation can be applied by this process has not tended to artistic refinement in design, but the great majority of cheap and useful glass articles of domestic use have been made available by the development of the pressing industry.

In the ordinary course, pressed glass is produced direct from the molten material, which is introduced into the presses either by gathering or by means of ladles, but for some special purposes glass is brought into its final shape by mechanical pressure after having first been allowed to solidify and having then been specially re-heated to undergo the pressing or moulding process. This is principally done in the case of the best kinds of optical glass, where the molten glass is first allowed to cool in the actual crucible and is then broken up into lumps of a suitable size, from which the more defective portions can be rejected, the more perfect portions only being heated up again in special kilns and then forced to take the desired shape by being pressed—sometimes with hand tools only and sometimes by the aid of powerful presses—into moulds of the required shape. Small lenses, however, for which the requirements of quality are not so high are sometimes pressed direct from small gatherings taken from the molten glass in the crucible.