Glass Manufacture

by Walter Rosenhain

The methods of bottle making are at the present time passing through what is probably a stage of transition. Up to the middle of last century the processes in use were little better than those of the middle ages; the first step of a more modern development of the industry took the direction of improved tools and implements for carrying out the old operations. More recently a whole series of inventions have been put forward with the aim of producing bottles by entirely different and wholly mechanical processes with the object of eliminating the uncertain element of skilled labour entirely. While it must be admitted that some of the earlier of these inventions proved to be brilliantly ingenious failures, there is little doubt that here, as in other manufacturing processes, the machine-made article will ultimately supersede the hand-made product. Even now, mechanical processes are largely in use both in America and Europe, and at some recent exhibitions machine-made bottles have been shown which in every point of quality were superior to the best hand-made goods.

The first stage in the production of bottles by hand, and also for most of the machine processes, is that of gathering the requisite quantity of glass. The bottle-blower’s pipe is between 5 and 6 ft. long, and is provided with a slightly enlarged end or “nose” upon which the glass is gathered. Three gatherings are generally sufficient for the production of ordinary bottles, but for extra large bottles, and especially for carboys, heavier gatherings are necessary, and for these the gatherer must go to the furnace four, five, or even six times. When the requisite quantity of glass has been gathered on the pipe the gathering is worked and rounded by rolling it either on a flat metal plate or “marver,” or in a hollowed block made of wood or more rarely of metal; by this process the glass is formed into a well-rounded, symmetrical pear-shaped body. The blower now distends the mass gradually by the pressure of his breath, at the same time swinging the pipe, the effect of these movements being to draw the bulk of the glass downwards, leaving a thinner and colder portion having the rudimentary shape of the neck of the bottle next to the pipe. In the oldest form of the process the next stage in the production of the bottle is accomplished by the aid of a cylindrical mould of fire-clay, whose diameter is that of the external size of the finished bottle. The pear-shaped bulb of glass is for this purpose re-heated at the melting furnace, and is then placed inside the fire-clay mould. By vigorous blowing, and a rapid rotation of the pipe and glass, the bulb is forced to assume the cylindrical shape of the mould, the glass forming the neck of the bottle being at this stage of the process too cold and stiff to be further deformed. The next step is the formation of the concavity found in the base of wine and beer bottles; this is produced by pushing up the hot plastic glass that forms the bottom of the bottle as it leaves the clay mould. This is done by a second workman using an iron rod known as the “pontil,” upon which a small mass of glass has previously been gathered. This mass of glass remains attached to the bottom of the bottle, which is thus for the moment fastened both to the “pontil” and to the blower’s pipe. The blower, however, immediately detaches the bottle from the pipe at the point where the neck of the bottle is intended to end, effecting this by locally chilling the glass—a process known by the descriptive term of “wetting off.” The unfinished bottle is now attached to and handled by means of the “pontil.” The neck is softened by re-heating it over the furnace, and is then moulded into the desired shape by the aid of specially-shaped tongs. Finally a thread of glass is wound round the end of the neck to produce the thickening usually found at that point. The finished bottle, still attached to the “pontil,” is now carried to the annealing kiln, where it is placed in position and detached from the “pontil” by a sharp blow, which severs the glass that had been gathered on the “pontil” from the bottom of the bottle.

The process, in the form described above, has been obsolete for many years, improvements, consisting of appliances for facilitating the various operations, having been gradually introduced. The most important of these is the substitution of metal moulds for the fire-clay moulds of earlier times. These metallic moulds are made to open and close at will by the action of a pedal, and are designed to give the entire bottle its final shape, except for the indentation of the bottom, although this is sometimes produced by a convex piece placed on the bottom of the mould. In the formation of the neck thickening, also, important mechanical aids have become almost universal. These last consist of tongs provided with rollers and arranged to rotate about an axis that terminates in a tapered spike which enters the neck of the bottle; by pressing the tongs together so as to bring the rollers against the outside of the neck and rotating the whole, the rollers are made to form the neck thickening in an accurate and rapid manner.

Important and valuable as these improvements of the ancient process of bottle-blowing undoubtedly are, they do not touch the main disadvantages of the process—disadvantages that seriously affect the economy of the process and the well-being of the workers employed upon it. It is consequently not surprising that a great number of inventors have laboured at the problem of the purely mechanical production of bottles. A large number of patents have accordingly been taken out in connection with bottle-making machinery. The first of these to attain any favour was that devised by Ashley, but although great claims were made for it, its use has not extended. At the present time, however, there are a number of bottle-works actually at work producing bottles by mechanical means; one of the most successful of these machines is that devised by Boucher, of Cognac. The products of this machine, exhibited in Paris at the exhibition of 1900, were equal, and possibly superior, to the best hand-made bottles. The Boucher machine, although by no means entirely automatic, requires no highly-skilled labour beyond that of a workman whose duty it is to operate the various levers of the machine at the right instant and in the proper order.

The details of the machine, as set forth in the patents and other published descriptions, are somewhat complicated, and vary somewhat in the different models; the general principle and mode of operation is, however, the same in all varieties of the machine, and we shall therefore give a brief account of it here.

In the Boucher process, the glass is first gathered from the furnace, but as no blowing-pipes are required, the gathering is done on a light iron rod, thus saving the gatherer much of the labour of carrying the heavy pipes. The requisite quantity of the glass so gathered is then dropped into the first or “measuring” mould of the machine, the “thread” being cut by hand by the operator. From the measuring mould, the glass is next caused to pass into the “neck” mould; the glass flows into this mould, and is further pressed into it by the aid of compressed air, applied above the free surface of the glass. At this stage the still liquid glass has the external shape of the neck of the bottle, but the mass of glass is solid, i.e., no cavity has yet been produced in it. The formation of the cavity is next begun by the action of a plunger which is driven into the “solid” mass of glass filling the neck mould, this plunger thus punching out the passage through the neck of the bottle. As soon as the plunger is withdrawn, compressed air is admitted into the cavity so formed, and the mass of glass is at the same time inverted, and that part occupying the position of what is to be the shoulder of the bottle is allowed to descend while being blown out by the compressed air. This process of distension is limited, and the desired shape is imparted to the mass by bringing towards it a third mould, by contact with which the glass is considerably stiffened—a row of jets of compressed air, impinging on the outside of the glass forming the shoulder of the bottle, being further used to stiffen the glass, once the requisite extension has been attained. The mass has now a shape very similar to that known as a “parason” in hand bottle-blowing, and is by this time decidedly stiff. It is now introduced into the finishing mould and is blown into perfect contact with the mould by powerful air-pressure, thus attaining the proper shape of barrel and base; the indentation of the base is, however, sometimes produced on a separate machine or press. During all these operations the neck of the bottle, which was the first part to be formed, has remained firmly held in the neck mould, and all the movements that have been described are performed by means of levers actuating movements of this mould as a whole, which, of course, carry the glass with them. The last movement of the levers, which releases the bottle from the finishing mould, also opens the neck mould, and thus leaves the bottle finished and entirely free.

It will be seen that the process adopted in this machine follows as closely as possible the various stages of hand blowing, but that the mechanical movements of the machine replace the laborious and difficult technique of the blower. One such machine is capable of producing as many as 120 bottles, each weighing 1¾ lbs., per hour, but this is accomplished only by having some of the moulds in duplicate and so arranged as to come into use alternately. The machine itself is attended by one “moulder,” who operates the levers, and by a youth, who carries the finished bottles to the annealing kiln, while, of course, the services of a gatherer are also required. The appearance of a bottle works equipped with these machines is in striking contrast to that of a hand-blowing works, where the stages around the working-holes are crowded with men doing arduous work under very severe conditions of temperature and atmosphere. Finally, it must be pointed out that the use of the Boucher machine is by no means confined to the production of the cheapest kinds of bottles, but that it has shown itself especially well suited to the production of champagne and other bottles that are required to withstand a high internal pressure, the machine-made bottles showing excellent results under pressure tests. The machine is also used for the production of moulded glass-ware of white glass, since it can be adapted to the production of any kind of glass vessel that can be produced by blowing into a mould.

The annealing of bottles was formerly carried out in large chambers or kilns of very simple construction, in which the bottles were stacked as made, the kiln being previously heated to the requisite temperature: when full, the kiln was closed up in a rough temporary manner and allowed to cool naturally, thus annealing the bottles stacked within it. In this branch of glass-making also, however, the continuous annealing kiln has superseded the older kinds, and continuous kilns are now almost universal in bottle-making. In these kilns, which consist of long tunnels, kept hot at one end and having a gradually decreasing temperature as the other end is approached, the bottles are stacked on trucks which are slowly drawn through the kiln from the hot to the cold end. At the cold end the trucks are unloaded and are then returned, by an outside route, to the charging end, but of course the bottles cannot be stacked on the truck until it has actually entered the hot end of the tunnel and acquired the temperature there prevailing. In a slightly different form of kiln, the bottles are carried down the kiln on a species of conveyer belt formed of iron plates, but the principle of all these appliances is similar even when used for very different kinds of glass.

In the account of bottle manufacture given above we have referred almost exclusively to the mode of production of the ordinary bottles used for the storage of such liquids as wine, beer, spirits, etc., and we will now deal with some other branches of manufacture closely allied to these.

An important branch of glass manufacture is the production of vessels of large dimensions. Those most closely allied to ordinary bottles are the vessels known as carboys, used for the storage and transportation in bulk of chemical liquids, and especially of acids. Formerly these were blown by hand in a manner closely resembling that used for ordinary bottles, but the weight of the mass of glass to be handled by gatherer and blower is very great, while the lung-power of a blower is not sufficient to produce the great expansion required. Formerly the only aid available to the blower was the device of injecting into the hot, hollow glass body, at an early stage of the process, a quantity of water or alcohol; this liquid was immediately vapourised by the heat of the glass, and if the blower closed the mouthpiece end of his pipe by placing his thumb over it, the expansive force of the vapour so generated served to blow out the glass to the desired extent. More recently mechanical aids to the production of these large vessels have become available, first in the shape of mechanical arrangements for relieving the workmen of the full weight of the glass and pipe by providing suitable arms upon which the whole can be supported without interfering with the blower’s freedom of manipulating the pipe and glass in the desired way; further, a supply of compressed air, which can be readily connected with the pipe at any desired moment, facilitates the blowing process.