CHAPTER I
Introduction and Preliminary Remarks—General Principles to be observed in Glass Working—Choice of Apparatus—Tools and Appliances—Glass.
Glass-blowing is neither very easy nor very difficult; there are operations so easy that the youngest laboratory boy should be able to repeat them successfully after once having been shown the way, there are operations so difficult that years are needed to train eye and hand and judgment to carry them out; but the greater number of scientific needs lie between these two extremes. Yet a surprisingly large number of scientific workers fail even to join a glass tube or make a T piece that will not crack spontaneously, and the fault is rather one of understanding than of lack of ability to carry out the necessary manipulation.
In following the scheme of instruction adopted in this handbook, it will be well for the student to pay particular attention to the reason given for each detail of the desirable procedure, and, as far as may be, to memorise it. Once having mastered the underlying reason, he can evolve schemes of manipulation to suit his own particular needs, although, as a rule, those given in the following pages will be found to embody the result of many years' experience.
There is a wide choice of apparatus, from a simple mouth-blowpipe and a candle flame to a power-driven blower and a multiple-jet heating device. All are useful, and all have their special applications, but, for the present, we will consider the ordinary types of bellows and blowpipes, such as one usually finds in a chemical or physical laboratory.
The usual, or Herepath, type of gas blowpipe consists of an outer tube through which coal gas can be passed and an inner tube through which a stream of air may be blown. Such a blowpipe is shown in section by Fig. 1. It is desirable to have the three centring screws as shown, in order to adjust the position of the air jet and obtain a well-shaped flame, but these screws are sometimes omitted. Fig. 1, a and b show the effects of defective centring of the air jet, c shows the effect of dirt or roughness in the inside of the air jet, d shows a satisfactory flame.
Fig 1
For many purposes, it is an advantage to have what is sometimes known as a "quick-change" blowpipe; that is one in which jets of varying size may be brought into position without stopping the work for more than a fraction of a second. Such a device is made by Messrs. Letcher, and is shown by e, and in section by f Fig. 1. It is only necessary to rotate the desired jet into position in order to connect it with both gas and air supplies. A small bye-pass ignites the gas, and adjustment of gas and air may be made by a partial rotation of the cylinder which carries the jets.
For specially heavy work, where it is needed to heat a large mass of glass, a multiple blowpipe jet of the pattern invented by my father, Thomas Bolas, as the result of a suggestion derived from a study of the jet used in Griffin's gas furnace, is of considerable value. This jet consists of a block of metal in which are drilled seven holes, one being central and the other six arranged in a close circle around the central hole. To each of these holes is a communication way leading to the gas supply, and an air jet is arranged centrally in each. Each hole has also an extension tube fitted into it, the whole effect being that of seven blowpipes. In order to provide a final adjustment for the flame, a perforated plate having seven holes which correspond in size and position to the outer tubes is arranged to slide on parallel guides in front of these outer tubes.
Fig. 2
The next piece of apparatus for consideration is the bellows, of which there are three or more types on the market, although all consist of two essential parts, the blower or bellows proper and the wind chamber or reservoir. Two patterns are shown in Fig. 2; a, is the form which is commonly used by jewellers and metal workers to supply the air blast necessary for heating small furnaces. Such a bellows may be obtained at almost any jewellers' supply dealer in Clerkenwell, but it not infrequently happens that the spring in the wind chamber is too strong for glass-blowing, and hence the air supply tends to vary in pressure. This can be improved by fitting a weaker spring, but an easier way and one that usually gives fairly satisfactory results, is to place an ordinary screw-clip on the rubber tube leading from the bellows to the blowpipe, and to tighten this until an even blast is obtained.
Another form of bellows, made by Messrs. Fletcher and Co., and common in most laboratories, is shown by b; the wind chamber consists of a disc of india-rubber clamped under a circular frame or tied on to a circular rim. This form is shown by Fig. 2, b.
The third form, and one which my own experience has caused me to prefer to any other, is cylindrical, and stands inside the pedestal of the blowpipe-table. A blowpipe-table of this description is made by Enfer of Paris.
There is no need, however, to purchase an expensive table for laboratory use. All the work described in this book can quite well be done with a simple foot bellows and a quick-change blowpipe. Nearly all of it can be done with a single jet blowpipe, such as that described first, or even with the still simpler apparatus mentioned on page 84, but I do not advise the beginner to practise with quite so simple a form at first, and for that reason have postponed a description of it until the last chapter.
Glass-blowers' tools and appliances are many and various, quite a number of them are better rejected than used, but there are a few essentials. These are,—file, glass-knife, small turn-pin, large turn-pin, carbon cones, carbon plate, rubber tube of small diameter, various sizes of corks, and an asbestos heat reflector. For ordinary work, an annealing oven is not necessary, but one is described on page 60 in connection with the special cases where annealing is desirable.
Fig. 3 illustrates the tools and appliances. a is an end view of the desirable form of file, and shows the best method of grinding the edges in order to obtain a highly satisfactory tool. b is a glass knife, shown both in perspective and end view, it is made of glass-hard steel and should be sharpened on a rough stone, such as a scythe-stone, in order to give a slightly irregular edge. c is a small turn-pin which may be made by flattening and filing the end of a six-inch nail. d is the large turn-pin and consists of a polished iron spike, about five inches long and a quarter of an inch diameter at its largest part. This should be mounted in a wooden handle. e and f are carbon cones. A thin rubber tube is also useful; it may be attached to the work and serve as a blowing tube, thus obviating the necessity of moving the work to the mouth when internal air pressure is to be applied. In order to avoid undue repetition, the uses of these tools and appliances will be described as they occur.
Fig. 3
Glass, as usually supplied by chemical apparatus dealers is of the composition known as "soda-glass." They also supply "hard" or "combustion" glass, but this is only used for special purposes, as it is too infusible for convenient working in the ordinary blowpipe flame.
Soda-glass consists primarily of silicate of sodium with smaller quantities of silicate of aluminum and potassium. Its exact composition varies. It is not blackened, as lead glass is, by exposure to the reducing gases which are present in the blue cone of a blowpipe flame, and hence is easier for a beginner to work without producing discolouration.
Further notes on glasses will be found on page 55, but for ordinary purposes soda-glass will probably be used.