A handbook of laboratory glass-blowing

by Bernard D. Bolas

The fragments of glass may be melted together on the end of a clay pipe-stem, care being taken to avoid trapping air bubbles as fresh fragments are added to the molten mass. When a sufficient quantity of glass has been accumulated, the viscous mass may be drawn out into a rod by bringing another pipe-stem into contact with the hot mass, rotating both pipe-stems steadily, and separating them until a rod of the desired size has been obtained.

If, on the other hand, it is desired to produce a tube from the mass of heated glass, the mass should be blown hollow before the pipe-stems supporting it are separated.

Methods of Manufacture.—When the student has familiarised himself with the more common operations and processes used in glass-blowing, he will be in a position to increase his skill and knowledge of special methods by a critical examination of various examples of commercial work. There are few exercises more valuable than such an examination, combined with an attempt to reconstruct the stages and the methods by which the article chosen for examination was made.

Obviously, it is impossible to give full details of all constructions in a small text-book; but it is easy to give an example of the constructional methods employed in the making of almost any piece of light blown-glass apparatus, and these methods should prove of special value when apparatus of a new pattern has to be evolved for the purposes of research. That is to say, one designs the apparatus required, applies known methods of construction as far as possible, and, by the examination of commercial apparatus having similar features, evolves the new methods required. For an exercise in such a process of reconstruction we may well take an ordinary commercial vacuum tube, such as that shown by a, Fig. 17.

Fig. 17

In the tube from which this drawing was made, it was found that the spiral in the middle bulb was of a slightly yellowish colour and gave a green fluorescence when the electric discharge was passed through the tube; that is to say, the spiral is made of uranium-glass, which is usually a soda-glass containing trace of uranium, and hence differing slightly in composition from the ordinary glasses. The two enclosed tubes which are bent into a series of S bends gave a pink fluorescence, which indicates lead-glass; and the remainder of the tube fluoresced with an apple-green colour; this suggests ordinary soda-glass. We have, therefore, a piece of apparatus in which three dissimilar glasses are joined, while, at the same time, that apparatus contains a number of internal seals, and it is not probable that the dissimilar glasses will have their coefficients of expansion so nearly alike as to permit of a stable internal seal being made if one part of the seal consists of a glass differing from that of the other part.

These considerations lead us to a closer examination of the joins where the dissimilar glasses are introduced, and we find that in no case is the internal seal made between dissimilar glasses, but that a soda-glass extension is joined on to both the uranium-glass tube and the lead-glass tubes at a point about half an inch before the internal seal commences. Careful examination of these joins shows that the change from one glass to another is not abrupt but gradual. Such a transitional joint may be made by taking a length of soda-glass tubing, sealing the end and fusing a minute bead of the other glass on to the sealed end, the end is then expanded and another bead of the other glass added, this bead is expanded and the operation is repeated, thus building up a tube, and, finally, the tube of the other glass is joined on to the end of this.

We are now concerned with the question of the insertion of the uranium-glass spiral into the bulb (see p. 38). Obviously the spiral is too large to pass through the necks of the bulb, and it is difficult to imagine that the spiral was obtained by the insertion of a length of straight tubing which was bent after entering the bulb; therefore, the only remaining method is that the spiral was made first and the soda-glass extensions fastened on, and that the bulb was blown, cut in halves and the spiral inserted, and the two halves were then rejoined. That this was actually the case is confirmed by traces of a join which are just visible round the middle of the bulb. The insertion of the spiral and the making of the first internal seal are shown by b, and c.

There is one detail in making the second join of the spiral to the bulb which calls for attention, and the small branch, similar to an exhaustion branch, at the side of the bulb provides a clue to this. If an attempt were made to complete the second internal seal through a closed bulb it would be impossible to obtain a good result, as the air-pressure in the bulb would not be under control when once union was effected, and further heating of the air in the bulb would cause expansion and perforate the wall near the second internal seal; we therefore make a small branch which can be left open and through which such air-pressure as may be found necessary can be maintained.