The third join, by which the lead-glass tube is joined to the soda-glass is made in stages similar to those in which the soda-glass and uranium-glass were joined; but the internal seal is most conveniently made by sliding a length of tubing over the lead-glass and fusing this tubing to the large diameter soda-glass tube to which the lead-glass is already joined. The first stage of this operation is illustrated by d. When this seal is completed, the end of the soda-glass tube is drawn off and sealed as shown in e, and at this stage a side tube or branch is joined on. The sealed end of the outer and large diameter soda-glass tube is heated until it contracts and fuses to the enlargement that has previously been joined to the lead-glass tube, and the end is burst out as shown in f. Another length of soda-glass is then joined on to the burst-out end, and this length of soda-glass tubing is drawn out to a thin-walled contraction; the non-contracted part is expanded to form the bulb, and a small exhaustion branch made on the side, the drawn-out portion being cut off, and an electrode, previously prepared by coating a part of its length with a suitable enamel, is introduced. The tube is tilted to keep the electrode away from the drawn-out end, which is melted off and sealed. A small perforation is made with a hot platinum or iron wire in the sealed end, the electrode is shaken into position, and the sealing is completed as explained on page 42.
The remainder of the tube, that is to say the lead-glass tube and the bulb on the other side of the middle bulb, is completed in a similar manner.
Summary of Conditions Necessary for Success in Glass-Blowing.
For the convenience of the student, it may be well to summarise the chief essentials for success in glass-blowing, and at the same time to add such brief notes on the various methods as may seem desirable.
Adjustment of Blowpipe.—The air jet should be clean internally, and so centered as to give a flame having a well-defined blue portion, the tip of the flame should not be only slightly luminous but purple in colour. In the case of a blowpipe burning oil or wax fuel the flame may be a trifle more ragged without disadvantage.
Bellows and Blowing.—The bellows should be adjusted to deliver air at constant pressure, either by insertion of a tap or, better, by attention to the wind reservoir if necessary. The movement of the foot in blowing should be steady, not jerky.
Heating Glass.—The tube or rod should be heated cautiously until it has reached its softening point in its thickest part. Steady rotation of the glass during the heating is almost essential.
Blowing a Bulb or Expanding a Join.—Prolonged heating is necessary in order that the thick parts may be heated completely through. Blowing should take place by stages, in order that the thin parts, which tend to expand first, have time to cool. The thick parts can then be expanded by further blowing and thus a bulb or expansion of even thickness can be obtained.
Cutting Glass.—The most useful method for general use is by means of the file or glass-blowers' knife. Either file or knife must be kept sharp by grinding. Neither file nor knife should be used on hot glass. The diamond and wheel cutter are useful for cutting sheet-glass, and when the diamond is employed a singing noise is an indication of a satisfactory cut.
Leading a Crack.—A crack may be led in any desired direction by means of a bead of hot glass or a small gas flame. The glass which it is desired to crack should be heated at a point slightly in advance of the crack, which will extend in the direction of the source of the heat.