Fig. 10.
Greater strength and elegance will be secured by preserving the external diameter of the tube unchanged throughout, as shown in [Fig. 10]. For this purpose heat the tube with the pointed flame, if it be small, or in the brush flame if it be of large size, constantly rotating it till the glass softens and the sides show an inclination to fall together, when this occurs, push the two ends gently towards A. If the tube should become too much thickened at A, the fault may be corrected by removing it from the flame and gently pulling the two ends apart till it is of the proper size. If the bore at the contracted part of the tube should become too much reduced, it may be enlarged by closing one end of the tube with a small cork, and blowing gently into the open end after sufficiently heating the contracted part. The tube should be rotated during blowing or the enlargement produced may be irregular.
Fig. 11.
When the external diameter of the tube is to be increased as well as its bore diminished, press together the ends of a tube heated at the part to be contracted, as already described, and regulate the size of the bore by blowing into the tube if at any time it threatens to become too much contracted.
Widening Tubes.—Tubes may be moderately expanded at their extremities by means of the charcoal cone (see [Bordering], [p. 31]). They may be slightly expanded at any other part by closing one end and gently blowing into the open end of the tube, after softening the glass at the part to be widened before the blow-pipe. But the best method of obtaining a wide tube with narrow extremities ([Fig. 11]) is to join pieces of narrow tube AA to the ends of a piece of wider tube B of the desired dimensions. The method of performing this operation is described under [welding], on [pp. 39]-[47].
