You closed small tubes in Experiment 5 by simply heating the end in the blowpipe flame. This method does not serve for large tubes, however, because it leaves a very large lump of glass which may crack on cooling or reheating.
Fig. 30. Attaching A Handle
Practice the following method of closing a large tube; first with a piece of No. 4 tube, and then with a piece of No. 6: Attach a working handle to the end to be closed, heat the tube 1/2 inch from the end in the blowpipe flame, turn constantly, and when soft pull apart until the tube has the shape 1, Fig. 31. Heat, turn, and pull the end away to leave the tube as in 2. Heat the end and blow out until it has the shape 3. The end is now closed and the glass has about the same thickness as the remainder of the tube.
Fig. 31. Closing A Large Tube
Fig. 32. Making a submarine