Hold one end of the six-inch piece of No. 2 in the tip of the flame (Fig. 8), and turn constantly until it is just red hot. Take it out and let it cool on the blocks. Do you find that the edges are smooth?
Repeat with the other end.
Repeat with both ends of the six-inch piece of No. 4.
If thick glass is heated quickly it may crack, because the hot exterior expands more quickly than the cooler interior and produces internal strains.
Fig. 8. Making the edges smooth
The No. 6 tube is comparatively thick and should be heated gradually as follows: Hold the end in the flame for about 1 second, then withdraw it for about 1 second; hold it in the flame again for 1 second, and withdraw it for 1 second. Repeat this eight or ten times, then hold and turn it in the flame until red hot.
Fig. 9. The Blowpipe Flame
Smooth both ends of the No. 6 piece in this way.