When you cut a tube either with a file or a glass cutter the edge of the end will be sharp but not smooth. All you have to do to round it is to heat it in the flame until it begins to melt, when the glass will run and make a smooth edge.

To Border the Ends of a Tube

To border a tube means to spread the edge out a little all around so that a cork can be put in easier or a liquid poured out better.

Fig. 88. bordering the end of a tube

Take a piece of charcoal and shave it down with your knife to form a cone the size you want the border to be. Heat the end of the tube by turning it in the flame until it is quite soft and then push in the charcoal cone, as shown in Fig. 88.

To Seal Off the End of a Tube

To close one end of a tube, or seal it off as it is called, heat it in a fish-tail burner just as I explained for bending glass.

When the glass begins to get soft pull the two ends of the tube apart until the walls of the tube are drawn together as shown at A in Fig. 89. Cut a nick in the thin solid part with a file and break it off. If you want a closed end as shown at B heat the end of the tube you have sealed off and press it down on a piece of iron.

Fig. 89. Sealing Off The End Of A Tube