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Updated: July 20, 2025
The two pieces which are to be joined are bent so as to just pass each other, marked at the right point with the glass-knife, and cut there, preferably with a small bead of hot glass. One or both of these tubes are now warmed to the softening point in such a place that the tubes can be made to meet properly, and the two cut ends pressed together.
=Cutting the Glass.= For this purpose a "glass-knife" is preferred to a file, if the glass is cold: if it is hot a file must always be used, and its edge slightly moistened to prevent drawing the temper. The glass-knife is simply a flat piece of hard steel, with the edges ground sharp on an emery wheel. The bevel of the edge should be from 30 to 60 degrees.
An old flat file can easily be ground into a suitable knife. The glass-knife makes a narrower scratch than the file but appears more likely to start the minute crack which is to cause the tube to break at that point, and the break is more likely to give a good square end.
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