Hi all,
I'm new here, and have been browsing, and I noticed a few posts on the subject of cutting glass tubing. This is how it's done by glassblowers:

• Score with tungsten carbide, or diamond – the score should be on the line of cut, and min. 1/8 circumference – longer is better
• wet score with water (saliva works fine) to prevent 'healing'
• touch small, hot end of glass rod to one end of score
• wait, applying light pressure. the score will crack and run around the tube
• if tubing doesn't separate on its own, apply axial tension, with very slight bend away from crack (in the 'opening' sense)
• Voila! The cut edge will need grinding/fire polishing to finish.

Small diameters (up to about 10 – 12mm) can be cut by scoring and pull/bending, without the need for heat, as long as they are long enough to get a good grip. Otherwise, heat is normally applied to the score by heating the end of a small dia. glass rod to molten and white-hot (oxy/propane, although for very small dia. rod, propane alone will get there) and transferring quickly to one end of the score line. Soldering irons aren't hot enough, I wouldn't think, neither burning meths, although a petroleum lighter fluid might be worth a try. Also worth trying would be a small blowtorch – larger dia. tubing is often cut using a long score and a very small oxy/propane flame while the work is rotated in a glass lathe.

Hope this is of some use – happy to answer questions if you have 'em.

Sean

(Glassblower, of sorts.)

Don't know about old glass – the thing about glass flowing isn't true, though, I'm afraid. Old window glass was made by dipping from a furnace and spinning out on the end of an iron – the windows were cut radially from the disc, which was thicker at the centre than the edges. Glaziers wisely put the thicker, heavier part at the bottom of the window.