I’ve tried this a couple of times and while I do get the glass to break at the string line, it never breaks cleanly; there are always cracks in the edge of the glass. Is there a trick to getting a smooth break?
You almost never get a clean cut with that kind of method. Some bottles have just the right kind of glass to break cleanly and need only a little grinding to get a smooth cut line, but that’s rare. If you can score the glass before using the string (or just heating the line with a candle or a cigarette lighter) it works a little better.
yes scoring a line all the way around with a file corner or glass cutter works. heat a spot on the line and it will crack.
I saw a YouTube demo of this:
score a line around the bottle, taking care to go just once around the bottle, not one and a quarter.
Start the water in the kitchen sink running cold (slowly).
Drizzle boiling water from a teakettle (slowly) over the scored section.
Run the bottle through the stream of cold water from the tap.
The trouble with YouTube demos is that you don’t know how many failed takes they had before the one they published.
I had to cut an open-ended cylinder of glass from a bottle/jar when I attempted to make a small Stirling engine (that project was not a success - but that’s another story).
I got through several dozen bottles in my attempts to cleanly cut one - I tried scoring with both wheel and diamond point type glass cutters, I tried holding the scored line over a candle, a gas flame, under running hot and cold water - in every case, the crack would propagate three quarters of the way around, then veer off line, ruining the piece I was trying to save.
In the end, I discovered a slightly more reliable technique - score the line around the bottle, immerse it in boiling water to a point just above the scored line, let it warm up, then plunge it into a bath of ice cold water - being very careful to plunge it exactly upright (so that the heated scored line enters the cold water all at the same time. Repeat until the end just falls off.
Even with this technique, it’s not always possible to get a perfect cut, just because mass-produced bottles and jars are not terribly uniform in their wall thickness, and may have been produced in a way that creates internal stresses that are fine in an intact bottle, but not in its incomplete parts.
I made my own Youtube video of the technique that worked best for me (hope it’s OK to link this here): http://www.youtube.com/watch?v=lzoOGbVEy8A