If you don’t have access to a glass cutter, you can break a glass bottle or jar cleanly using a piece of string and a little heat.
- Wrap the string around the jar several times before tying it off.
- Slip the string off of the jar and soak it in acetone or nail polish remover. When it is saturated, slide it back around the jar and position it exactly where you want the glass to be cut.
- Use pliers to hold the jar while you ignite the string with a lighter. The flame will spread more evenly if you rotate the jar as the string burns.
- When the flame is about to burn out, dunk the jar into a bucket of extremely cold water. The glass should crack evenly along the string leaving an open end to the jar.
Watch the video for details.
I think you should remind folks of some safety basics, like eyewear at least.
The break did not look even at all, and there was no mention of sanding the edges.