During the 70's hobbyists were cutting bottles to make glasses, vases, bowls, etc...
Tie heavy twine around the cut line. Soak in lighter fluid, light, let burn, then plunge into a bucket of cold water.
The bottle should part right at the string line. Then use a Dremel tool to smooth the cut.
You've got lots of bottles...experiment !
I seem to recall that one could use nichrome wire to do that, with better results. (Cleaner edge, less cleanup.) The wire is wrapped tightly around the bottle to be cut. An isolation transformer drives your Variac which powers the nichrome. The wire creates a very localized strip of hot glass. Dipping the bottle in water causes a clean fracture at the heat line.
Grind the lip of the glass for a flat surface and rounded corners.
Use all safety measures, like eye protection and gloves!
--Winston
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