Some day you may find yourself without the normal tools and devices that you might normally carry. In an emergency situation, when you have no knife, you can improvise a cutting tool by carefully breaking a glass bottle. As I have begun to learn flint knapping, I’ve begun to collect the bottoms of glass bottles as a modern material to work with. I’ll hold a bottle by the neck and strike the bottom rim of the bottle on the edge of a rock or concrete structure. This is done with more of a quick wrist action vs. an arm swing. This breaks off the bottom. Then I gently strike the side at a diagonal angle which leads to a sharp edged blade.
Any broken piece of glass has a sharp edge, but having an unbroken bottle neck to use as a handle is a real asset. Goggles and gloves are highly recommended for this process. Use extreme caution when performing this procedure and when using the resulting tool.
You can always take a piece of broken glass and haft it to a stick or other hanle and have a useable knife also. It is similar to how the ancient cultures used their stone knives.
To break the bottoms out of the bottles I put a bolt or drill bit in the bottle and place my thumb over the hole in the neck and shake the bottle up and down. It usually will break the bottom out intact.
I had to try the metal rod shaking method, but my results were only marginal (practice make perfect?). I had used the blunt end of a punch/drift, but I misplaced it, so instead I tried a tire-iron… Bingo! I slip the wedged end of the iron into the neck of the bottle until it reaches the bottom’s inner edge. Then, while holding the tire-iron handle, gently tap the bottom, and Voilà! a glass medallion for knapping pops off.