The next breakthrough: Tempered Glass
The story of tempered glass can be traced back to the middle of 17 century when there was a prince named Rupert in Rhein who performed a fascinating demonstration. The prince put a molten drop of liquid glass into icy water, which then was transformed into a very hard droplet of solid glass. This high-strength piece of glass had a long tail that looks like a tadpole.
These glass teardrops, shaped with a long and curved tail, were known as Prince Rupert's Drops.
One of them could withstand a blow from a hammer on the rounded end of the droplet without breaking. Or it can be pressed with up to 20 tons of force, and even shooting it with a gun won’t do it a lot of damage.