Prince Rupert's Drops are tiny pieces of glass with a head and a relatively long thin tail. They are formed by molten glass drops cooling very quickly in water. Since the outside cools first, it contracts onto the inside. This means that the outer skin of the glass is in tension because of the incompressibility of the inner core. Since the head is egg-shaped, a relatively strong blow with a hammer will not shatter one because the stress is distributed around the outside (and actually relieves some of the tension). However, a slight crack caused by breaking off the tip of the tail will propogate up to the head almost instantaneously. As soon as there is an outlet for the pressure inside, the skin is broken and the whole thing will explode.
scrap glass rods
800 to 100 mL beaker
Assemble the ring stand so that the bunsen burned is clamped about a foot and a half above the lab bench. Angle it about 45 degrees. Place a large beaker about six inches deep with water under the area of the flame. Carefully melt the end of a piece of scrap glass rod until it begins to melt. Make sure the drop lands in the water.
Be VERY careful - the glass can still be extremely hot when it looks normal. You must wear goggles during this lab. It is not unusual for most or all of your attempts to fail (the best year was fall 2000 when we had two successes in a triple period class!).
Collect all glass shards with a dustpan and brush, and dispose of them in a cardboard box marked "broken glass."