Creation and testing of tempered glass drops (“Prince Rupert’s drops”)

MATERIALS

Rod of soda-lime glass (ordinary stirring rod)

High heat torch (at your local hardware, look for the YELLOW gas tanks – depending on your local laws,

this may be some version of MAP gas or propylene gas. A special torch tip is required.)

Large beaker of water, room temperature or colder

Pretzel jar demonstrator (any clear plastic container with a small hole drilled in the lid)

Hammer

METHOD

Wear goggles!

Hold the torch so the flame is more or less horizontal. Heat the last ¼ to ½ inch of the rod, rotating

the rod slowly as you do. The aim is to keep the molten section from drooping. Excessive rotation will cause the

molten section to twist on itself. This is not good.

Once the end is molten, hold the rod vertically, about 4-6 inches over the beaker, and stop rotating it so

the drop can separate. Keep the drop in the flame at first. As the drop separates, keep the flame pointed

towards the end of the remaining rod (i.e., so the drop falls out of the flame). It is important for the drop to

fall fairly quickly, which is why you are keeping the flame on the thinning part of the glass. As the drop hits the

water, move the flame aside.

With practice, you will end up with drop in the water and a thin tail connecting it to the rod. You can

safely sever the tail by quickly flaming it where you want the break.

Successful tempering is indicated by the presence of a vacuum bubble in the drop, due to shrinkage of

the inner glass once the outside shape has formed.

TESTING

Wear goggles! Observers should also wear goggles if they are standing close.

Tempered glass is resistant to impact. To show this, place the drop on a sturdy surface, hold it down by

the tail and tap with the hammer on the main part of the drop. Be cautious increasing the force of the blows.

Tempered glass breaks explosively when the tensioned interior is penetrated. To show this, thread the

tail of the drop through the hole in the pretzel demonstrator so the main part of the drop is inside the jar.

Snap the tail (it will be hard to do because the tail is also tempered). Don’t blink.

The pieces of shattered glass are not sharp and can be safely rubbed between your fingers.