I’m going to show you two videos this week — both about an interesting early form of tempered glass called Prince Rupert’s drops. Allegedly first made in the 1640’s by Prince Rupert of the Rhine (in the modern-day Netherlands), Prince Rupert’s drops are made by dropping hot molten glass into cold water. The glass rapidly cools into a tadpole shape with a long, thin tail. Since the outer part of the drop cools first, it’s compressed when the inner part cools and contracts later (leaving the inner part in tension).
As a result of this odd combination of stresses, a Prince Rupert’s drop can take hammer blows without breaking, but will shatter explosively if the tail end is snapped off with even gentle finger pressure. A high-speed video study discovered that the “crack front” unleashed by snapping off a drop’s tail propagates through the drop at speeds of up to about 1900 m/sec (4200 miles per hour).
The first video is a quick presentation on the glass by the Corning Museum of Glass in Corning, New York:
The second video (longer, but with no narration) was produced by the Museum of Glass in Tacoma, Washington:
Hat tip: forgetomori