Asked by: Mary Lenham, Andover
Everyone who’s dropped a glass object knows how the fragments sometimes end up astonishing distances from the point of impact. It’s as if something was propelling them. And there is an extra source of energy, in addition to the kinetic energy generated by the fall. It comes from the thermal stress left in the glass after it was made.
Glass objects are made from sand, soda ash and limestone heated to 1,700°C. They have to be specially treated – ‘annealed’ – during manufacture to help release the stress trapped inside; if the glass is cooled too quickly, it can crack before leaving the factory. But dropping it afterwards can release residual stress.
A dramatic demonstration of the effects of improper annealing is provided by the tadpole-shaped glass objects known as ‘Prince Rupert’s Drops’, named after a 17th-Century German aristocrat who gave a set to King Charles II for entertainment. They are created by dripping molten glass into cold water, causing the drops to chill rapidly on the outside, but much less so internally. As the interior cools and contracts, it pulls on the outer surface, creating a huge amount of thermal stress. Snapping the tail of the drops causes cracking that unleashes the pent-up energy, making the drop explode spectacularly.