From P. Poulton
As you indicated in your article, the mechanisms producing compression and tension zones in force-cooled glass are well-known (“Why do teardrops explode?”, 11 February and Letters, 11 March).
In the mid-1960s when I was an undergraduate in the Department of Metallurgy at Sheffield University, we were obliged to attend lectures on a Saturday morning at the Department of Glass Technology. This gross and traumatic imposition was, however, enlivened by some excellent demonstrations.
One of particular note, reflecting a larger version of Prince Rupert’s drops, involved a standard milk bottle which had been heated in a furnace and then air cooled on the outer surface.
The lecturer, who may even have been the Professor of Glass Technology, hammered a nail into a piece of wood with the milk bottle. He then put a nail into the milk bottle, in contact with the tension zone of the inner surface, shook it about and the milk bottle shattered into numerous fragments.
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