Hot Glass, Cold Water: Experiments in the History of Glass Fracture 1 Gerhard Eggert gerhard.eggert@abk-stuttgart.de State Academy of Art and Design Objects Conservation AmWeißenhof 1 70191 Stuttgart Germany Keywords: fracture, craquelure, thermal toughening, Prince Rupert’s drops, Bologna flask, cremation Abstract Prince Rupert’s drops and Bologna flasks are predecessors of modern thermally toughened glass. As in societies of learned men some 300 years ago, demonstrations of their surprising behaviour encourages a closer look. The same is true for the formation of a superficial crack network during quenching of hot glass with water. These experiments may inspire the study of glass craquelure to uncover information hidden in them. Therefore, short instructions for such demonstrations are given. Introduction What amazement, even bafflement, must have come over all those natural philosophers when tear drops were demonstrated to the Royal Society in 1661 or shattering bottles to the Bolognese Academy in 1745! Both glasses withstand a sharp blow with a hammer, but noisily explode into tiny peaces when struck at the wrong spot. Curiosity led to systematic experimentation by scholars which resulted in a better understanding of these apparently contradictory phenomena. This lecture builds on the seventeenth and eighteenth century tradition of scientific education through demonstration experiments in learned societies and private salons. It derived from the study of fracture patterns in historic glasses (Eggert 2006) in order to understand the hidden information in them. An archaeologist described a Roman glass bottle as ‘fractured like Sekuritglas’, a trade mark for thermally toughened glass. Rottländer (1990) connected such finds from the Rhineland with the ancient story of unbreakable glass and postulated a Roman quenching technique for the production of glass with higher fracture strength. This hypothesis has long been refuted in detail (Eggert 1991), the ‘on a second view’ different crack pattern being just one argument. Nevertheless, these works pointed to glass crack patterns as a source of information and led to a study of the history of thermal glass toughening. Modern thermal toughening (‘tempering’) of glass to increase fracture strength was invented in 1874 by Alfred Royer de la Bastie, who quenched very hot glasses in hot oil. At the moment when the glass ‘solidifies’ (i.e. when the stresses can no longer relax by viscous flow) the outer zones are colder than the interior (Figure 1a). If the outer and inner layers consisted of 8