A Bologna bottle, also known as a Bologna phial or philosophical vial, is a glass bottle which has great external strength, often used in physics demonstrations and magic tricks. The exterior is generally strong enough that one could pound a nail into a block of wood using the bottle as a hammer; however, even a small scratch on the interior would cause it to crumble.
Contents
It is created by heating a glass bottle and then slowly cooling the outside whilst rapidly cooling the inside. This causes external strength and internal stress such that even a scratch on the inside is sufficient to shatter the bottle.
The effect is utilized in several magic effects, including the "Devil's Flask."
Manufacture
To create the desired effect, the bottles are rapidly cooled on the outside and slow cooled on the inside during the glass-making process. This causes the outside to be extremely hard, and the inside to be soft and susceptible to damage which can release the powerful internal stresses. The glass is not annealed. Reheating the glass and then allowing it to cool slowly will remove the unique properties from the glass.
Uses
Because of the seemingly paradoxical nature of the glass (being both extremely durable and extremely fragile), Bologna bottles are often used as props in magic tricks, where the bottle can be shattered by rattling a small object inside it.