Bologna bottle

Common card · No. 242413

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. It is created by heating a glass bottle and then rapidly cooling the outside whilst slowly cooling the inside. This causes external compression and internal tension 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".

Read on Wikipedia