In 1797 Matthew Boulton was authorised by the government to strike copper pennies and twopences at his Soho Mint, in Birmingham. It was believed that the face value of a coin should correspond to the value of the material it was made from, so each coin was made from two pence worth of copper (2 ounces). The large size of the coins, combined with the thick rim where the inscription was incuse i.e. punched into the metal rather than standing proud of it, led to the coins being nicknamed Cartwheels. Many have survived being used as weights for kitchen scales, and thus battered and worn.