The movable metal type invented by Gutenberg was very innovative in that multiple pieces of the same type were produced. According to one record, Gutenberg acquired coin-minting techniques and worked as a mirror manufacturer in Strassburg around 1439. Since he had metal carving skills, it is believed that he was able to carve letters into hard metal to make punches, the base of type. Using these punches, pieces of type of the same shape were produced through the process of developing matrices and casting the type by surrounding the matrices with L-shaped blocks. Matrices were concave moulds created by knocking an iron punch into a piece of copper. Melted alloys consisting of lead and other kinds of metal were poured into these matrices and cooled down to produce the pieces of type. The matrix casting device shown to the left was developed to ensure smooth production.