Crystal is a special glass, the brilliance of which is the result of adding lead oxide to the glass mixture of silica, potash, and soda. There are three types of crystal. If the lead content is between 10 and 24%, it is crystalline. Above 24%, it is lead crystal. Finally, superior crystal, like that of the Lalique crystal factory, exceeds 30%. Added as an oxide, lead makes the glass more malleable and lowers its melting point from 2,730°F (1,500°C) to 2,550°F (1,400°C). The addition of metal oxides or rare earths during the melting of the crystal colours the material. The crystal is ready to be worked 20 to 40 hours after firing.