Cream chargers (standard 8 g bulbs) are typically made of 100% recyclable steel. Usually nickle plated to make them shiny and resistant to rust. Some cream chargers are also painted instead of nickle plated.
Some cream charger bulbs are made from aluminium. However, aluminium cream chargers are not common.
Some new style larger cream chargers (such as the 580 gram cream chargers) are also made of steel that is typically painted.