Bitumen is also used for waterproofing and sealing roofs and boats and to coat buildings. In technology, bitumen plates are used to muffle sounds in machines such as computers and dishwashers. In early photography, bitumen was spread on pewter plates which were then exposed to light, resulting in black and white images being formed. Bitumen is derived naturally from petroleum or through distillation. Naturally occurring bitumen is sticky and tar-like and must be heated or diluted before it can flow. The refined form is the leftover product from the distillation of crude oil.