A mold is formed when a deceased animal is buried by sediment from the Earth. As the animal starts to decay, a chamber remains in the shape of the animal. A fossil cast is made when minerals are deposited in the mold.
There are two types of molds created by a fossil: external and internal. The external mold is made when the organic materials on the creature decay, leaving behind an imprint of the external shape and characteristics of the animal. And internal mold is created when a hollow organism fills with sediment and minerals after it dies. As the shell decays, an imprint of the organism is left in the minerals that creates the fossil.
Though casts can be made through natural processes, many of them are made though artificial means. Students and researchers create a mixture using clay and plaster of paris to cover the fossil. The concoction is allowed to dry before being removed. This creates an exact replica of the fossil that is more durable than the original, allowing for it to be handled and studied without compromising the original artifact. These synthetic casts can be made using both external and internal molds of the fossil.