Covering fossils in plaster is a common technique used to protect and preserve them. The main reason for covering fossils in plaster is to stabilize and protect the fossil from further deterioration. Fossils are often found in fragile and delicate states, and plaster provides a protective layer that helps to prevent further erosion or damage caused by exposure to the elements or handling. Plaster also provides a hard, durable surface that makes it easier to transport and handle the fossils without causing additional damage.
Additionally, the plaster can be used to make a mold of the fossil. This allows for a replica to be made which can be used for research, study or display, without risking damage to the original fossil.
Finally, plaster is also useful for reconstructing missing or broken parts of the fossil, which can make it easier to study and interpret the fossil.
In summary, covering fossils in plaster is necessary to protect and preserve them, to make a replica, and to reconstruct missing or broken parts.