The making of replicas of the original fossils, is pretty much the say way, just different materials, in which the fossils originally have been formed and preserved.
When an organism dies, be it a plant or an animal, it gets fossilized only if very quickly after the death it is covered with sediments or mud. The sediments or the mud protect the plant or animal from the outside influences, so once it is kind of in a vacuum, or protective layer if you will, in which parts of the plant or animal that are long-lasting are preserved.
The clay and plaster used for the replica, work in the same way, because they wrap around the fossil tightly and isolate it from outside influences, with the result being different though, as they just make a copy of the fossil.
