Science Class Activity: How to Make Fossils

Fossils are the preserved remains of plants and animals that lived in the distant past. Fossils provide valuable information about living things from long ago, and evidence of how they have evolved over time. By studying fossils, we can learn about extinct animals such as dinosaurs, as well as understand how Earth's surface has changed. For example, finding fossils of sea creatures hundreds of miles away from the ocean tells us that at one time there was an inland sea in that location.

Fossils may be formed when a plant or animal dies and becomes buried in sediment. Some fossils are made when the empty spaces or pores of the organism are filled in by minerals in a process called permineralization. In other cases, the solid parts such as bone or shell are actually replaced by minerals from the soil. Mold fossils are created when the remains of a plant or animal are completely gone, and all that remains is an indentation in the rock in the shape of the organism. Cast fossils are created when an indentation is filled in with minerals.

How to Make Cast Fossils from Plaster of Paris

In this activity, students will make cast fossils by using a natural object to make an indentation in modeling clay and then filling it in with plaster of Paris.

Materials:

Plaster of Paris
Water
Modeling clay
Petroleum jelly
Small natural objects such as sea shells
Paper cups
Plastic spoons
Newspaper or paper towels

1. Combine the plaster of Paris with water at a 1:2 ratio (1 part plaster of Paris to 2 parts water) in a paper cup and mix well with the plastic spoon.

2. Allow the students to choose their own sea shell or other object to make their fossil.

3. Give each student a piece of modeling clay and have them knead it until it is soft and pliable.

4. Coat the sea shell with petroleum jelly and firmly press it into the modeling clay to make an impression. The petroleum jelly prevents the shell from sticking to the clay.

5. Remove the sea shell carefully to create a mold in the shape of the shell.

6. Fill the impression in the clay left by the shell with plaster of Paris. Smooth the plaster to form a flat surface.

7. Place the clay and wet plaster on a newspaper or paper towel and allow it to harden at least overnight, or preferably two or three days.

8. Carefully peel the clay away from the hardened plaster to free the fossil. The shape of the shell should be clearly visible.

Sources:

Personal experience
http://www.enchantedlearning.com/subjects/dinosaurs/dinofossils/Fossiltypes.html
http://www.discoveringfossils.co.uk/whatisafossil.htm
http://www.fossils-facts-and-finds.com/what_is_a_fossil.html