Making "Fossil" Casts
demonstrationactivity exercisesuggested grade levels: 7- 8

view Idaho achievement standards for this lesson

Fossils are interesting records of the past that took a very long time to make, but they are also beautiful to look at. In his project you can speed up time and make a fossil mold or cast in just a few days. Your cast will have the same details and delicate patterns that make real fossils so fascinating.

Lump of plasticene (modeling) clay the size of your fist Dull table knife
About 1/2 cup (63 g) of plaster of Paris (sold at hardware and hobby or craft-supply stores) 1/4 cup (59 ml) of water
2 paper cups; bottom should be 2 to 3 inches (5 to 7 cm) in diameter Spoon
A well-formed seashell, small bone, seedpod, or other natural item  

Review the section on Fossils using the Digital Atlas. To get there: Click on Atlas Home, mouse-over Geology, then click on Fossils. This section will serve as a review for you before having your class do this activity.
1. Review as a class:
   1. Conditions needed in order for fossilization to occur.
   2. Types of fossilization.
   3. Examples of fossils, use pictures from the Digital Atlas on large screen
   4. Be sure to discuss with your class the difference between real fossilization and making a cast of a natural object.
2. Make a ball of clay and flatten it until it's about 1 inch (2.54 cm) thick and smooth on top. Trim the circle of clay with the knife until it fits into the bottom of the cup.
3. Slide the clay into the cup, flat side up. Carefully press the object you want to fossilize into the clay until it's half buried. Then carefully lift the object out of the clay. You will be able to see an impression or print of the object.
4. Pour 1/2 cup (63 g) of plaster of Paris into the other paper cup. Add 1/4 cup (59 ml) of water to the plaster of Paris and stir until the mixture is smooth. Leave it alone for five minutes.
5.After five minutes, the plaster of Paris mixture will have thickened. Pour it into the other paper cup right on top of the clay. Let this sit for an hour without touching it.
6. After an hour, the plaster of Paris should be almost completely hard. It will feel cool and you will still be able to make marks in it, so be careful with the next step. Carefully tear away the sides of the paper cup and remove the clay and plaster. Holding the clay part with one hand and the plaster part with the other hand, gently separate them.
7. Clean off the clay part and put it away. You can use it for other projects. Use the knife to carefully trim away any rough edges from the plaster fossil cast. Smooth out the edges, then let it dry for a day or two until it no longer feels cool when you hold it against your cheek. Be sure to let it dry slowly and not in an oven or in the sun. Drying it too quickly could cause it to crack.

Handouts/Activity links:
These are links to access the handouts and printable materials.

Related Lesson Topics:
Geology: Geology Topics

Lesson plan by Alan Anderson, Gwen Diehn and Terry Krautwurst with permission from Geology Crafts for Kids, 2001
Idaho Achievement Standards (as of 7/2001) met by completing this activity: