Badlands National Park Lesson Plans: Fossils-Making Plaster Casts

Making Fossil Casts

Age/Grade
Grades 1 through 3

Objectives:

Describe how fossils that are not actual remains of the living organism form.

Create a fossil mold and cast.

Length

45 Minutes

Location

Classroom

Background

Fossils are not always the actual remains of the living organisms.   Many fossils are just copies called imprints, molds or casts.   Imprints are impressions made by organisms in soft mud that were preserved when the mud solidified.   Imprints can be traces of an animal’s activity, rather than its actual remains.   The hardened tracks of animals or the burrows of prehistoric worms in solidified mud are examples of fossil imprints.

Molds are made when organisms are totally or partially buried in mud that hardens into rock.   Over time ground water may dissolve the organisms, leaving cavities shaped like their bodies.   Both imprints and molds are mirror images of the organisms.

If a mold was later filled with mud or mineral material, the hardened filling is called a cast.   It is a reproduction that has the same outer shape as the organism.   A cast looks like the organism itself, not like its imprint.   Paleontologists make casts of fossil molds by filling them with liquids, such as plaster, that harden.

Materials (per student0

Modeling clay about the size of a large walnut (Should be about twice the size of the seashell or other object to be cast)
Paper plate
Small seashell (may be purchased at a craft store) or another distinctively shaped object
Petroleum jelly, such as Vaseline
7 ounce paper cup
Plastic spoon
Plaster of Paris (available from craft store or hardware store)
Tap water

Vocabulary

Fossil
Mold
Cast

Method

Comparison -- Modeling -- Art

Procedure

Place 4 spoonfuls of dry plaster of Paris in each cup.

Squeeze the clay until it is pliable.

Place the clay on the paper plate.

Coat one side of the seashell with petroleum jelly.

Press the lubricated side of the shell into the clay.

Carefully remove the shell from the clay.

Observe the imprint of the shell in the clay.   Compare the imprint in the clay with the shape and texture of the outside of the shell.

Place two spoonfuls of water in each cup and stir until smooth.

Pour the plaster mixture into the shell imprint in the clay.   Note: Throw the cup and spoon away and do not wash any plaster down the sink as it can clog the drain.

Allow the plaster to harden.   This will take about 20 minutes.
Gently separate the clay from the plaster.
Compare the shape and texture of the outside shell with the shape and texture of the outside of the plaster cast.

Extension

The imprint in the clay and the plaster cast are both examples of how fossils form.   Pressing the shell into the clay represents burying the shell in mud.   In nature, the mud would have hardened into rock around the shell.   Removing the shell from the clay represents how the shell dissolves over long periods of time, leaving a cavity called a mold in the rock.   The mold produced is a mirror-image imprint of the shell’s outside surface.   In nature, this mold would have been filled with sediment, or small particles or rock and minerals that are deposited by water, wind, or ice that hardened into rock.   The Plaster of Paris, like sediment, hardened – but in a much shorter period of time.   The plaster is a replacement of the shell and is called a cast.
Resources
"Fossils Tell of Long Ago," by Aliki.   Harper Collins: New York, 1990.

See the "Critical Issues sections of the Badlands" National Park website (http://www.nps.gov/badl/exp/issues.htm#paleontology)