Fossil Casts, Molds, and Impressions

Want to explore fossils in a hands-on way but can’t break the bank with casts? Teach your students how to make their own fossil casts! These fossil casts, molds, and impressions use white glue to create these fun hands-on fossils.

This easy guide comes from the Kentucky Geological Survey so check out their post for more details!

Grades: 3-5

Background

Fossils are natural preservations that give scientists insight to the ecology and structures of ancient organisms. Fossils form over the process of millions of years and can only form under certain conditions. There are two common kinds of fossils that paleontologists encounter: body fossils and trace fossils. A body fossil is an exact replica of a living organism, and is what you often see in natural history museums. Trace fossils are like a footprint, and are often found in the form of tracks, trails, and burrows. In order for a fossil to form, an organism had to have been buried under a layer of sediment. When the organism slowly decomposes, the sediment hardens and leaves a hollow space to be filled with different material, like rock. If water filters through the sediment as the organism decomposes, it will leave behind a perfect impression, creating an external imprint.

Museums will most often display plaster casts of fossils in order to preserve and study the originals. In order to make these casts, scientists use plaster of Paris, fiberglass, clay, or other similar materials in order to create a perfect impression of a fossil. First, the mold needs to be formed that will be filled with materials. Once the mold is formed, liquid polyester or other materials are used to fill the mold and create a replica of the original.

What You’ll Need

  • White glue
  • Objects to make impressions. Small, low-relief to flat objects work the best
  • Cotton ball
  • Coin
  • Blade of grass
  • Leaf
  • Shell or other hard natural object like a paper clip or nail
  • Modeling clay to make impressions
  • Copy of worksheet

What to Do

How to make the fossil casts:

  • Students should shape their clay into a pancake shape.
  • Students should take an object and place it on a desk or in their hand.
  • Press the clay into the object on the desk.
  • The impressions should not be too deep (the deeper the impression, the longer it will take for the glue to dry).
  • Slowly and carefully pull the object out of the clay. Try not to have the clay stretch or smear when you remove the object.
  • The impression of the object in the clay forms a “mold” of the object even if the object is gone.
  • Now take white glue and fill the mold.
  • Let the glue dry. The time it takes to dry depends on the depth of the impression
  • When the glue has dried, peel back the glue shape from the clay. The glue shape is a “cast” of the object.
  • Sometimes there is excess glue around the “fossil.” Cut away the excess glue with your fingers or scissors. Many fossils have excess material around them and have to be cleaned to see the original fossil.

Connecting Concepts

Analyze and interpret data from fossils to provide evidence of the organisms and the environments in which they lived long ago.

Disciplinary Core Ideas

LS4.A: Evidence of Common Ancestry and Diversity

  • Some kinds of plants and animals that once lived on Earth are no longer found anywhere. (Note: moved from K-2)
  • Fossils provide evidence about the types of organisms that lived long ago and also about the nature of their environments.

 

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