Activity 4: Molds & Casts

Hip Wimpee (Hippocampus)



Fossils are remains of life from the past. The most common kinds of fossil preservation are molds and casts. The activity will give students the chance to make modern day molds and casts.

Instructional Method: Activity

Goal: To teach students two common fossil types; a mold and a cast.

Objectives: Students will be able to:

  • Distinguish between a mold and a cast.
  • List the properties of a mold and a cast.
  • Describe which fossil gives more information about the once living object.


Setup: 45 min
Activity: 20 min
Discussion 20 min

Materials Needed:

  • Modeling clay
  • Plaster of Paris
  • 12-16oz. plastic soda pop bottles
  • Small paper cups 6 oz. sized
  • Leaves, plastic dinosaurs or bugs, shells and twigs
  • Water Bucket or bowel to mix plaster in
  • Paper towels
  • tooth picks
  • labels



hard parts

soft parts



trace fossils


Fossils are ancient remnants of once living things. They are our only link to the living past. They tell us of ancient environments and organism habits. Many times they are used to help distinguish periods of geologic time. The absence of fossils in rock can make it difficult to be sure what took place in the past.

Fossils can range in size from 6-foot leg bones to microscopic animal shells. Most fossils we find today are of an organism's hard parts. Hard parts give scientists an idea of how strong the animal was, what may have been feeding on it, what it ate, the speed of the organism, etc. Hard parts are what most people think of as fossils. In reality they are the most rare. They consist of shell, bone, scales, leaves, wood, teeth, claws and horns.

An organism's soft parts are less commonly preserved, but when they are they can tell scientists a lot about the organism. Soft parts can tell the environment and the organism's sensitivity to its surroundings.

Trace fossils are the most common fossils found in nature. They are impressions left in the sediment from once living things. Common trace fossils are footprints or walkways, resting spots, living burrows, feeding burrows, casts and molds. They are not the original parts of an animal. A trace fossil is preserved when mud or dirt that was disturbed by something living hardens and keeps its shape.

There are two main kinds of trace fossils, molds and casts. A mold forms when something is pressed into soft mud and removed by decomposition or pulled out, leaving an impression of the object. A cast is a 3-D example of an object of the past created when a mold fills up with sediment like mud, sand or volcanic ash.

Fossils can form when hard parts of a fossil are replaced by other minerals (see Let's make a fossil #1). This is known as permineralization and petrification. It occurs when fluid flows through rock and dissolves the original animal part while depositing a new mineral in its place. If replacement happens slowly, the original structures in the bone, shell, or plant material can be preserved exactly. A common example of replacement is petrified wood. Original parts are good at telling scientists how the organism lived and what diseases may have affected it.

The following activity allows students to make molds and casts in Plaster of Paris. They will see the difference between the two fossil types and determine which gives them more information, the cast or mold.

Instructional Procedures:

  1. Cut plastic bottles into 2 inch rings. Mark each student's name on the side of the ring.
  2. Cut paper cup tops off leaving approximately 1 inch to the bottom. Mark the student's name at the bottom of the cup.
  3. Divide clay into portions that can be pressed into the bottom of the plastic soda bottle rings, leaving about ½ inch layer at the top of the ring.
  4. Press clay into the ring bottoms. Make sure the clay is as smooth as possible. Any marks in the clay will appear when the fossil is made.
  5. Have students choose objects they will press into the clay or Plaster of Paris.
  6. Press objects into clay and remove. Try to keep the image as clean as possible. If it does not look good, smooth the clay and repress the image.
  7. Mix Plaster of Paris. Make only the amount you will be able to pour in 2 min. Clean the bowl and remix the plaster. If the bowl is not clean the next batch sets up too quickly.
  8. Pour plaster into the rings with impressed images. Continue until all the plastic rings are full.
  9. Now, pour Plaster of Paris into paper cups.
  10. Let plaster setup for 2-3 min. It needs to stiffen up to the point where the plaster will keep the shape of the object. When it reaches this point, have students press the object into the plaster carefully and let sit for a minute or two.
  11. Carefully remove the object.
  12. Let all molds and casts sit for at least 30min.
  13. When hard, remove paper cup from around the plaster and push plaster and clay through the ring. Remove clay from plaster.


Students have just made a cast and a mold. Write on the bottom of each fossil their name and if it is a cast or mold. Have students form two lists to describe what they see in the two 'fossils', hair, shape, texture, size, color, etc. Have students write on paper which fossil gives them the most information, mold or cast (3-D) and why.


To make tracks, try to get animals that you can let walk or slither across the wet plaster. If you do, make sure the plaster is firm enough for the animal to walk on. Insects are easy to find.


Have students trade fossils with another student for observation purposes. Describe what you see in the new fossil and what it is or may have done while it was living, i.e.: walking, swimming, meat eating, vegetarian eating, defense, etc.

Included National Parks and other sites:

Agate Fossil Beds National Monument
Alibates Flint Quarries National Monument
Dinosaur National Monument
Florissant Fossil Beds National Monument
Fossil Butte National Monument
Grand Staircase-Escalante National Monument
Hagerman Fossil Beds National Monument
John Day Fossil Bed National Monument
Petrified Forest National Park


dinosaur eggs
dinosaur skin
dinosaur trackway

Utah Science Core:

4th Grade Standard 4 Objective 1,2


Last updated: February 24, 2015

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Bryce, UT 84764


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