Lab: Mold and Cast Fossil - A Simulation Model

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Lab Mold and Cast Fossil - A Simulation Model

• Brian Finley and Erik Frey
• Science 7
• Computers and Technology
• Spring 2003

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Background

• Fossils are remains or traces of dead organisms.
  They help scientists learn about living things
  that existed in the past.
• Permineralization and Replacement Many fossils
  are shells and bones that settle into sand or
  mud. Over the years the sand or mud turned into
  rock.
• Mold If the original shell or bone is dissolved
  away from its surrounding rock, the hollow
  impression is called a mold.
• Cast If the mold is later filled with mineral
  matter, a cast is formed. The cast is a copy of
  the original shell or bone.

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Molds/Impressions

• Bones and shells fall to the bottom of a body of
  water.
• Water may completely dissolve the bone or shell.
• Over time, once the object is dissolved, only a
  mold, or hollow shape in the shape of an
  organism, is left in the sedimentary rock.
• Sometimes fine sediment will fill the inside of
  the hard part, creating an internal mold.

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Casts

• Sometimes molds become filled with sediment from
  the water, creating a cast.
• A cast is and exact copy of an object that forms
  when minerals fill a mold and harden.

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Materials (Items/Group)

• Shells/Plastic Fossil Specimen (1)
• Sheetrock Mud (or Plaster of Paris) (1/2 cup)
• Cups (2 plastic, 12 oz)
• Plastic Bowl (1)
• Petroleum Jelly (1tsp)
• Water (8oz, plus clean-up)
• Stir Stick (tongue depressor)

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Procedure, Part A (Day 1)

1. Working with one partner, cover the fossil
   object with a layer of petroleum jelly.
2. Slowly mix water with the powdered sheetrock mud
   (or Plaster of Paris) until the mixture is the
   consistency of toothpaste (not so thin that it
   pours out of the cup, but almost to that
   point).
3. Scoop the liquid plaster into the plastic bowl.
4. Place the object on the surface of the liquid
   plaster. Let the plaster dry for at least twenty
   minutes.
5. Remove the object and clean it. Its impression
   in the plaster is called the mold.
6. Clean up and let the mold dry and completely
   harden.

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Sample Plastic Fossil
This shark tooth is approximately 2 long
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Creating the Mold/Impression
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Procedure, Part B (Day 1)

1. Coat the entire top layer of the mold with
   petroleum jelly.
2. Prepare more liquid plaster (as done in Part A).
3. Pour the plaster over the impression in the dried
   plaster, filling it to about 1/4 above the top
   of the impression.
4. Let the new plaster dry overnight.

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Two Casts
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Procedure, Part C (Day 2)

1. After the plaster is thoroughly dry, separate the
   two plaster blocks. The dried plaster on the top
   is the cast, while the hollowed-space piece was
   the mold.
2. Compare the mold, the cast, and the original
   object.

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Sample Casts/Original Item
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Removing the Cast from the Mold
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Discussion Questions

• 1. Which of the plastered objects is most like
  the original object?
• 2. How does the mold object differ from the
  original object?
• 3. In the activity, what corresponds to the soft
  mud or layer of soil?
• 4. Which of these objects represents the mold.
  How might the mold be created in nature?
• 5. Which of these objects represents the cast?
  How might the cast be created in nature?

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Other Hands-On Experiences
Other simulation models might include making
other types of fossils. For example Insect in
Amber model involves using clear Elmers glue
(hardens in two days), or actual rosin (melts on
a hot plate and cools within a couple of hours.
Students can imbed real bugs, rubber bugs,
leaves, etc The Lost Wax Experiment better
simulates a mold and cast fossil with
permineralization. Refer to Eriks PowerPoint.
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Culminating Assessment

• Set up a practical lab with samples of the
  various types of fossils. In lieu of actual
  fossil samples, you could use photos. For each
  sample/photo, have students list the type of
  fossil it is (mold/impression, cast, carbon copy,
  petrified remains, actual remains, insect in
  amber, etc) and how it was probably formed.