Casting Fossils
Introduction: Fossils are the remains or traces of plants and animals that lived in
the past. Most fossils are found in sedimentary rocks that were once under
water. When marine or fresh water organisms died, they sank to the bottom and
were covered with mud. The soft parts of the organism decomposed; the hard
parts were covered with sediment.
In time, the sediments hardened into sedimentary rock. When the layers of the
sedimentary rock are broken apart or eroded, they expose the fossil impression,
called a mold. Sometimes the mold filled with mineral deposits from the water
and became hard, forming a cast.
Other fossils are formed with living things were caught in tar pits or hardened
tree sap called amber.
Purpose:
 to understand the process by which fossils are formed
 to identify fossils and place them in the periods of history in which they
lived
Materials:
 Plaster of Paris-Home Depot 5lbs for $3.99 or 25 lbs. for $7.00.
 Optional: sand
 Plasticine clay (You could use any modeling clay that is oily base so that
the clay will stay supple or soft and not harden. You need to release the
plaster of Paris from the clay mold.)
 Fossil Replicas Wards Scientific www. wardssci. com 36V244 ( if fossil
replicas are not available, sea shells may be used, but they should be
rubbed with cooking oil.
 Plastic food containers and lids- Smart and Final
One 32 oz. Plastic container per group
One 16 oz. plastic containers or measuring cup per group
One 8 oz. plastic containers for each cast. (3- 6/group)
Procedure:
1. Place the clay on the bottom of the 8 ounce plastic container to make a 1 inch
layer. Smooth out the clay to cover just the bottom of the container. Make sure
to press the clay firmly into the container so it will not leak underneath and
enclose the clay after it is poured and molded.
2. Push the fossil replica into the clay to make an impression of the fossil. Push
firmly on the clay to get a good impression. It is all right to push the fossil
deeply into the clay so the plastic container on the bottom is exposed. Remove
the replica fossil with the tip of a ballpoint pen. The Plaster of Paris will stop at
the plastic and still get your fossil imprint.
3. Make several impressions of the fossil in different orientation such as the sides,
bottom, and top to get a realistic impression of a cluster of the same fossils. Also
your students will see how the fossils looked on all sides.
(Remember archeologist found fossils in clusters of the same types of animals or
plants, which they lived together in communities to survive. Do not put
different type of fossils together into one mold. These mixed fossils probably
were not found in the same layers and may not have lived at the same period.
Then you would have an unrealistic look at the different types of fossils and your
students could not do the identification exercise.)
4. Fill your 16 oz. plastic container with plaster of Paris and pour into the 32
ounce plastic container. A little sand may be added at this point to make the
plaster look more like sandstone.
5. Pour and stir just enough water into the 32 ounce plastic container to make a
thick, creamy and not watery mixture (like a thick milk shake).
6. Pour the thick mixture over the clay to about 1 inch thick.
6. Let the plaster of Paris dry (over night or more) and pop out the fossil casting
with the tip of a butter knife. Allow it to dry in the sun, if some of the surfaces
are not
completely dry.