Fossil Frenzy

    Kids, let's learn how fossils are formed and preserved.  

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

     Use fresh play dough that is soft and pack it into an empty margarine tub until the tub is half full. Make sure the surface is very smooth. Coat a twig, shell, or chicken bone with a thin film of vegetable oil. Press it into the play dough to make a clear, deep imprint. Remove the item and throw it away. Let the play dough harden for a day or two in order to make your special mold fossil.

    Coat your hard mold fossil lightly with more oil. Measure 1 cup plaster of Paris and 1/2 cup water and mix them together. (Be sure to follow all precautions on the plaster of Paris label). Pour the mixture on top of the mold fossil and let it dry. Carefully separate the plaster piece and notice your new cast fossil, which has the outward shape of the original item. The play dough represents the soft mud of ancient times. Plants and animals made imprints in the mud. If nothing collected in the prints, the mud dried and made a mold fossil. If sediments filled the imprint, a sedimentary rock formed with the resulting cast fossil.

    Plaster of Paris is made by grinding a clear, shiny crystal called gypsum (calcium sulfate, CaSO4 .2H2O) into a powder. The powder is heated to dry out all the moisture. This dry powder changes back into a solid when water is added, but it never looks clear and shiny again. Some heat is given off during this phase change!


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    March 1994


    References: "WonderScience", 1993, 8(1); J. VanCleave Earth Science for Every Kid, 1991, p. 48; J. VanCleave Chemistry for Every Kid, 1989, p. 144.