Waco Gem and Mineral Club, Waco, Tx

3.2 Types of Fossilization


3.2: Types of fossilization and making or excavating fossils

Forms of Fossilization

Fossils are the preserved remains or evidence of past life (plant and animal). These include actual remains of the plant or animal (such as teeth), carbonized impressions, molds and casts of shells and other body parts, etc., as well as evidence of an organism‘s activity, such as chemical traces, burrows, footprints, or coprolites (known as trace fossils). Following are some common forms of fossilization.

Molds and casts. Calcareous shells may dissolve, leaving a cavity in a rock that is later filled with sediment or minerals, forming a mold and cast of the original organism. Only the general shape and form of the original organism is left.

Mineralization or petrification. Original shell, bone, or wood may be infiltrated or totally replaced by a mineral that seeps into pores via mineral-laden groundwater. When this happens, scientists can observe even tiny details of cell structure.

Re-crystallization. Shells may re-crystallize, leaving original shell material but in a different mineral form. For instance, many shells are formed from calcium or aragonite, which changes to calcite during fossilization.

Carbonization. Between layers of finely bedded shale, original organic material may be compressed and distilled away, leaving only a thin film of carbon on a bedding plane, as often happens with leaves and insects that fossilize.

Original remains. Sometimes, animal or plant remains may undergo little to no alteration at all. Such is often the case with fossils such as teeth that are resistant to decay. Or an animal like an insect may be captured in sap, which hardens into amber, creating a natural time capsule that preserves the original organic material. (Scientists have been able to extract bits of ancient DNA from such insects!) In Siberia, creatures such as woolly mammoths have been found locked in ice that has remained frozen since the Ice Ages.

Making a Fossil

This activity simulates how fossils in the forms of molds and casts are created.


  Plaster of Paris Small Cardboard Containers
  Jug of Water Shells, Leaves, or Fossil Models
  Modeling Clay Paper Towels
  Vegetable Oil Masking Tape
  Paintbrust (1-inch wide) Pen or Marker
  Paper Cups Roll of Large Paper/Newspapers
  Dowels or Sticks (optional) Paints and Paint Brushes










This can be a messy procedure, so start by protecting your tabletop or other work surface by spreading out a roll of paper, newspaper, or some sort of drop cloth.

Place a chunk of modeling clay into the bottom of a small cardboard container (the cut-off bottoms of individual-serving milk cartons, paper cups, or Pringles potato chip cans work well) and press into a flat, smooth surface.

With your 1-inch wide paintbrush, brush a light coating of vegetable oil across the surface of the clay. This is to make it easy to remove your fossil model and, later, the plaster cast. Otherwise, the clay will stick.

Have kids select the fossil they wish to make. Use real leaves or seashells or plastic models of fossils. Such models often may be found in museum gift shops. Ward‘s Natural Science also sells a set of plastic fossil models. Ones that seem to be most popular with kids are trilobites, ammonites, and shark teeth.

Press the fossil model or seashell into the clay and then remove it to create a mold.

Mix and stir plaster and water in a paper cup with a dowel or stick to the consistency of a thick milkshake. Pour it into the mold created in the clay. Use the dowel to get all the plaster out, and if you‘re making a number of fossils and will need to re-use the dowel, wipe it clean right away with paper towels before the plaster hardens on it.

Gently tap the bottom of your container with the clay and plaster several times against the tabletop to ensure that the plaster completely fills the mold and to remove any air bubbles in the plaster.

It takes about 15 to 20 minutes for the plaster to dry enough to complete this project, and if you‘re working with a lot of kids, it‘s easy to mix up which fossil belongs to whom. Have kids write their names on small strips of masking tape with pens or markers and affix them to their fossil containers. Set all the containers aside to dry. During this drying period, you should have another activity; otherwise, you‘ll hear is my fossil ready yet? about 200 times. This is a nice activity to do prior to a meeting; once the meeting is over, before everyone goes home, you can return to the fossils. Or, after setting everything aside to dry, you might show a video about fossils and cap it off by having everyone unveil and share their newly minted fossils.

Once the plaster has dried, tear the cardboard container and peal the cardboard away, leaving a layer of clay attached to a layer of plaster. This gives you a chance to talk about layers of sediment and to show kids how fossil-bearing sediments usually (but not always) form in discrete layers.

Peal the clay away, and your kids will find a cool fossil in their slab of plaster. Many kids then write their names on the backs of their fossil slab.

Optional. Have kids paint their fossils. Glossy or flat enamel paints (the kinds used with plastic model airplanes and cars) work well in shades of black, gray, brown, or beige. Craft stores often carry textured and paints, so kids can paint the surface around the fossil to resemble a real matrix. I‘ve also found a pearly coating at one craft store. I painted it over an ammonite cast that I had painted a brassy brown. The pearl coating gave a glossy, iridescent sheen just like real mother-of-pearl. Experiment with different sorts of paints and coatings like these.

I‘ve also been told about a fossil-making activity that calls for mixing together one cup of used coffee grounds, one-half cup of cold coffee, one cup of flour, and one-half cup of salt in a mixing bowl. Knead this into a dough, flatten it on a sheet of waxed paper, and cut out small squares or circles. Press objects such as scallop, snail, or clam shells into the dough, remove the object, and either allow the dough to harden for a day or two or bake it briefly in an oven. The resulting items have the look and feel of a real fossil in matrix. Or, for yet another, simpler process to make fossils using just self-hardening clay. (see Activity 10.4 Sedimentary rocks: Making fossils)

Note: You can use any of the activities described above to help kids satisfy requirements toward earning both their Fossils and Earth Processes (Activity 10.4.b) badges simultaneously.

Making "Tri-lo-Bites"

Even more fun than making plaster fossils—especially for younger kids—is making "Tril-o-bites". Dennis Gertenbach of the Flatirons Mineral Club in Colorado sent this activity. Haul out your grandma‘s Christmas cookie recipe, gather kids in the kitchen, and work with them to fashion, bake, and decorate cookies with frosting in the forms of trilobites, ammonites, star fish, dinosaurs, and other fossil creatures. In the process, they‘ll be learning about the names and shapes of different fossils in a way that should leave a good taste in their mouths when everyone gets to eat their fossil creations!

Excavating a Fossil

This activity gives kids a really fun way to learn about the basics of fossil excavation without even leaving home! Mix together plaster, sand, and, optionally, a bit of diatomite. (Diatomite is available with swimming pool supplies in hardware stores.) You may need to experiment to get the right proportions of plaster, sand, and diatomite and the right consistency. Pour it into a small container. Set a fossil into this matrix. Lightly spraying the fossil with a vegetable oil like Pam will make it easier to chip out. You might use a real fossil (a crinoid stem fragment, a brachiopod, a shark tooth, etc.) or a small plastic dinosaur skull or skeleton from a toy store. (I‘ve obtained some at our local 99¢ Store.) Pour more matrix to cover the fossil. You may want to keep a tip of the fossil emerging from the top so kids will know where to begin excavating. Once the matrix has dried, it can be removed from the container. Make one for each kid in your group. Give each a nail to excavate fossil treasures. Then have them learn about the fossil each has excavated.