Activity 3: Permineralization

Hip Wimpee (hippocampus)



Fossils are remnants of once living things. One way fossils form is by permineralization. This is when the pore spaces of a bone are filled with mineral rich water and internal crystals begin to form. Eventually the whole bone becomes rock. The activity uses a sponge dipped in wax to imitate permineralization.

Instructional Method: Activity

Goal: To teach students how permineralized fossils form.

Objectives: Students will be able to:

  • Make a comparison between their new wax sponge fossils and how mineral rich water crystallizes in the pore spaces of an organism's bones.
  • List the steps in how a bone becomes permineralized.
  • List organic body parts besides bones that can become permineralized.


Setup: 15 min.
Presentation: 30 min.

Materials Needed:

Paraffin wax, 1 block (2" x 2" x 6") per 10 students 1 Nonabrasive household sponge per student (the bigger, the better) Heat source and a pot/pan for boiling paraffin wax Rubber gloves and safety goggles Tongs Permanent Markers and Scissors Various colors of food coloring (optional) * Stencils of different fossil shapes download file Wax paper


calcium carbonate












Permineralized fossils, such as petrified wood, are often some of the most beautiful fossils. They are often filled with multicolored crystals. The various colors can be traced to specific types of mineral impurities (reds = iron, greens = copper, yellows = sulfur, etc.).

Sections of Petrified wood, large piece in foreground laying on it's side with an end toward the camera view.
Sections of petrified wood.

Jorge Dauerer

Most fossilized dinosaur bones and wood are permineralized. Permineralized fossils are created when voids and cavities in bone and wood, such as cells, are filled with minerals. The most common mineral in permineralization is silica (SiO2) or quartz. Oftentimes, the original cellular structure (pattern) is preserved. In the case of petrified wood, it is often possible to identify the tree genus. The various colors observed, especially in petrified woods, are due to trace amounts of other elements such as iron (Fe) and copper (Cu).

Eventually, all carbon-based tissues will rot, completely decomposing. But through a complicated chemical process called replacement, even things like cell walls and cell membranes can be preserved by trading their carbon molecules for rock forming minerals. When this happens, a large amount of biological detail is preserved, especially when the replacing minerals differ from minerals that caused the permineralization. For example, when silica-rich water is the permineralizing agent and iron pyrite replaces the tissues, paleontologists can see internal cellular structure. They can even distinguish between different cell organelles! This is only possible when the fluid space within individual cells is also permineralized. This high quality form of permineralization is called petrifaction and commonly occurs in petrified wood. So to be petrified, objects end up containing none of their original chemistry.

Although permineralized fossils are usually the most durable fossil type, they are not impervious. Extreme heat and/or pressure created by igneous or metamorphic processes can dissolve and melt them. Rainwater, being slightly acidic, will dissolve fossils permineralized with calcium carbonate. Furthermore, the same mechanical weathering that exposes them from their surrounding rock will also eventually destroy them.

Permineralized fossils are highly sought after by both rock hounds and paleontologists. While paleontologists value these kinds of fossils because of the detailed biology they record, collectors like them for their beauty and durability.

In this activity the sponge represents animal bone and the paraffin wax is the permineralizing agent.

a represention of three sponges that can be used to create fossils. The left sponge(blue) is blank for young ideas, the middle sponge (yellow) has a drawing that could be used to represent a tooth. The right sponge (pink) has a
three sponges that represent what can be used to create fossils


Instructional Procedures:

  1. Present background information to students.
  2. Pass out sponges, fossil stencils, and markers.
  3. Instruct students to use the stencils to trace the shape of a fossil of their choice or encourage them to draw one of their own design.
  4. Have students cut fossil shapes out of their sponges.
  5. Instruct students to write their names or initials on their fossil to prevent ownership confusion later.
  6. Cut a large enough piece of wax paper to accommodate the cooling of all the sponge fossils.
  7. Place paraffin block(s) in a pot on high heat. Add just enough water to keep the wax from burning (usually ¼ cup per block of paraffin is sufficient).
  8. Put on rubber gloves and safety goggles.
  9. Bring wax to boil and then back the heat off to minimize hot wax splatters.
  10. Use tongs to submerge each sponge fossil for 5-10 seconds in the boiling wax. Squish the sponges so that every pore absorbs wax.
  11. Place each sponge fossil onto the wax paper for cooling, being careful not to squeeze the sponge, or some of the wax will be lost from the sponge.
  12. Once cool, return the sponge fossil to the owners for their inspection. What once was soft and squishy should now be hard and brittle.


Ask students to explain how permineralized fossils form using either the sponge example or one from real life. Encourage the students to speculate as to what will happen to their sponge if it gets hot again or exposed to water. Help them draw analogies as to what natural processes can destroy real permineralized fossils.


Add different colors of food coloring to multiple pots of boiling wax to simulate different kinds of mineral impurities. As directed by the students, dip different portions of the sponge fossil in different colors of wax. Alternatively, experiment with squeezing the sponge in one color and then just quickly dipping in another, in an attempt to simulate different internal permineralization.


Have the students cut out their fossils so that the leftover portion of the sponge is one intact piece. Use the internal portion of the sponge and the external border to illustrate the concept of molds and casts. Tour a museum or natural setting where various kinds and colors of permineralized fossils can be observed. Initiate and guide a discussion about the conflict created between souvenir hunters / rock hounds and paleontologists. Explain how permineralized fossils are exceeding in value to both groups of people yet for entirely different reasons. Invite the students to come up with ideas and policies that might satisfy both kinds of people's needs.

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

Utah Science Core:

4th Grade Standard 4 Objective 1,2


Last updated: February 24, 2015

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