How to Make a Mold of a Fossil

Make a fossil mold or cast the way that professionals do. In grade school classrooms, people often use the old burlap and plaster method, but get relatively poor "resolution." If the authenticity and detail of the fossil cast is important to you, make the fossil more life-like using a technique to bring out the authentic details of the fossil.

Difficulty: Moderate

Instructions

Things You'll Need:

• Clay
• Wooden crate
• Dental alginate
• Fiberglass strings
• Polyester resin (or plaster)

1. Gently clean the fossil with a clean sponge to remove any of the matrix still attached to the cast. Make a clay bed around the fossil on a supporting surface (like a wooden crate). This bed should cover half of the fossil, leaving the fossil as shallow as possible both in the clay and above it. Make ridges in the clay so that when the mold is finished, the mold can be filled with plaster or liquid polyester resin.

2. Pour on dental alginate (found in dental supply catalogs) for a detailed look. Make this alginate from powder you buy from a dental supply catalog. Make sure that you pay careful attention to the amount of time it takes to dry and that the bubbles are removed using a source of vibration.

3. Cover the alginate with fiberglass strings to help the mold retain its form. Make sure that the sheets lay on the alginate in a pattern that has a crossing overlap to it. Make sure that the layers are embedded into the alginate and that they are supporting it. If necessary, the fiberglass can have a layer of polyester resin brushed onto the fiberglass to create a stiffer backing.

4. Remove the mold after hardening. Pry both sides apart gently and make sure that the fossil is not lifted out of the protective layer by the cast. The cast should be rubbery and flexible if dental alginate is used and the alginate will recover if gently pried away (use care not to rip the cast or disturb the fossil). Don't separate the fiberglass layer from the alginate layer that supports it while prying the layers off of the fossil.

5. Make a mold of the other side that was not in the clay, making sure that the fossil is clean before making the cast.

6. Press the two molds together so that they hold when the liquid (such as plaster or polyester resin) is poured into the mold through the channels made earlier.

7. Gently split the mold open and remove the excess material from imperfections in the mold and the channels in the mold. This can be removed using an air scribe and then the fine detail can be ground down using a dremel.

Tips & Warnings

• Do not do these activities unless the bone is completely stabilized and hardened and that the matrix (rock and dirt) has been removed. Do not do these activities without first protecting the fossil by properly hardening and stabilizing the fossil. Do not get the fossil wet unless absolutely necessary since it weakens the fossil and surrounding matrix.
• Casting fossils is not an exact science and there is always a degree of risk when a fossil is cast. Even experts damage fossils when collecting or casting them. Make sure that you use an expert if you are unsure of your abilities and have not practiced. The materials used to make the cast can alter the fossil, causing damage and staining the fossil in some instances.

Resources

• Fossil Casts (see page 63)
• Professional Paleontology Supplies (stabilizer and hardener)
• eHow: Making Fiberglass Resin (filling for cast)
• Resin Casting
• Making Skeletal Casts (see page 330, last paragraph)