Latex Mold and Cast Making
This is the process to follow when making a mold of a fossil or bone. After the mold is made
follow the directions for cast making and make many casts (duplicates) of the fossil.
- A fossil or a bone from an animal such as a chicken.
- 8 or 9 oz. paper cups
- SMOOTH-ON brand SMOOTH-CAST 300 www.Techno-Industrial.com
- SMOOTH-ON brand Mold Release
- SMOOTH-ON brand So-Strong color pigment (opt.)
- Place the least important side of the fossil on a flattened piece of clay, then place the
clay on a piece of cardboard.
- Dab a layer of latex on the fossil filling in the spots that are not covered. Make sure
that the latex is not painted on too thin or too thick.
- If you see brush marks, itís too thin. Make sure that you leave an edge along the base of
the fossil that will stick out of the mother mold *(which will be made later).
- Let the latex dry completely before painting on additional layers of latex. Drying it near
a warm open window speeds up the drying time.
- It is recommended that you put on at least 4-7 layers of latex, thoroughly drying between
each coat. The more coats that are placed on the fossil, the more durable the mold
- When the last layer is dry, peel the latex covered fossil off of the cardboard. Do not take
the fossil out of the latex yet. Completely dry the latex that was sitting on the
- The next step is making a mother mold. Find a box as close as possible to the size of the
latex mold, making sure that no latex mold edges touch the side of the box. Cut the top of the
box off. Line it with plastic wrap or aluminum foil.
- Mix up a batch of plaster that would fit into one half of the box. Stir to the consistency
of whipped cream. Pour the plaster into the box. 1. Place the latex and fossil into the box
with the open end touching the side. Keep as close to one half of the latex mold above the
plaster. Hold the latex mold edge flush against the box until the plaster has
- When this has completely dried, cover the entire plaster surface with Vaseline to prevent
- Make another batch of plaster that would fit into one half of the box. Stir to the
consistency of whipped cream. Pour the plaster into the box. Place the plaster and mold
combination on top of the new plaster so that the other side of the mold is immersed the
plaster in the same way as before. Let this dry completely.
- When it is thoroughly dried, pry the two plaster pieces apart.
- Take the latex covered fossil out of the plaster. Carefully take the fossil out of the
latex. Sometimes the latex needs to only be rolled back slightly in order to pull the fossil
out. Other times you need to roll the latex almost completely off the fossil in order to
- Place the latex mold back into the mother mold. Use duct tape or strong rubber bands to
hold both halves of the mother mold together. Spray mold release into the latex mold.
- Mix up an amount of SMOOTH-ON brand, SMOOTH-CAST 300 following specific directions on the
jug. Estimate what you would need to fill one-half of the mold. This would be the amount of
Part A. (Do not pour water into your mold as that causes a reaction with the SMOOTH-CAST.)
The mix is a 1:1 ratio based on volume. Pour the same amount of Part B in one paper cup as
poured of Part A into the other.
- If you want the cast to be colored, this is the time to add the SMOOTH-ON brand, So-Strong
color pigment. The pigment is added to part B. It is a very strong pigment. Add it slowly,
until the desired color is shown, especially with small molds. The color will appear slightly
lighter than the color in the cup. If there is no pigment added, the cast will be cream
- Pour part B into part A. Stir until completely mixed. Slowly pour part of the mixture
into the mold. Roll the mixture around in the mold making sure all the tiny or unusual areas
- Then finish filling the mold. As the reaction happens, heat is given off and the mixture
will begin to set. After this happens it will take 3-15 minutes to set, depending on the size
of your mold.
- Take your latex mold out of the mother mold. Peel the latex back carefully and pull out
the cast of the fossil. You now have a duplicate of your original fossil.