Molding and Casting Fossils

Fossil footprint (left), latex mold (middle), and cast (right)

People often feel duped, even betrayed, when they come across a cast (replica) in a museum. True, the real thing is maybe more awe-inspiring, but the fact is they are not always practical or feasible to display. Actual fossils are irreplaceable, often delicate, and very heavy. Casts therefore are extremely important as they resolve all of these problems and can be made relatively cheaply while preserving all the detail of the real thing.

This is a short tutorial to teach the beginner how to make a simple one-piece mold and cast of your favourite specimen.

STEP 1: Once you have selected your piece to be molded, the first step is to apply an anti-adhesive – something to prevent the molding material from sticking to your fossil. Here, I used a spray-on anti-adhesive called Ease Release™ 200, but you can use specialty glues such as vinac, which can be painted on or even Vaseline on small items.

Tools of the trade: an anti-adhesive (left) and a two-part silicone rubber compound.

STEP 2: For the mold, you will do well to invest in some high-quality silicone or latex compound. I use OOMOO® 30 by Smooth-On, which is a two-part compound that must be thoroughly mixed (in a 1:1 ratio) prior to application. Smooth-On offers a great range of products with different specs to suit different tasks. I would also recommend Dragon Skin®, which is very strong and won’t tear like OOMOO. The great thing about silicone products is that they won’t bond to anything (except more silicone or if your fossil is quite porous) and will yield hundreds of perfect casts, unlike latex, which gradually loses it’s detail and therefore has a limited shelf life.

STEP 3: With your now-mixed silicone, pour a thin layer onto the fossil. Pour it slowly and steadily and allow it to flow across the surface of the fossil. This will help reduce air bubbles that can ruin an otherwise good mold. You can use a paintbrush or spatula to help spread the silicon into hard-to-reach places. You may need to do several coats (allowing it to dry in between coats) in order to make the mold thick enough. Because some products tear or sag, you may need to add a reinforcing layer of tough, permeable material such as cheesecloth (muslin) before adding the next later of silicone. Chux® cloths also work really well.

Note: Depending on the size/shape of your fossil it may be necessary to build a barrier around your fossil that keeps the silicone in and/or allows you to fill it up with silicone. A plasticine barrier or a couple of sheets of perspex joined with plasticine work well for this.

Pre-mixed latex is poured directly onto the fossil. Strips of Chux® cloth were added between layers two and three.

STEP 4: Once the mold has dried and cured, simply peel it off the fossil being careful to avoid tearing the mold or breaking the fossil, especially if there are small, delicate parts that protrude into the silicone.

STEP 5: Making the cast. What you make the cast out of depends on what you want to use it for and also your budget. Plaster of Paris is cheap and readily available; however, it is relatively heavy and chips easily. Various plastic and resin compounds (check out the Smooth-Cast® and TASK® Series from Smooth-On) for lighter-weight, more durable options. Once mixed, these can be poured straight into your mold making sure it’s on a stable, level surface. If your casting material is quite runny (like plaster), gently tap around the edges of the mold. This will help remove any trapped air bubble leaving you with a better looking cast.

Plaster of Paris makes good cheap casts.

 STEP 6: After the plaster (or plastic) has cured, invert the mold to remove your cast and you’re done! Alternatively, you might like to paint it to make it more realistic.

Painting your cast can really bring it to life. Most types of paint will do the trick although watercolours won't bond to plastic or resin casts.