Molding And Casting Rocks

My last post dealt with creating rocks by hand modeling. So this time I thought I'd touch on a couple ways of making rocks by casting them from molds.

Sometimes while walking the beach I'll find a rock or small piece of driftwood that calls to me. I don't know where I'll use it but if it is the perfect scale for using in a sculpture I'll hang onto it.

On occasion I'll make a mold of these found rocks. The rock I'm holding in the photo had barnacles adhering to it which I thought would make an interesting addition to a marine diorama.

Generally the materials I'll use for simple molds are liquid latex or 100% clear silicone caulk. For my mold separator I take a glass baby food jar and put a blob of Vaseline petroleum jelly in it.

To this I add a few drops of Naphtha. I keep adding the fluid and stirring the mixture till the Vaseline is thinned down to a brushable consistency.

If the model isn't too intricate another economical separator to use is neutral colored shoe polish. You apply it to your model and then buff it with a soft cloth.

On rocks I stick with the Vaseline mixture and just take an old bristle brush and apply it directly to my model.

After the separator I brush on multiple coats of liquid latex. Usually about 8 coats though that can vary due to the size of the mold. When the latex has cured I apply a bit of my separator to the outer surface of the mold and make a mother mold which will support my flexible latex mold. Sometimes on small molds I'll use Apoxie clay to make my mother mold. On larger molds I'll use Bondo Auto body filler.

Once the mother mold has hardened it is removed and the latex mold peeled away from the model.

The above photo shows a latex mold of the rock and its two piece mother mold. The rock was cast using a mixture of casting resin and Bondo auto filler.

The yellowish casting in my hand was made using Polytranspar Liqua-Cast from Wasco: WASCO -- Wildlife Artist Supply Company

When I don't have time for the multiple coats required for building up a latex mold I'll use 100% clear silicone caulk.

I'll use the same Vaseline mold separator on my model and apply a small amount of the silicone with a caulking gun.

Using a disposable brush I smooth the silicone over my rock model. Because the silicone is so thick this technique doesn't work well on highly intricate models. But for this river rock it worked fine. I try to get an even application of about a 1/4" thick.

The mold I'm holding is made of silicone caulk with a two piece mother mold of Bondo Auto filler.

When it comes time to cast the molds I'll brush a bit of separator in and use Apoxie Sculpt clay pressed carefully into the mold.

Other times I'll cast a mixture of casting resin/Bondo into the mold to make my rock. This is a picture of a small latex press mold and my cast rock.

In the photo below the lighter colored rock is Apoxie Sculpt while the darker one was made by pressing stoneware clay into the river rock mold. The size difference in the rocks is caused by shrinkage during the firing process.

Another option when it comes to making molds is to use the type of materials offered by companies such as Smooth-On - Mold Making and Casting Materials for a World of Applications!

The photo below shows a mold of the same barncle covered rock made using Smooth-On rubber in a plastic container in one pour.

After it cured the mold was partially cut apart to free the model. This method leaves much less seam line to clean off your casting then a traditional two piece mold would.

My King Kong Casting Gets Some Teeth

After I prepainted the teeth I cast for Kong it was time to install them in his mouth. Using a Dremel tool with a cutting bit I carefully removed the excess material around the mouth.

Once I had access to the mouth I began building up the gums and interior of the mouth using Apoxie Sculpt clay. This is a two part modeling material that is mixed in equal amounts by hand. It sets in a few hours by chemical reaction.

When I was satisfied with the gums I marked off the locations where I wanted the teeth to go. Next I took the tongue that I had sculpted the evening before and set it into the wet clay. One of the benefits of using Apoxie Sculpt is that it acts like an adhesive and will bond strongly to most materials.

I made a slight depression for each tooth and sunk them into the gums. Using a stiff brush and some water I smoothed the gum area around the teeth and the tongue. After the Apoxie Sculpt set up I painted a base coat of acrylic pinkish flesh on the gums and tongue. Here are a couple photos of Kong with his new choppers.

Casting Kong's Head

After I finished building up the latex rubber mold to about an 1/8" thick I made a mother mold. A mother mold is made of a rigid material and supports the flexible latex mold while it is being cast to prevent distortion. For this piece I used plaster of Paris and strips of burlap to make the mother mold.

The casting material I used is called Polytranspar Liquid Cast. It is a two component product which requires mixing together equal amounts of Part A and Part B. The reason I like it is because it sets up within 3 minutes, has little odor and can be removed from the mold in a short time.

I sprayed the inside of the two mold halves with a light coat of mold release and clamped the mold together. After mixing the Polytranspar liquid cast I slowly poured it into the mold trying to avoid causing air bubbles. When I had poured about 8 oz. into the mold I began to rotate the mold by hand to make sure all the surfaces of the mold were evenly covered. I continued doing this for several minutes till the material set up.

Here is a picture of Kong's head fresh from the mold and ready to be cleaned of the flashing left on the casting from the mold seam. Next post I'll get down to installing some teeth in Kong.