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.
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
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.
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.