At this point, I knew my head didn't look right, but I wasn't sure what was wrong exactly. I needed a reference, so I looked at pictures of faces in magazines and refined my head.

The finished head.

Next, I prepared to make a silicone mold around the head. The traditional way to make a silicone mold is to buy 2-part RTV silicone and pour it over the master. I'm sure that method works extremely well, but Scott Holden showed me a cheaper method, and I was excited to try it out.

Because my head had details on both sides, I needed to construct a 2-part mold. With thick paper and a hot glue gun, I constructed a thin temporary wall to establish a clean border between the two halves.

The face half came first.

To try the silicone puck mold-making method, prepare a tub of very soapy water. Soapy water will tend to keep the silicone from sticking to your hands.

Slice the tip off of the silicone tube and push a little out of the tube.

I used pucks about the size of an Oreo Cookie. Slice them off into the water with a plastic knife or stir-stick.

Warning: The solvent used in silicone sealant could be dangerous. I believe, based on information on wikipedia, that the reaction product of silicone for consumer use is nontoxic acetic acid (vinegar). It sure smells like vinegar. However, if the solvent is something else, it might be toxic.

http://en.wikipedia.org/wiki/Silicone

With soapy hands, pick up the silicone puck and place it onto the surface of your master model. Smooth it flat onto the surface with a finger, obtaining 1/4th or 3/16ths of an inch of thickness. Smooth more wet silicone pucks next to the first one, overlapping the edges until the entire surface is covered.

In about 10 minutes, the entire face was covered. I used one tube of silicone and nearly ran short by the end.

I let it dry overnight.
I was tempted to continue with the back half, but I decided to make a "mother mold" next. The mother mold helps support the thin silicone mold and to keep it from stretching out when I mold a head out of expanding foam.

My first attempt to make a mother mold consisted of crafting a cardboard box which fit closely around the clay head. Unfortunately, when I filled the box with expanding foam, I used too much. As the foam expanded, the clay master was lifted out of position.

After that failure, I realized it would be almost impossible to correctly estimate the precise amount of expanding foam needed to fill the box. My next idea was to flip the head onto its back and spray on a mother mold from the front side, hoping that the resultant shell would be strong enough to support itself. I also tried adding a more substantial separation wall out of clay, but I lost patience with trying to get it to adhere to the head and removed it.

At each stage, I wanted to tear off the mold and see how it turned out, but I patiently waited until the silicone mold was dry, and the foam mother mold, and the back silicone and the back foam mother mold, until finally I could remove all of the molds and see if I had a mold that I could use.