Have you closely looked at any kind of cheap rubber skull, mask, or other trinket? There's usually a seam that runs down the object somewhere. That's an imperfection that shows where two halves of a mold came together when the product was formed (it's really noticeable in the skull JPEGs at the top of this page). Hopefully, the model skull you are using has this seam like the one I used. If not, just wing it.

Start by taking the scrap wood pieces (simple 1/2 inch plywood should work fine) and build a box that will fit around the entire skull, leaving about a 2 inch border on all sides (the box drawn in these diagrams is not to scale, and the sides should be tall enough to completely contain the skull). The face of the skull will be the first half of the mold, the back of the skull will be the second half. Put the box together with small wood screws or double-headed nails and you will be able to take the box apart after making the first half of the mold in order to rebuild it to make the second half. In the bottom of the box, cut a hole with a jigsaw (I used a reciprocating saw) that will roughly fit the size of your skull's face. Place the skull in the hole, face up, looking out at you from the inside of the box. Fill any gaps in the hole with nondrying clay, in addition to molding a "clay tunnel" from the bottom of the skull to the edge of the inside of the box. When your mold is complete, this tunnel will allow you to spray in your foam to make your skulls.. You may also use the clay to seal the seams of the box itself to prevent the RTV rubber from leaking out of the box after you pour it. Rig the box on top of a couple of bricks so it will be able to lay flat & stable, since the other half of the skull sticking out the bottom of the box will cause the whole thing to wobble. Now mix your rubber and catalyst together well, following the directions on the label, and pour it over the face of the skull in a steady stream, until the entire face is coated. Work with small batches at a time so you don't waste the RTV rubber. Mix a little, pour it over the skull, wait an hour, mix a little more, pour it over, and so on. When you have a fairly thick coating allow the rubber to cure at room temperature for at least 24 hours.

The next day, check your mold to see it has cured properly. It should feel solid, yet flexible. Don't take apart the box yet. Now what you need to do is create a "mother mold" out of plaster that will keep the RTV mold from expanding and stretching when you create your foam skulls. Take a little of the liquid dish soap or vaseline and grease the insides of the box that are not covered with rubber. This will keep the plaster from sticking to the box. The plaster will not stick to the rubber ( nor will anything else). Now mix some of your plaster with water according to the directions on the label, and pour it in the box, covering the entire mold/face. Allow it to cure for at least an hour, then you should be able to take apart your box and restart the process over to create the second half of your mold (the back of the skull).

Now you should have a completed skull-making mold and mother mold. When sandwiched together, the two halves will form the shape of your original skull. Using any kind of a casting agent (I use foam in a can, but you can use plaster, or clear silicon, or a variety of other types) you can now pour it into your mold, let it cure, then pop out your finished copy.

Now for Evil Jay's shameless advertising plug. This is Great Stuff Minimal Expanding Polyurethane Foam Sealant. This is the foam in a can product that I use to make my skulls. There are many different brands, but this one seems to work the best. You can find it at any home improvement store or local hardware store. CAUTION: this stuff will stick to your skin and not come off for months. You must wear gloves and eye protection when using this stuff. If you get this in your eye you will be going to the emergency room. Use caution when using this product!

Start by placing each half of your RTV rubber mold into the plaster mother molds. After shaking the can well, start slowly filling the "face" half of the mold with foam until you have almost filled the entire half. Now add the other half of the mold to the first one and bind the two together with a few bungee cords. Turn the whole mold upright, and fill the remaining space with the rest of the foam until it reaches the top (you are squirting the foam through the "tunnel" you built with clay when you were constructing your molds). The foam will expand and begin to pour out of the top after about 30 minutes. If you can, every once in a while, scrape this extra runoff away with an old spatula or putty knife to allow surface air to enter the hole. Be sure to wear gloves when doing this. Now comes the hard part: waiting. I usually let the mold sit undisturbed for at least three days to a week. Once you feel confident that the foam has cured, start by detaching the bungee cords, lift off the back part (not the face half) of the plaster mother mold, then slowly peel off the back RTV rubber piece. I usually let the face half remain in the mold for another hour or so, just to see if any uncured foam in the middle of the skull "erupts" from the back of the head. Then, when I'm brave enough, I remove the "face" mother mold, and pull and pry the rest of the skull out of the mold like popping an ice cube out of an ice cube tray. Let the foam skull sit for another day to make sure it is completely cured and then you are ready to paint and detail it.

You now know all that I can teach you about making skulls, my evil minions. If you want more information, I suggest picking up a copy of "The Prop Builder's Molding & Casting Handbook" by Thurston James. I learned the entire process from this book. Have fun!