The mold halves are then mated together. Some extra denture adhesive or CA glue can be useful here for holding the alginate halves in place. Note that the two holes in the nose area need to be filled; you can use alginate or denture adhesive for this. Be careful not to press too hard and distort the nose. We recommend mold straps to hold the halves together while you're casting the positive.
Mix up a batch of UltraCal. This is a dense gypsum cement which is very good for reproducing small surface features. You could also use other mixed stones, such as plaster. UltraCal is a fine gray powder and should be sifted slowly into a bucket of water. The finished mix should be about as thick as pancake batter.
Pour the UltraCal into the uprighted head mold. Using some towels around the base of the mold can help to steady it. After adding enough to fill the head about up to the eyes, use your hands to move the UltraCal slurry inside the mold. Splash it (carefully) around the inside of the mold to ensure that all of the alginate surfaces are coated. This "splash coat" will help to pick up details and prevent surface air bubbles.
Add more UltraCal and continue to distribute it by hand. Pull it to the upper parts of the mold and allow it to flow back down. As it dries, the UltraCal mix will become thicker. The reason for all of this work is simply to help reduce the weight of the finished head. You can just pour a bunch of UltraCal in the mold and create a solid bust; it's easier and it works. But the finished head will weight a lot more and be a pain to move anywhere.
As the UltraCal is finally setting, do some last smoothing on the inner surfaces. This view of the finished cast, versus the original mold, gives you an idea of how thick the inner walls are. Leave the cast like this to dry for a couple hours. You can gauge the progress by feeling for the heat released by the UltraCal as it cures. It is safe to remove the headcast once the inside surfaces have cooled to room temperature. It's done!
Here's a shot of the workspace after we finished four headcasts. This gives you some appreciation for the mess involved in such a project. So plan wisely, stock lots of paper towels, use large dropclothes, and have friends to help out.
Several of the heads resulting from the day's work are left to cool on the lawn.
