We use a special technique to help bond the alginate to the plaster we'll be applying as an outer mold. To do this, we attach terry cloth to the alginate. Since the alginate has already set to a firm state, we need to soften it a bit so that we can get the cloth to adhere. This is done with a spray bottle filled with a special alginate softening chemical (ours is from Douglas and Sturgess -- see the Materials Sources page).
An old towel is cut into small squares which are embedded in the alginate with the aid of our spray chemical. The terry cloth hooks can get a decent hold in the alginate. The plaster, which doesn't bond at all with alginate, is able to connect with the exposed surface of the squares and hold our finished mold together.
The outer plaster mold is done in two halves. To prevent accidentally going beyond the centerline, we use a piece of SaranWrap. This is also used later to prevent the the halves from bonding. For this part, just concentrate on laying down the plaster bandages so that they overlap about the first inch of the plastic wrap.
Plaster bangages are applied to make the mother mold directly on top of the alginate. These bandages are available in large rolls. Cut a length of bandage, dip it in water and then squeeze it lightly so its evenly damp. It's very helpful to have a separate person cut and lay out lengths of bandages.
Apply at least three layers of bandages all around the front half of the person. Try to create a smooth line around the edges of the mold. Reinforce around the bottom edge, too, as this will be the upper lip of the finished mold when it comes time to fill it. Rubbing your fingers over the bandages helps to smooth them together and distribute the plaster well. Leave the first half to dry (10-15 minutes).
Once the first half is dry to the touch, create the second plaster half-mold. To keep these bandages from sticking to the earlier ones, use the plastic wrap again. Fold it forward, over the dry plaster mold, to create a barrier. Use a butter knife to pre-slice the alginate; make a shallow or intermittent cut where the edge of the first mold is. Then add bandages to create the mold for the back half of the head.
Once both plaster halves are dry. carefully remove the entire mold. The alginate is attached to the inside of the plaster shell (via the terry cloth), so it will come off with the plaster mother mold. This is why it's important to score the alginate before finishing the plaster layer; the cuts will encourage the alginate to tear long the chosen line with relatively clean edges. The loose edgesof the alginate can be mated with the plaster using denture adhesive.