This isn't the only way to mold a sculpture for mask making, but it's what I did for the Old Hag posted earlier this month (sculpted in WED clay), and it works pretty well.
First a light, even coat of crystal clear (Krylon is the appropriate brand to use, but I had Rustoleum on hand and it worked fine). This is to seal off the water content of the clay so it doesn't interfere with the plaster later. Ooo, shiny! Make sure to give it plenty of time to dry before proceeding.
Back inside. I decided to mold this one laying down, so I propped the sculpture face up on a piece of bubble wrap (to protect the back and keep it from rolling), then put some bricks and blocks of wood to fill in most of the space before I build the clay wall.
I used EM-210 White Clay (which is a fairly dark grey, for whatever reason) to build the mold wall.
I added keys and pry points, then did another round of crystal clear to seal the EM-210. Then a coat of mold release. Usually I use Epoxy Parfilm. This one is called Frekote Lifft. It seemed to make the plaster start setting up faster on the surface. I'm not really sure what was going on, so I'll have to experiment. Some kind of mold release is important, though, as you'll see later. Without it, the crystal clear would be going right up against the plaster, and can sometimes stick together.
So, plaster time. I'm using Ultracal-30. It's stronger than hydrocal, though it is less absorbent, which means when I run latex in the mold, it will take longer to dry, but I'd rather that than a mold that doesn't last as long for this.
I also added an "acrylic bonder and fortifier," Silka brand, from the cement department at Home Depot to the first batch on each half of the mold. This adds some extra durability. About 50/50 with warm water. Then the ultacal is sifted in.
...until you get a "dry lakebed"
I mixed by hand this time, but using a paint mixing drill attachment is more efficient. Let it cream (when it just thickens a little), then brush on with a chip brush. This is two batches of plaster with two layers of burlap inside. plaster, burlap, plaster, burlap. The burlap is wet slightly to help the plaster absorb into it, then dip the strips in the bucket of plaster. It takes some practice to figure out how to do it well. I'm still learning. The burlap, by the way, is another thing that will slow the drying of latex, but it adds durability.
Once the plaster was set, I flipped the whole thing over and removed the EM-210 to be able to make the mold of the back of the head.
You can see how cleanly it came away. This is because the sculpture was sealed before building the mold walls.
A healthy coat of vaseline over the exposed stone, and then the back of the head was molded the same way as the front (acrylic bonder in the first layer of plaster, two layers of burlap-soaked plaster, and a final "beauty coat" of plaster).
When the mold was opened (which took some careful prying, but that's what the pry points were there for), the clay peeled right out because of the combination of sealer and release.
Casting the first copy.