Pile water-based clay onto a head armature and sculpt the face mask you would like to make. Cover any exposed parts of the armature with a thin layer of clay so it does not get damaged by the plaster.
Spray the clay mask sculpture with clear sealer and let it dry completely.
Cover your workspace with newspapers. Pour a small amount of water into a bucket, no more than a few cups, and mix gypsum powder into the water until you have a soupy plaster mixture.
Paint a thin coat of the plaster mixture onto the clay, covering the entire mask area. Make sure that you get plaster into the crevices of the design. Let this coat dry for an hour.
Cut a piece of burlap into small strips.
Make another plaster mixture and paint a second coat over the mask area. Place strips of burlap over the coat while it is still wet, so that the burlap sticks to the sculpture.
Paint a third coat of plaster over the burlap. Let this coat dry to the touch.
Add another layer of plaster and burlap, like you did in steps 6 and 7. Let the plaster dry completely, for at least a day.
Grasp the edges of the plaster mold and pull the mold away from the clay sculpture. You will likely damage the sculpture in the process, as water-based clay is very soft.
Pull any extra pieces of clay out of the mold. If clay residue has stuck to the inside of the mold, gently clean it out with a small amount of water. Let the mold dry again before use.