What type of clay are you using? and what is it's shrinkage percentage rate? Are you going to fire the mask afterwards? Was the clay liquid or workable clay?

When you put the clay into the mold spray mold with WD-40 and take the clay out of the mold when it has firmed up somewhat, but is not completely hard - the nose and other parts sticking up would cause the clay to pull apart and crack when it shrinks as it is drying. Also the clay should be covered with plastic and let to dry very slowly and evenly - this helps prevent cracking.

Check with a local ceramic artist near you for more info on working with clay.

If you had the clay in just the inside of a bowl, this would not be a problem as the clay could shrink in any direction it wanted to, but with a mask there are parts that keep it from shrinking evenly.