Additional info - working with Styrofoam-

Forgot to say, when using the Styrofoam. You need to fit the Styrofoam round the armature you can use duck tape around it, then to hold together use STUFF-IT spray foam insulation inside the areas were the Styrofoam does not meet. Then you let dry, then cut off the extra foam.

Helpful Hints, working with styrofoam- under the clay

Saving on the cost of clay and for the sculpture to weigh less use styrofoam over the armature.
When you use Styrofoam under the clay, the helpful hint is to melt brown wax and then brush on the Styrofoam. Someone else said they put Saran wrap over the foam and them apply the melted wax.

Hope this helps.