clay

Technically you can harden the clay by cooling it down in a freezer for a while. However it will get soft once it heats up again. Another way to do it is to melt the clay to the armature. When this clay cools it does become slightly harder until you knead it in your hands a lot. If you want hard parts you might consider sculpting the forearms, upper arms, lower legs and upper legs in polymer clay you bake. Then add soft clay where the joints are so they can bend. That is a bit tricky to do but has been done by some studios.