Some doll makers use scrunched-up foil as their under-sculpt. Others use epoxy sculpt products.
A relatively new product, Sculpey UltraLight, is a lightweight and extremely soft clay that bakes so hard that it won’t crack or break, even in larger pieces. The manufacturer said that UltraLight is a great replacement for foil under-sculpts on wire armatures and when covered with a layer of polymer clay, the UltraLight chemically bonds together with the covering clay. It is much lighter then epoxy sculpting products.