Most polymer clay dolls have a metal armature embedded inside. Armatures can be made from a variety of metals; common types for small dolls are aluminum and galvanized steel wire, aluminum tape, copper tubes, and aluminum foil.
Aluminum tape is used to secure wires together. All aluminum tapes are rated for maximum temperature. Some take higher temperatures than others. If baked at a higher temperature than rated, the tape will bake without damage to the aluminum surface but the glue becomes soft and melts. A tape rated at a temperature lower than that used to cure polymer clay (up to 280 degrees F) does not seem to react badly to that firing temperature because covering the tape with aluminum foil and masking tape may shield the tape from the higher temperatures.
Copper Tubing is used to increase strength and/or to function as a stand or connection to a base. Wire is threaded inside the tube and is secured by tape or crimping.
Aluminum Foil is used to pad the armature. It bulks out the doll body to reduce the thickness of polymer clay covering the surface. You need to compress the foil lightly so air pockets have room to expand inside the body during baking (air expands when heated and could crack the clay). Some doll makers do not recommend using aluminum foil for small dolls.
Masking Tape or Floral Tape is used to cover all metal surfaces of the armature (wire, tape, tubing, and foil) to keep the polymer clay from picking up oxidation from metal surfaces. Wrapping armatures in masking tape also keeps your hands clean and avoids getting the clay dirty while sculpting. Covering the armature helps hide the metal armature under the polymer clay where it comes close to the surface. For small dolls, masking tape is also used to secure wires together.