Modelling wax is ideal to use as a core. You can use this material immediately without taking into account drying. Take the required amount and make desired core shape by heating the wax with your hands. Form Art Clay silver clay or other metal clay around it. Place the piece over aluminium foil and heat with a heat tool or hair dryer. Let the wax flow completely out of your piece through an opening of about 2mm. Then bake the piece with sufficient ventilation to prevent wax residue from burning. (14cmx7.5cmx4mm)