Cork clay is the perfect filler to create hollow shapes with metal clay. After creating the shape with cork clay, you must allow it to dry for at least 24 hours. Do not force-dry the cork clay with a heat gun. After it's dry, coat the cork clay with metal clay.
Cork clay creations are usually fired in small jewelry kilns such as the Paragon SC-2 or Evenheat Kingpin 88.
It is important to fire cork clay in a well-ventilated area. Open windows and circulate the air with a fan. To prevent the cork from flaming out, always keep the kiln door closed between 500° - 800°F. Also, to give the cork enough time to burn out, fire no faster than 1500F per hour.
Leave the kiln’s vent plug out from the beginning until around 800 °F. Hold at that temperature for 1 hour. After the hold, insert the vent plug and continue to fire the metal clay to the clay manufacturer’s recommended temperature.
⚠️Caution: When venting materials such as cork clay, remove the vent plug at the beginning of the firing. If you forgot to vent the kiln at the beginning of the firing, do not remove the plug later. Removing it later during the firing may cause the organic materials to flame up for a moment. If you ever remove the vent plug and the cork clay flames up, keep the kiln door closed so the flames can die out safely.
Do not fire cork clay in a microwave kiln (the type that is placed inside a household microwave oven). The microwave kiln fires too fast to properly burn out the cork clay.
Just for fun... cereal, pasta, and bread are other good core materials. Do not use wax or Styrofoam as a core as they emit harmful fumes.