Things You Will Need
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Make a small box with ceramic pottery clay. The walls of the box should be about about 1" thick. Fill the box with wet clay.
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Coat the object you want to use to create the mold with oil and press the object into the wet clay. You can use any cooking oil (such as olive oil or Pam) as a mold release. Paint on the olive oil. Spray on the "Pam".
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Remove the object and let the clay dry thoroughly.
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Fire the mold in a kiln to at least 1800F to ensure they the mold will tolerate thermal shock while being used to fire cast glass. Note that that the clay will shrink (usually about 10%) during the bisque firing.
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To create a mold using a "ceramic slip cast" technique, begin with the same small box. Coat the object with oil and plant it upside down in the bottom of the box. Pour molten clay over the object until the box is full. Allow the clay to dry thoroughly and remove the object from the now dry clay. Fire to to at least 1800F until the mold is bisque.