1. Imbed the model in modeling clay up to the parting line. Leave bumps and hollows in the surrounding clay to allow the completed mold halves to interlock.
2. Make a box out of cardboard as a form for the mold. Allow about 1/2" of space around the model except where the funnel is to be. Allow 3/4" to 1" on that side.
3. Shape a half funnel out of clay. This can be skipped if you choose to cut it out of the completed mold later.
4. Add any vents using pieces of wire.
5. Apply thinned petroleum jelly to the model. Allow it to dry.
6. Apply petroleum jelly to the inside of the box. Mix the amount of liquid RTV rubber needed for this half of the mold per manufacturer's instructions. Pour on a small amount and use a brush to coat the model, making sure there are no air bubbles trapped on its surface. Pour the remainder of the rubber. Allow it to cure overnight (about 12 hours).
(First half complete.)
7. Remove the box and the modeling clay. Invert the completed mold half and reassemble the box around it. Complete the clay funnel.
8. Apply thinned petroleum jelly to the model and the normal to everything else. Allow the model to dry.
9. Pour this half as you did in step #6.
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