Spray the model or sculpture that is to be molded with a liberal amount of spray-on polyurethane rubber release agent. Allow it to dry for several hours.
Paint the sculpture with three coats of liquid polyurethane rubber molding material, allowing each coat to dry partially for four hours before applying the next coat. This forms the intermediate rubber mold for the wax model.
Peal the rubber intermediate mold carefully off the model. Small areas of a model, like hands or fingers my need to be massaged or twisted to get them to release the rubber mold. Turn the rubber mold inside out and use paint thinner and a rag to remove the release material.
Place the rubber mold into the center of a pile of rags to support it. Melt and pour liquid casting wax into the rubber mold, filling it completely. Insert a piece of wire or coat hanger into the wax for a holding wire. Allow the wax to cool for eight hours.
Make a sand-casting bucket by drilling a 1-inch hole in the side of a 5-gallon plastic bucket, 2 inches above the base. Insert the nozzle of an air compressor through the hole, and lay the nozzle on the bottom of the bucket. Fill the bucket with 3 inches of ceramic silica sand, and cover the bucket with a heavy rag.
Cut the rubber molding material away from the hardened wax convex copy of the sculpture. Pour the liquid silicone adhesive into a small plastic bucket. Hold the wax model by the wire and dip it into the liquid silicone adhesive, being careful not to wet the flat bottom of the wax model. Allow it to drip off for 30 seconds.
Turn on the air compressor, and pull back the rag covering the top of the 5-gallon bucket. Wait until the bubbling sand begins to swirl around in the upper part of the bucket. Insert the adhesive-covered wax model into the bucket. Without touching the sides or the sand surface, pull the rag over your hand, and the swirling sand will begin to attach itself to the wet silicone adhesive.
Hold the wax model inside the bucket for three minutes. Remove the sand-covered wax model and hang it up to dry for one hour. Repeat the process 19 more times, producing a half-inch thick silica/silicone mold covering the wax positive model.
Pull the wire from the wax. Wrap a coat hanger around a cubic or cylindrical shape or small box to create a small frame that the mold can rest upside down on while it is being heated. Place the mold into a small metal pan and place it in an oven at 450 degrees Fahrenheit for two hours. The wax will melt and run out of the mold. The result is a concave silica mold that can be poured with molten metal -- brass, copper, aluminum or pewter -- to cast a near-perfect reproduction of the original sculpture or figurine.