Measure and draw a line along the middle length of the pedestal. The line needs to divide the entire pedestal in half lengthwise.
Lay on your work table the column pedestal you want to make a mold of. Make a foam board or plywood box frame that fits around the column but leaves at least a 5-inch space all the way around the column. Expect there to be more than a 5-inch space on the lateral sides. Make the width of the box at least 5 inches taller than the width of the column.
Roll out two or more slabs of clay that will fit into the bottom of the box frame. Press all areas of the clay, especially the edges, into one flat slab filling the bottom of the box frame.
Cut a 5-inch piece of PVC pipe with the hand saw. Cut the 5-inch piece again lengthwise to create half a fill tube for your mold. When the two-piece mold is joined, the molded PVC pipe will make a complete fill tube.
Lay the column in the box on the clay slab. Lay the halved 5-inch piece of PVC pipe at the top or bottom of the column. Place the curved plastic side upward and the open side toward the clay base.
Add balls of clay and smooth them around the pedestal until it is embedded in the clay up to its halfway mark. Even the clay out so it lies flat against the pedestal at this mark. Add small amounts of clay to fix the PVC tube in place.
Mix the liquid rubber compound in the quantities specified on the packaging. This type of compound is sold in a two-part kit. Spray mold release agent on the column and pipe so you can easily pull off the finished rubber mold.
Brush the liquid rubber on the exposed half of the pedestal and PVC pipe. Brush the rubber about two inches out onto the clay base on all sides, creating a rubber edge around the column. Allow the rubber layer to dry for an hour. Add another six to 10 layers in the same way. Allow the mold to dry overnight.
Remove the rubber mold from the column by gently pulling up a corner and lifting the entire rubber mold from the column. Lift the column from the clay, turn it over and embed the previously exposed side you made a mold of. Place the PVC pipe in the same location as on the previous mold half you made. Spray this side of the column with mold release agent.
Brush the liquid rubber layers on and allow each layer to dry again. Be sure to brush the rubber out two inches around all sides of the column, allowing for the 2-inch edge you need. Allow this half to dry as you did the first. Remove the mold as you did previously. Join the finished mold halves together and press firmly along the entire 2-inch mold edges. Band the mold together with rubber banding.
Mix your column-making compound according to the package instructions and pour in the rubber mold to make the column. Allow the mixture to set for the time specified by the compound instructions. Remove the mold halves to obtain the molded pedestal.