How to make homemade modeling clay?

How do you make a mold of a modeling clay sculpture?

• I made a car design out of a wax based non-hardening modeling clay. I'd like to make a mold of it so I can cast my design into a more durable material. Any advice? All tutorials on youtube show how to make a mold of a very solid object but mine will not harden so I don't get a second chance at making the mold.

• Answer:

  You need to make a flexible rubber or silicone mold (RTV = room temperature vulcanizing) - the mold needs to be flexible for removing the original and for casting up copies. You could use a brush on liquid latex such as "Mold Builder" by Castin' Craft, available at most craft stores like Hobby Lobby and Michaels - this you can do by building up the mold in layers, letting at least 12-24 hours (air dries) pass between layers (you will also have to ruin cheap brushes or use urethane sponges to coat the original model with rubber). Smooth-On has a variety of products for your needs - they also have booklets, tutorials and workshops that you can attend or videos you can buy. You might try to find a local distributor for their products in your area or order online. For your positive copies I would recommend a polyurethane resin plastic such as Smoothcast 300 series or even plaster or polystone. But you will have to make a "box mold" out of wood or foamcore board and if you plan to do it in one piece you will have to have a way to remove the original sculpture and also have a way to pour material into the empty mold "shell" to create your positive copies. Usually a flat bottom is the best for this.