Creating identical copies of existing objects or parts is a professional-level task, but can also be done at home by any beginner. This process is called casting, and consists of pouring a material like plastic, rubber or latex into a mold to harden. In order to cast, however, you will need to first make a mold that is best suited to the object you are casting. One of the simplest mold materials is plaster, which is best suited to copying clay objects. To make your own plaster cast mold, follow the instructions in this article.
Instructions
1. Sculpt a model object out of clay, or choose an existing model object in another non-porous material. If you are working in clay, coat the sculpture with a clear sealer like Krylon Crystal Clear. If you are not working in clay, coat your model object with a mold release agent. Let the model dry completely.
2. Find the middle line of your model, all the way around it, that splits your model into two parts. Along this line, create a thin wall of modeling clay that separates the two halves of your model. Make the wall a few inches tall, or at least as tall as you are going to make your mold.
3. Set your model in a clean, well-protected workspace. In a bucket, mix up a small batch of plaster by combining gypsum with water. You will want a smooth, creamy texture while working with the plaster mix. Make sure that there are as few bubbles as possible, as these compromise the strength of the mold. Using a paintbrush, apply plaster to one side of the model object, from one end of the modeling clay wall to the other. Let this base coat, or impression coat, cure completely.
4. Paint another layer of plaster mix over the impression coat. While it is still wet, place small strips of burlap over the plaster, covering your model and allowing the burlap to soak up the plaster. Let this layer cure, then apply another layer of plaster mixture over it. Continue adding layers of plaster mix and burlap strips until you have the desired mold thickness. Thicker molds are best for larger objects, as more layers of burlap help strengthen a mold.
5. Cure the finished side of the mold, then repeat Steps 3 and 4 on the opposite side, applying layers of plaster and burlap until you have matching sides. Let the mold cure completely, then pry the halves apart and remove the modeling clay. You should now have two matching negative forms, which can be strapped together during the casting process.
