Gather Materials and Prepare Partition
To create a simple plaster mold, you will need the object to be molded; clay; two disposable containers; Vaseline, mold release or orange shellac; plaster; water; and a brush. In the case of large molds you may require additional clay or wooden strips.
Examine the object to be molded. The mold will be two parts, so you must decide where the center line, or partition line, will be located. The easiest way to do this is to hold the object parallel to the table top and envision the object cut in half horizontally. Draw a line on the object to mark the partition line. You will create a plaster mold of the top half of the object first. To do this you will need to make sure the object can stand level unassisted. Build thin strips of clay around the partition line. Press the object into the clay and smooth the joint between the block of clay and the partition line clay. There should be clay extending at least 1 inch (more for large objects) around and underneath the object.
Prepare the Mold
Plaster is liquid, so it needs a container to contain it until it dries around the object. If possible, find a disposable container that snugly fits your clay-embedded object. Alternatively, you can use a larger container and fill any empty spaces between the object and the container walls with clay. If the object is especially large or if you cannot find a suitable container, you can use strips of wood and additional clay to create a container around the object. If you create your own container make sure the walls are at least twice as high as the highest point on the object so that there is enough room to hold all the plaster.
Your object should be embedded in the clay, and the clay should be securely in the container with no spaces between the clay and the container wall. Only the top half of the object--the half you will be molding--should be visible. Use the end of a pencil to make four depressions in the clay around the object. These depressions will help you fit the two pieces of the mold back together later on. Cover this half of the object and the exposed clay and container walls with mold release spray. Make sure to coat all exposed surfaces. If you do not have mold release spray you can use orange shellac or Vaseline to create a barrier between the object and plaster. This barrier is necessary to prevent the plaster from sticking to the object and ruining the mold.
Prepare and Pour the Plaster
Mix the plaster powder with water according to the directions on the package in a disposable container. Stir thoroughly to remove any clumps or bumps. If the plaster has bubbles gently tap the side of the container. The bubbles will rise and pop. Try to remove as many bubbles as possible before the plaster starts to set.
Using a soft brush, gently brush a thin layer of plaster over the object. Add several more layers of plaster, each progressively thicker. Once the object has a thick coating of plaster, you can pour the plaster over the object and into the mold container. Allow the plaster to dry completely.
You can now remove the clay and container from around the object. Try to preserve the container for making the next half of the mold. Make sure the object is free of any clay. Lay the object plaster side down in the container. Fill any spaces between the plaster mold edges and the container wall with clay (if your container has perfectly straight sides there won't be any spaces) and coat the container walls, plaster and object with mold release. Make sure to thoroughly cover the plaster or the mold will stick. Apply the plaster as before.
Once the plaster is dry you can pull apart the mold halves and remove the object. The mold is finished.
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