There is a variety of modeling clay that you can use to sculpt a Halloween prosthetic. There is no right or wrong kind of clay, but there are several things to consider when selecting one, including what the prosthetic will look like.
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Water-based clay is a fairly familiar product. Most people have played with it in school, and it is available in art stores, craft stores and even in the craft sections of some general superstores. You can form this clay into a shape quickly, but it can be somewhat difficult to add fine detail into because of its softness. Water-based clay dries out with exposure to air, so it must periodically be sprayed with water. It also must be wrapped in wet paper towels and placed in a plastic bag while not being used.
Oil-based clay comes in a wide variety of colors and consistencies. The largest benefit to oil-based clay is that it does not dry out, allowing you to take more time with your sculpting without having to regularly wet it. It can also be softened and melted by heating it, allowing you to quickly create a rough form, then add finer details once the clay has cooled and hardened. Oil-based clay can be reused again and again, allowing you to create many prosthetics with a single batch of clay.
Sulfur vs Sulfur-Free
Oil-based clay comes in two varieties: sulfur and sulfur-free. Sulfur clay is less expensive due to the presence of sulfur-based fillers. If you are planning to make your prosthetic out of platinum-based silicone, however, this clay is not an option. The silicone will not cure in a mold that has been contaminated with sulfur. In this case, you must use the more expensive, sulfur-free variety of clay.
Oil-based clay is available in several hardnesses. Softer clay is much easier to work with, but is also very easy to dent with a stray knuckle or sculpting tool. Soft clay is also very difficult to add fine detail or hard edges to. Hard clay is on the other end of the spectrum. It is much more difficult to work with, but is ideal for sculpting flat surfaces and crisp edges. This is the material from which scale models of car prototypes are often sculpted.