When it comes to sculpting clays, there are a number choices as well as considerations. For most sculptors and mask makers, oil based clays are the preferred medium, as they don’t dry out or shrink. Though there are water based clays that work well for this purpose they are often too soft to hold fine detail. Therefore, the clay of choice is Plastilina
Plastilina is a wax and oil-base modeling material most often used for both artistic, special F/X and prototype modeling. Plastilina has a few names such as plasteline, plasticium, and plasticine. The main ingredients are wax, oil, and clay flour that is used as a binder. All Plastilina clay is manufactured by heating, and then cooled and extruded into shape. Plastilina cannot be fired. It does not harden and will always remain the same consistency as it was when first used.
If a permanent piece is required a mold of plaster or rubber must be made and then cast to obtain a duplicate piece
Plastilina can be categorized into three grades: professional grade, school grade and industrial grade. The professional grade Plastilina commonly contains sulfur, to make the smoother, more homogeneous texture required by professionals.
ROMA Plastilina is an oil- and wax-base modeling material preferred by many professional sculptors. A non-sulfur professional grade material in medium consistency, called Prima Plastilina, is also available. Roma Plastilina comes in 4 consistencies; #1 is the softest, up to #4 which is the hardest. The one most used by mask makers is #2, as it is soft enough to work with, yet firm enough to hold detail well. Caution: do not attempt to melt Roma, due to the odor it will leave behind.
The school grade, or amateur grade Plastilina, does not contain sulfur and tends to be stiffer and harder to model with the fingers. School grade Plastilina comes in a variety of striking colors and is used in the Claymation field of movie production and advertising.
Chavant is a well-known manufacturer of industrial grade Plastilina. This clay is usually very hard and needs to be heated prior to use. This type of Plastilina is most commonly used by designers, in the automotive field for car models and is not available to the retail consumer. But, many hobbyists, industrialists, and FX pros alike, prefer Chavant clays.
To soften Plastilina, if not being used at room temperature, 68° - 70°, place the material approximately two to three feet from a 60-watt light bulb for about twenty minutes. To change the grade consistency, soften the clay as instructed above by placing it near a light bulb and knead in small amounts of oil until an even consistency is achieved.
While some people microwave Plastilina, it is not recommended for Plastilina containing sulfur since people tend to overcook the material. If you do attempt to microwave Plastilina containing sulfur, be careful - the Plastilina is melted to a more liquid state and since the component parts contain sulfur, will create an unpleasant odor in the oven or microwave.
On the other hand, non-sulfur Plastilina can be placed in a microwave oven to soften it for use, but since microwave ovens have different power levels and wattage varies, there is no one set rule for successfully heating the Plastilina - accomplish this by trial and error.
To harden Plastilina, place it in the refrigerator for thirty to forty minutes before working with it. There is no technique for permanent hardening. To change the grade consistency, soften the clay as directed above and add softened Microcrystalline Wax in small amounts to the material by kneading it into the softened Plastilina until the desired consistency is achieved.