Here's another entry from Dick Johnson's Encyclopedia of Coin and Medal Terminology.
A soft material used by sculptors to form the model of an intended numismatic or medallic item. It is more than the substance found in nature, although natural clay has been used for three-dimension modeling since ancient times. Modern modeling clay has beeswax or paraffin and oils added to it in formulas closely guarded by the sculptors who originally prepared it (or the companies who now manufacture it). When clay is formulated for professional use by artists it is called plasteline.
The properties of modeling clay are that it remains in a semi-rigid state once it is formed; it is easily molded, it works well with fingers or tools to be altered at will. It must hold a hard surface once the artist is satisfied with the desired shape and be able to make a casting, usually in plaster. Also, it can be used over and over again (provided the most rudimentary care is taken not to contaminate it). When a sculptor's estate comes on the market his supply of modeling clay is eagerly sought after by other sculptors, it is so nondestructible.
Plasteline is made in four grades of hardness (dyes are sometimes added to color code these grades). The softest is for large sculptures where masses can be formed quickly. The hardest form of plasteline is the most ideal for the relative small mass for coin and medal models, but more importantly because it holds a sharper edge and more precise detail can be molded into its surface.
Modeling clay differs from porcelain clay (which is used only once and is to become the ceramic object being formed). It is possible for porcelain clay to be used for coin and medal modeling but generally a harder clay is more desirable (the difference in the clay and reduction from oversize patterns are two reasons diestruck items can have finer, sharper detail than ceramic ones).
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