Sculptamold, Hydrocal, Plaster of Paris?

Can someone give a brief explanation as to when you would use these? I am about to embark on my first layout and I need some advice. From what I can tell, sculptamold is used mostly for terrain

What kind of plaster?

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Sculptamold

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Sculptamold incorporates the best features of clay, plaster and papier mâché. In dry form, this white, non-toxic compound is quickly prepared by adding water. It clings to almost all clean surfaces, including glass, and does not shrink, so it may be applied to an armature or core without cracking. It sets in 30 minutes and may be carved, sanded or sawed when dry. Finished pieces are very light-weight, strong and durable.

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