What exactly are you trying to do ? Create a mold from a clay sculpt to create a latex mask or a hard bio mask ?
The answer to this question will lead you into your material choice. Remember you make a hard mold (plaster) to cast a soft material (latex)
You make a soft mold (silicone / latex) to cast a hard material. A soft mold will also most likely need a hard outside shell.
Plaster is ok for making mold in a pinch but Ultracal is A LOT better and either one will need a fiber reinforcement like burlap.