clay for molds?

what kind of clay should i use for making a vacuform mold for storm trooper armor? wood is too hard for me.

I suggest that you use resin, mixed with aluminium powder. Make a mold of the positive, then pour in the resin/powder mix into the mold.

Then you'll have again a positive, but in resin.

Clay is definately not the right stuff to use as a plug for a vacforming process.

You can use plaster too.

no, i mean not to actually form the plastic over, but as a cast...like just using it to design, then use resin and pull the clay out and fill it with fiberglass or something, etc.

water clay will work fine for what you are doing. It is quick to shape and toughens up as is drys. You can vacuform over it wet and use the pull as a mold into which, stone is poured. Sand the stone plug smooth and you have your hard tool for more vacuforming.

Hope that helps.