I used to make aluminum molds from plaster of paris all of the time to cast aluminum and copper. The key is that it has to get baked out in an oven above 400F for a couple of hours (depending on the size of the mold). It will give you flawless castings if you do it correctly. I used to use the lost foam casting method and then dip it multiple times. Usually used POP or Premixed Drywall mud. Both worked well.
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