I've got a shape that I want to have cast but I don't know how to achieve this:

this shape

I know the basics of regular casting but this shape has an open interior.
The picture is the master shape.
How can I make a plaster mold for it, in order to be able to pour a liquid clay in it?
And how will I be able to get it out without damaging the hardened clay?

Are there any known ways of achieving such a casting? And - especially - what would the correct terminology be to make me able to search more about that certain way of casting?

2 Answers 2


It looks like you could blow up a ballon to get the shape of the interior. Then glue hex shaped objects to the surface of the ballon in the pattern of the holes. Then build a wire framework over the tops of the hex shapes to hold them in position and to support your mold's outer shell. Finally apply that outer shell as a series of glued vinyl sheets or paper mache. Once that outer shell is dry, you can reinforce it with clay or silicon.

You should then be ready to pour in the liquid clay.

This will be a sacrificial mold, so you will mostly need to destroy it to get your casting out. To de-mold, remove the reinforcing material and paper mache, then the wires. Then pop the balloon and push out the hex shapes.

The exterior of your casting may have some lines from both the wire framework and any seams in the vinyl/paper mache. It might also have bits of vinyl or paper mache stuck to it unless you washed the interior of the mold with lubricant before adding the clay. Those problems should all be pretty easy to fix with a little sand paper.

  • you may want to fill the balloon with lightweight sand or water as air will tend to compress and distort.
    – John
    Oct 26, 2017 at 20:23

Let's go through this step by step.

For your plaster outer portion (mold), I recommend at least 8 pieces (2 pieces if it is single-use and you can break it up, in which case make sure to use a soft plaster). And you are going to put a styrofoam of balloon center in it when you go to cast.

  1. Make a 2 piece blank (master) of your shape around a solid center sphere
  2. Pour your plaster in two sections
  3. Open it and remove your blank and your solid center
  4. Put your styrofoam ball (or balloon) and plaster mold back together
  5. Pour your cast.

This does depend on how hot your clay is. If it is hot enough to distort the styrofoam sphere, replace styrofoam with a water or sand filled balloon instead. Then you can pop and drain it afterwards. Don't use an air filled balloon - it will compress, distort and shrink with the weight and heat of clay.

Also, the thicker your final shape the more pieces your mold may need, especially if you intend to reuse it. Otherwise it will lock around your shape and you will not get it out without damaging one or the other. 8 pieces is best for a reusable, think of 8 cubes stacked to make one large cube.

Your blank (also called the master) will need to be get out without damaging the the plaster. This is when styrofoam, sculpting foam, unfired sculpting clay, or any other soft media will work best.

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