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Jon W

Slip Casting Model (Master) Material

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Newbie here. I'm going to try slip casting and was wondering about the materials used for my master (is that the right term?). I'll be doing small figurines (about 7 to 8 inches tall) and they'll be separated for mold making and pouring. The sections will be head, torso w/out arms, arms, and finally legs w/feet from the waist down. I see videos of plaster molds made with the master formed using modeling clay that is leather hard and was wondering if that is the only option? I've made a couple of models already using a high quality air dry clay that has hardened and then been coated with clear acrylic spray. I was going to make silicone molds and resin pour them but my daughter does ceramics and I love the slip casting process. Are these "hard" masters usable or are they too brittle? Is the acrylic coating an issue concerning the plaster? The coating makes them quite strong actually. Thanks!

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Got the answer at Davens here in Atlanta. I was told you can make a plaster mold out of pretty much anything and the acrylic coating won't affect it at all. 

douglas likes this

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I think the answer you received needs some qualification— while it may technically be possible to make a plaster mold using any model, some materials are much easier to use than others. The two main concerns when making plaster molds are: (1) the model shouldn't be made of a porous material (or the plaster will stick to it) and (2) the model should give slightly (or you will have trouble getting it out of the mold intact, especially if the form is complex).

 

Examples of materials that are too porous to be useful are bisqueware, papier mâché, and untreated wood. Applying a mold release agent, like murphy oil soap, will only help so much with these materials.

 

Examples of materials that may be too stiff to be removed from the mold without breaking either the model or the mold are thin glass or ceramic and wood (particularly fragile wood, like balsam).

 

Better options for making models are green clay, modeling clay (oil-based), or carved plaster. If you have an existing model that is too porous or stiff, make a new model of it using silicone.

Edited by Achilles

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