Plaster Moulding and Release Agent Question

[RobertMuldoon]

I have a baked Sculpey model which I want to make a plaster mould out of, so that I can then use that mould for casting clay replicas of the original piece. The model itself is pretty simple and has a flat back, and so I can make a one piece mould by just pouring the plaster on top of it.

What I’m struggling to find any advice on is what mould release to use to stop the original model stick to the mould, but without affecting the absorbency of the plaster so it will still dry out clay well when I come to cast from it. Anyone got any suggestions on what to use?

Thanks!

 

[frosty]

I would suggest making a simple silicone mould instead, you may have trouble getting clean castings from a plaster mould, silicone you will have no problems with and no release needed

 

[ChickenHaunt]

I would suggest making a simple silicone mould instead, you may have trouble getting clean castings from a plaster mould, silicone you will have no problems with and no release needed

Moreover, because you've baked the Sculpey, you now have a rigid sculpture. You can make a rigid mold of a rigid sculpture, but you need to be 100% certain there are no undercuts. If you have even slight undercuts, you'll be much better off with a silicone mold.

That said, vaseline makes a great release agent for plaster.

 

[Astromechfan]

Awesome Molding advice all around! Happy I found this thread.

 

[martinwilson]

Hey Robert,

I wish I had seen your query earlier - but I thought I'd reply anyway - better late than never.

I have made many plaster molds like the one you are describing for casting porcelain and other ceramic slips. For those who recommended making a silicone mold instead - this will not work with wet clay (or slip) as the mold needs to be porous in order to dehydrate the cast.

With regards to the release agent, plaster inherently does not stick to plasticy\waxy surfaces - especially if they are perfectly smooth. A convex plastic object that is polished up to a nice shine will almost self-release from plaster, but I always give a quick spray of universal mold release anyway. Baked Sculpey is therefore a good material to begin with, but if there is any surface texture or any complex shapes with steep relief then the plaster will mechanically grip to the object. Obviously any undercuts make a one-piece rigid mold impossible.

Unwanted surface texture in the original object can be eliminated with a couple layers of clear glossy spray-paint and\or polishing with a hard release wax like one would use for a fiberglass mold. Deep relief is more difficult to get around - but not impossible. I once made a plaster mold of a crocodile skin by first making a silicone mold, then making a silicone positive from that, and then making the final plaster mold from the flexible silicone positive. If you use this silcone/plaster method you can reproduce the most beautifully fine textures and details in your clay casts.

All that being said - don't worry too much about the release agent reducing the porosity of the plaster mold - it actually takes a lot of grease to completely seal plaster. So if you need to use a thin layer of vaseline that's actually fine - and any detrimental effect can be mitigated by rinsing the plaster mold in warm (dilute) detergent immediately after de-molding (obviously don't scrub the surface or else you will scrub away detail).

I hope this helps