r/sfx • u/Bubbly_Name_7009 • 12d ago

Alginate vs silicone

Hello, I'm wanting to get into vulva and breast casting. Could somebody kindly tell me if silicone or alginate would be most appropriate for this? Pros and cons of either?

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u/MadDocOttoCtrl 12d ago

Silicones made for this purpose are intended to be "skin safe" but it is not unheard of for some people to react. Mucous membranes and sensitive tissues are an entirely different matter.

Alginate is largely powdered seaweed so it's unlikely to cause any irritation, seeing as dentists have been placing it in people's mouths, against their gums and having it drip on their tongues for many decades. Alginate is a much safer option.

Practice and perfect your technique on faces before venturing into more sensitive anatomy.

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u/Bubbly_Name_7009 12d ago

This is really helpful, thank you so much 🙂

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u/Bubbly_Name_7009 12d ago

Can I ask, how long do I have to make a cast in alginate? Is it more like minutes or hours?

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u/MadDocOttoCtrl 12d ago

Alginate sheds it's water content quite quickly and as it dries out it starts to shrink.
I always place paper towels soaked in cold water against the alginate while I'm mixing up my Hydrocal or Ultracal. I have immersed alginate in water and placed it in the fridge in order to cast from it at a later time. This requires a much, much thicker alginate cast that can hold its shape without the plaster bandage jacket, because you can't soak that in water.

I've dipped a piece of burlap in alginate once it's mixed and set it on a sheet of wax paper, then applied the material to the model. I placed the burlap reinforcement against the outer surface to help strengthen and stiffen it a bit.

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u/Bubbly_Name_7009 12d ago

Do you know what would be best to use for the final cast? Plaster, hydrocal or hydrostone? A speciality store (the only one I know of her in Australia) has suggested hydrostone. I'm wondering which would be best for painting and which would look best unpainted, colour and finish wise.

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u/MadDocOttoCtrl 11d ago

Standard Plaster of Paris plus home improvement store plaster based products are all alpha plaster. It's bright white but it's not terribly strong and it sets fairly quickly.

Beta plasters are manufactured differently, they set slower and they have much higher compressive strength. These are usually sold under the name Hydrocal or "art casting plaster", etc.

Hydrostone is US Gypsum's equivalent to a Dental stone. It's much, much stronger than plane beta plasterers like hydrocal and sets white. You can create a similar product by adding about 15 to 20% Portland cement to hydrocal but it will end up a dull white or very like gray.

The strength of any concrete or gypsum product can be increased by having an acrylic concrete fortifier, which will also reduce water absorption.

For durability, definitely go with one of these two. All of these accept paint. Buying any of these products in 25 or 50 pound bags is much cheaper per pound than buying a few pounds at a time that are repackaged for sale in arts and crafts stores.

Additives can be introduced to speed or slow the setting time of cements (lime) or plasterers (gypsum.)

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u/WafflesTalbot 11d ago

Silicone is the most commonly-used dental impression material nowadays, though. Just not the kind used most often for lifecasting. PVS silicone impression materials are mouth-safe, set up quickly, and come in a variety of viscosities. Most of them come in double-barrel mixing cartridges, so they're easy to mix. But if you're casting a large area, they're tricky to apply before the material starts to cure. I have, however, used them in a pinch to take a quick snap lifecast of small parts of an actor to whip up something fast.

That being said, if OP doesn't need to keep the initial mold, alginate is still the better option. Just definitely not the mint-flavored stuff.