Hi Everyone!

I’m brand new here, and working on my first project… Unfortunately I went BIG.

I also made a mistake. I created an original sculpt out of plasticine. I decided to create the mold out of mold craft latex (which worked really well) and a mother mold on top of that from plaster.

What I didn’t realize is that I wanted to cast the piece in Flex Foam It III. Which apparently sticks to latex even when you use an insane amount of mold release. I now have to pick out the pieces of foam by hand before I can save the mold.

My question is, can you cast a layer of mold craft latex, in a latex mold and then I add the Flex Foam? I figure if flex foam bonds to latex so well, it will likely take the new layer of latex as a “skin” BUT what should I use as a release between the two layers of latex?


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Hi there. Your sculpt looks good. I hope you can clean the mould u p alright to be able to use it again.

There are a couple of approaches to your problem, apart from the obvious taxi driver reply of 'I wouldn't choose to start from here!'. It depends on what you want your final piece to be made of. I am assuming you want a latex skin with flex foam inside it.

It might be possible to cast latex inside latex, although I think there are big risks of it all sticking together and making an unrescuable mess. To try out whether this might work, do a test with some latex and release agents, and then proceed very carefully.

Another approach might be to use the latex mould to cast a new clay sculpt, which you can then make a plaster mould from, this being the ideal material for casting liquid latex inside. You can warm Chavant clay until it is very soft, and then brush and press it into the mould, building up a good thickness. If you then put the mould halves together, you can pour a plaster block inside the clay. The detail may need quite a bit of cleaning up, but you effectively get your sculpt back. Then proceed to make a plaster (dental stone, Ultracal, Crystacal R NOT plaster of Paris) mould in as many parts as you need, and finally slush mould the latex inside and foam inside that.

The principle to always bear in mind is to cast something flexible inside something rigid and vice versa. Hope this helps.

Thank you for the reply! 

I think I'll be doing a LOT or testing. I'll have to find some of the plaster you're talking about as all I have is plaster of paris, which is why I didn't use it as the main mould.

I'm wondering which of the two products below would be better as they are both available locally:
Hydrostone (Southard)

Or the Ultracal

Either way I'm going to have to modify the sculpt at some point. 

This is a large piece, so I am wondering if the extra strength is worth the extra weight.

It looks like I have some experimenting to do! 

Hi Sarah

The Ultracal is mould-making plaster and is the best for your purposes. Plaster of paris is too weak to give crisp fine detail. When you make a mould using Ultracal you can reinforce it with burlap or glass fibres, so it does not have to be too vast or thick. You do need to have 'keys' so the pieces fit perfectly together.

The Chavant clay is able to be melted with heat or in a microwave... carefully, and then used with great care so as not to get scalding hot sticky clay on your hands. It cools quite fast. Comes in soft, medium and hard.

I've finally gotten my hands on some Ultracal 30, and am being smart this time with creating test pieces. I have the plasticine from the original sculpt - not Chavant brand, but from my experimenting got quite hard after melting it, and it should work well. 

But creating a new mold on the plasticine... I have a few additional questions (sorry!)

- What would you use as a release on the plasticine before putting on the plaster?
- Would the plaster reinforced with glass fiber be strong enough to bolt the pieces together? For the amount of detail I have, I think I will need more than 2 pieces that can just be strapped together. 

Simon Tytherleigh said:

Hi Sarah

The Ultracal is mould-making plaster and is the best for your purposes. Plaster of paris is too weak to give crisp fine detail. When you make a mould using Ultracal you can reinforce it with burlap or glass fibres, so it does not have to be too vast or thick. You do need to have 'keys' so the pieces fit perfectly together.

The Chavant clay is able to be melted with heat or in a microwave... carefully, and then used with great care so as not to get scalding hot sticky clay on your hands. It cools quite fast. Comes in soft, medium and hard.

Hi Sarah

To answer your questions directly.

  • I generally don't use much in the way of release agent on the sort of clay that I use. This is because the Ultracal generates quite a lot of heat as it sets, and I like to open the mould while it is still a little warm. If I have difficulty getting the clay out, for sample from a deep area where using a tool to dig it out would likely damage the mould surface, I put the open mould into the oven at a little over 100 degC to soften the clay - not for too long as you don't want the clay to become a sludge that then gets absorbed by the plaster! Just five minutes or so. If you want to use some release agent, a spray release is good, Vaseline is not so good as it leaves streaks. Can just about be used if thinned with white spirit. BUT.... When you come to making the second half of your mould, you must use a release agent where plaster is coming into contact wiuth plaster, or it will fuse together. The simplest way is one of the best - use some water clay diluted in a pot with water to make a 'slip' or slurry. Brush this onto the plaster, which will absorb the water readily. The first time you do it, it will look like it could never work, but it does, and leaves a close join. Just be sure not to leave any plaster areas still pristine white, not even the tiniest.
  • When you have mixed your Ultracal to a creamy consistency (powder into water, 3:1, so don't overfill the mixing bowl, brush the first layer into the mould to get rid of air bubbles that tend to linger in deep folds. Then add more plaster to build up around 25mm thick. Finally, add a layer of glassfibres and some more plaster on top. By now it shouold be thickening, so you don't run the risk of pushing the fibres down to the surface of your mould. If I am using Lego to give a nice square mould (BTW it needs generously vaselining to fill the little gaps between the bricks), I usually try to fill it up and get a flat top, to make the eventual mould stable for working.
  • To answer about bolting a mould together - you would never boilt a plaster mould together. Plaster has not very much strength in tension, most of it is in compression. But I do clamp plaster moulds, either in a vice or using G clamps. Still, one has to be careful not to over tighten, or it may crack. Let's go back a step.... what you seek is a perfectly-fitting mould that has very fine seam lines, and so this comes from making a mould that fits properly together. Squeezing the mould will not give better detail within the mould - this is entirely dependent on the quality of sculpting, whether you brushed the first layer of plaster in carefully, and on the quality of the plaster (Ultracal will take and reproduce very fine detail). OK, so there are some tips for making a plaster mould that fits together well. First, make sure you have some 'keys'. These are formed by making depressions in the clay surround for the first half of your mould, that become corresponding bumps in the second half. Everyone has a favourite way to do them, have a look at some YouTube videos to see. Second tip - when you are setting up to do the second half of the mould, put some little bits of clay in a couple of places around the edges of the mould. These will become hollow, and should be slot-shaped, so you can put a screwdriver tip into them to prise the mould apart. Makes life a whole lot easier. Third tip - the quality of your seam-line will be higher if the surface area of plaster-to-plaster is less. Imagine if you made a small sculpt inside a big 2-part mould. There would be a very large mating area of plaster, so however much pressure you applied it would be difficult to get a good seam-line. So when setting up for the second half, you can put some more clay about 6 to 10mm away from the edge of your sculpt, which will form a hollow in the mould, so that only the area surrounding the sculpt and the keys are actually touching. Then all the pressure you bring to bear on the mould will be compressing that seam-line. One word of caution, though. It is possible to end up with a mould that is not very stable when put together, so it should be done with care.
  • Last thing to say: I like to use water clay to make the bed around my sculpt, and preferably of a different colour to the sculpt itself. The reason I like this is because I can wash the water clay off without damaging the sculpt, or even touching it. I can also make sure that it is softer than the sculpt, and can brush it into the gaps if necessary.

Hope this helps!

Thank you for all of your great advice! As time and $ were a factor, I thought I would try one last aproach before resculpting it. 
I cleaned and coated the latex mould in 3 coats of clear coat, then I baby powdered the whole thing (blowing it off outside so I could still breathe). I then poured in MORE liquid latex. Now I know the foam sticks to latex like glue, BUT latex doesn't stick to latex provided it's treated properly first. So I made a "skin" with 5 layers of liquid latex. Once cured I poured in the foam. 

IT WORKED! There were a few spots that stuck because I didn't get enough clear coat or baby powder into the points of some horns, but I was able to cast the mask! I've attached a picture of it (From halloween... because I wore it on a large harness I designed). It still needs further painting, but the latex skin actually gave it a much more reptile rubber skin. 
I should mention that in order to paint on the latex without using rubber cement paint, I did coat it in 77 spray glue first. And once that was dry I painted. 


Well done! That is absolutely stunning and worth all the perseverance. Lovely job, if a bit big for stopmotion!!

Great sculpt with a lot of detail, I can see why you really wanted to salvage your latex mould!

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