Silicone Caulk and Cornstarch Molds – Take 2!

Casting a Complex Octopus
Behold, the Octopi!

I tried combining caulk and cornstarch back at the beginning of my 100% Silicone Caulk mold making adventures… and it was dismal failure. It never set completely and ended up sticking to the sculpture.

But the more I did research into the casting and molding world the more I came across articles and YouTube videos insisting it worked, so I figured it was time to try again!

The tl;dr story is that this works exceptionally well for making molds of sturdy objects that you are looking to cast in resin. It doesn’t work well for for fragile objects, because of the amount of pressure required or for plaster casting (without a release agent) since that sticks to the molds (boo!).

As always, click the pictures to embiggen! 🙂

Materials Matter

Silicone Caulk and Cornstarch Attempt 2

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Note: Do NOT use the Silicone II! It will not set properly with either of the methods.

What appears to be the secret of success this time (at least for me) is that I’m not using just caulk and cornstarch…

1 oz shotglass Silicone Caulk and Paint
Mix just a dab of acrylic craft paint

Instead of mixing the caulk directly into the cornstarch, I first mixed the caulk with a few drops of acrylic paint. The paint appears to serve the same function as the dish soap does in the water bucket method and causes  the caulk to set properly. As you can see from the photos it’s very easy to tell when the two are mixed together well.

I used a toothpick dipped in the paint before using it to mix with and didn’t have any issue with the caulk sticking to it. I do wonder if you could do basically the same thing using dish soap instead of paint, but the color is so thin I don’t think it would show well.

Silicone Caulk and Paint Mixed Together
This is what it should look like well mixed

My method of mixing the cornstarch and caulk was slightly different than last time. My primary mixing tool was a wooden mixing stick instead of my hands, which made for easier cleanup and I think easier mixing as well. I haven’t tried the credit card/plastic card method, but it’s something I want to try later.

I started out with a small pile of cornstarch on the paper plate. Then scooped the caulk out onto the small mound I made and covered it with more cornstarch.  After that it was just adding cornstarch to the top and carefully squishing it together with a wooden mixing stick until it was non-tacky enough to mix by hand. Then I kept kneading the caulk until it was no longer tacky to the touch.

Mixing Silicone and Cornstarch
Messy, but effective!

The first time I tried this I really wasn’t paying much attention to how much cornstarch I was using or how long I took to mix it. But I think I ended up adding too much and mixing too long because by the time it wasn’t tacky it was really starting to set. The later attempts (non-orange molds) I tried a little less of both things and they were still very firm, but more malleable.

Keep an eye on how firm this is getting, once it gets to the tipping point it sets up FAST. (It also sets up with a lot less of the ammonia smell, but I’d still recommend doing this outside for larger batches.)

Making the Mold

Silicone Cornstarch Paint End Result Only Slightly Tack
Orange you glad I tried again?

At this point I really had to press the sculpt into the caulk, which means I couldn’t use something with parts that might break (hence the cat). I don’t normally do these types of molds with the other method, so I ended up creating a lip around the piece trying to make sure it covered the object properly.

The resulting mold (after completely curing) was very stiff and would flex, but it took a lot more force than the plain silicone molds. This made it difficult to demold the plaster casts without breaking, even when I left them to fully harden overnight.

Silicone Caulk Paint Cornstarch
First Mold attempt

The lip made the points break off when I tried to demold the plaster mix. (Darnit!) So I worked on carving the lip back with an xacto knife. That worked well, but interestingly I was able to sand the mold using the dremel and a sanding drum– although I wouldn’t recommend doing it in a non-ventilated area!

I don’t know if the bits are toxic, but better safe than sorry.

100% Silicone Caulk Mold with edges sanded off and plaster cast residue
Mold with edges sanded off and plaster cast residue

The other issue I hit was that the plaster and wood glue mix had a bad tendency to adhere to the mold itself. The residue does scrub off with a toothbrush and warm water, but it’s still not something I want to have to deal with when I have alternatives. I could also mitigate the issue by dusting the mold with cornstarch before using it, but combined with the stiffness my final call was that this shouldn’t be used for plaster at all.

Thankfully I had plenty of resin to play with! 😀

I was wondering if the hardness of the caulk came from adding too much cornstarch, so the second time I only added enough to get it to ‘not sticky’. It was a smidge more flexible, but not enough to notice.

Cornstarch and Silicone Caulk Molds Redux
Take 2 on the pressmolds

Yup, there’s something about these molds that just doesn’t like plaster and wood glue mixture. I have to demold them when it’s still wet or they stick to the mold. They do scrub off with a toothbrush and warm water, but it’s still not something I want to have to deal with when I have alternatives.

So I tested it out with the resin and it works perfectly!

Resin Hello Kitty casts and 100% Silicone Caulk Mold
Hello Kitty Army ASSEMBLE!

There is a little sticking to the mold with the resin, but it’s very minor and seems to occur mainly in pinch points. It means these molds will wear out faster than their silicone counterparts, but since they are cheap and easy to make, it’s not much of a downside.

I was able to cast the Hello Kitty base I sculpted for a friend (which will soon become many fandom characters) as well as a much more complicated octopus. The firmness of the molding material is great for the more 3D items like the octopus and I have a feeling they’d do great for casting things without a flat back.

But that’s a post for another day! *goes off to happily cast a small penguin army*

Link Love and Six Degrees

Here are some of the videos and posts that I used in researching this method. Check them out! 🙂



7 responses to “Silicone Caulk and Cornstarch Molds – Take 2!”

  1. Moonlight Avatar

    can you paint it once the silicone is cured? I want to make something that the main color is blue, but I want to add black details to it, will it stay? Will it come off? Is it waterproof?

    1. Martha Bechtel Avatar

      I don’t think paint would stick well to the silicone. Since it’s waterproof and flexible, the paint will crack off when it was moved. But it could be possible if you are making an item that won’t be moved much once it is painted. The other thing you could try would be to make two colors of silicone and add the details like you would with sculpey clay. Silicone loves sticking to silicone, so it would be hard, but possible! 🙂

  2. Pam Avatar

    FOUND THIS HELPFUL-THANKS!

  3. Lisa Ann Hainline Avatar

    try a mold release with those materials that are sticking.

    1. Martha Bechtel Avatar

      I’m not sure how well a mold release would do with plaster, but it’s worth a try! 🙂 (I actually didn’t own any mold release until I started working with the OOMOO 30.)

  4. Jerry Avatar

    What brand of resin did you use? I tried this before, and the resin never cured.

    1. Martha Bechtel Avatar

      I use Alumilite White (Amazing Casting Resin). It’s a two-part 1:1 mix and I get it from Michaels, but you can get it online as well. I know I’ve had problems with it curing if the materials are older (when you start to get particulate in the yellow Part B) or if I get the ratio wrong.

      What brand were you using? Maybe I can experiment some and see if i can try and recreate the issue! 🙂

      https://www.alumilite.com/products/casting-resins/alumilite-white