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! 🙂
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- Silicone I All Purpose 100% Caulk (Don’t use II!)
- Acrylic Paint (any type, but cheap craft paint works well)
- Argo Cornstarch
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…
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.
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.
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
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.
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.
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.
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!
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! 🙂