Casting in Silicone molds using Plaster of Paris and Wood Glue

Plaster casting artwork from 100% Silicone Caulk Molds
Messy is Good!

Update: Come check out the experimentation that led to my current recipe!

At last, the casting post (that isn’t Hot Glue) is upon us!

As you probably guessed from my Making a 100% Silicone Caulk Mold and the follow-up post, I’ve been happily dabbling in the world of ‘making art desks messy’ for a few weeks now.

As I mentioned in the earlier posts, I’m focusing in on the cheaper casting methods right now because I’m still in the learning curve. I’m not willing to waste a $20 resin kit, but I’m sure as heck willing to play with a $5 bucket of Plaster of Paris.

So time to share what I’ve learned… *rolls up sleeves*

Water + Plaster = Fun!

Materials Needed for Plaster of Paris and Wood Glue Casting
Plaster of Paris, Elmer’s Wood Glue, and brushes you will never use for painting again…

Mixing water with Plaster of Paris is relatively easy. (Thankfully!)

You will need either a disposable or flexible container to mix in because there will always be some that doesn’t pour out. If the container is flexible, you can just pop the plaster out. I’ve had some luck reusing the plastic cups, although it turns out even a third of the cup is much more plaster than I need at one time.

You measure out the cold/room-temperature water (1 part) and then slowly sift in the plaster (2 parts). You want to let it sit for a minute and then stir just a smidge, since heat makes it harden faster and it can causes bubbles, but you are aiming for a nice fluid mix.

Wet the silicone molds and then shake off the excess water.

Then you slowly pour the mixture into the dampened molds, making sure to tilt them to release air bubbles caused by any deep pockets in the mold.

Other than a few air bubbles –which you can get rid of by tapping or vibrating the mold after the pour– most of the learning curve was how much plaster to mix up. I almost always had more plaster than molds, which is how I learned the joys of Painting With Plaster, which I’ll cover a bit further down this post.

But casting with just plaster makes for very very delicate magnets. So I went looking for things to strengthen it with.

If your local stores don’t carry everything, here are some Amazon Affiliate links! (And yes, I get kickbacks from Amazon if you buy through these links, but it won’t cost you anything extra.)

Water + Plaster + Wood Glue = Fun?

Plaster of Paris and Wood Glue casting failures
Plaster Casting FAIL

Elmer’s Wood Glue seemed the most obvious answer: it dilutes with water, hardens to a sandable surface that takes paint well… what’s not to love?

Surface tension.

You see plaster inhales as much liquid as it can get it’s little powdery teeth into. In water this isn’t an issue since the surface tension of the water isn’t stronger than plaster’s desire to drink.

To create the mixture, you add wood glue to the water and stir until it’s dissolved. If the mixture has too much glue, when you add the plaster you can end up with little pockets of plaster, like lumpy gravy. Mixing in all those blobs adds a lot of bubbles and those bubbles hang around longer because… surface tension!

Plus the bubbles can cause setting issues, as you can see in the image. The end result looks more like moon rocks than plaster. They are nice and solid, so I can go back in with the mixture and paint in filler to bring it smooth, but that sort of kills the time-saving aspect of casting things.

On the other end of things, too much water means the glue floats to the surface and the plaster sinks to the bottom. It’s still stronger than just plaster, but not quite what I wanted. (Plus the layer of yellow is harder to sand off.)

This mixture takes longer than normal plaster to set, so I’d normally give it 12 hours in the mold. That’s 12 hours before I get to find out of the batch was good or bad.

All in all, it’s something that drove me nuts trying to perfect…

Water + Plaster + Wood Glue + Wood Glue = Victory!

Plaster of Paris and Wood Glue casts and finished items
Plaster of Paris and Wood Glue Casts

So I came up with a Cunning Plan!

The cunning plan basically consisted of mixing 1 part glue to 4 parts water and then adding 1 part plaster, mixing slowly, and then adding a small amount of plaster at a time until I got the consistency I was looking for.

Then pour (and tap) and pour (and tap).

Working slowly and doing a lot of small items helped. There might be a few at the very beginning or end that turned out badly, but the ones in the middle come out pretty close to perfect.

Once they’ve had a day to dry I go back, sand and fix any divots.

Then –here comes the magic– I start painting them with a mixture of 50/50 of glue and water. This step goes VERY slowly and takes 3-5 coats (let them dry for a good 2 hours between coats).

Overkill? Maybe, but it makes a nice firm surface to paint on and it makes them a lot sturdier. I’ve dropped the magnets countless times now and the only thing that seems to damage them is cutting pressure, not blunt.

Painting With Plaster

Plaster of Paris and Wood Glue castings of the simple horse head mold
Large Horse Head Magnet – Variations

One of the fun side effects of my plaster learning curve was that I realised you can paint with plaster!

I had originally cast the horse heads so that I could then modify them with sculpey, but while playing around with some leftover plaster in the cup I figured out that I could paint on new plaster. That allowed me to modify the sculpts without having to change mediums.

I had been using a thin slurry of plaster to fill in divots and pinholes, but as the plaster thickened I realized I could sculpt with it. A few experimentations later, I had the perfect way to make nice flowing manes.

Plaster loves bonding to plaster (even with the wood glue) so it creates this nice strong connection between the original and the base. Using wood glue and Sculpey didn’t have the same solidity– but this worked so well I even grabbed a toothpick and made a unicorn! ๐Ÿ˜‰



25 responses to “Casting in Silicone molds using Plaster of Paris and Wood Glue”

  1. Scott Avatar

    Have you tried embedding some sort of fiber in the plaster? Some loose-weave canvas (perhaps that plastic stuff used for various yarn arts) or even just some loose bits of yarn?

    1. Martha Bechtel Avatar

      Not yet, but that’s actually a great idea! I was working on the mix ratio post this weekend, so I’ll grab from fiber and toss it in for a test. I think I’ve got some yarn floating around in the horde that is my workbench. *digs*

      1. Jerry Smith Avatar

        I just started working with plaster of paris to create sugar skull magnets. Your article was a great help

        1. Martha Bechtel Avatar

          I’m so glad they were useful! I’ve had a LOT of fun playing around with the mixtures and techniques while making the magnets. I’m planning on expanding into ‘traditional’ casting this year (rubber molds and resin casts) but I’m always going to go back to plaster when I want to play around! ๐Ÿ˜€

    2. chrisg Avatar

      Try using plasters strips scrim cloth or bandages

  2. Aimsaim Avatar

    Wow! Fantastic stuff !
    This is so chock full of useful information , I can’t wait to dig deep and start utilizing the resources you have given me . Your Exuberance is refreshing.
    The guide is really informative and useful. Thanks so much for providing this for free. I understand that the casting is done when the plaster mold absorbs the water from the casting slip. Does that mean the plaster mold can be used for only 1 cast?

    1. Martha Bechtel Avatar

      I’m glad that you found it useful! ๐Ÿ™‚

      I’m not sure what you are asking about the ‘casting slip’? The molds I have been using are silicone and can be used multiple times (sometimes hundreds) before they finally wear out.

      The plaster that is poured into them can only be used once, there is no way to turn it liquid again after it has hardened. If you add water to the hardened plaster it will turn to mush (which is why I make sure the magnets are well sealed!)

  3. Ben Avatar

    Can you use silicone to mold an already painted plaster sculpture? Will it affect the acrylic paint even if its sealed with a clear coat?

    1. Martha Bechtel Avatar

      I’ve actually tried this! ๐Ÿ™‚ …and if you use the silicone and water method then there is a high likelihood that it will destroy the thing you are casting. I use a Krylon spray sealer and a Liquitex brush on sealer and both of these fell apart after being in contact with the soapy water silicone. The plaster item underneath also suffered damage, so it’s not something I would recommend trying.

      I haven’t tried this with the method that uses cornstarch, paint, and caulk but that creates a very tough mold so the piece would have to be able to withstand the pressure without cracking. I hate to say it, but you might be better off with a commercial molding product like you can find in the craft stores as those are a lot softer (from what I have heard).

      1. Ben Avatar

        Thanks!!

        1. Adrian Avatar

          What about trying to use proto putty to mold already created plaster. It is silicone dries like silicone mold after ten minutes but it is moldable just like play doh but not as moist more dry like plasticine. YouTube search for Grant Thompson proto putty.

          1. Martha Bechtel Avatar

            This is actually the silicon caulk and cornstarch mixture that I also tried! Although I used craft paints instead of food coloring. ๐Ÿ™‚ Silicone Caulk and Cornstarch Molds โ€“ Take 2!

            While it would bypass the water issue, the problem I had this with mixture is that it firms up -very- quickly, When I tried making press molds with it, it broke my sculpey master. I’d be very leery of trying it with a smaller plaster item for fear of breaking.

            Here’s the video link for Grant Thompson if anyone else was looking for it! ๐Ÿ™‚

            https://www.youtube.com/watch?v=7fwytA5r2Mw

  4. Brad Avatar

    Interesting suggestion. I was glad to learn that surface tension causes problems when mixing plaster and glue. Have you tried adding a drop of dish soap to the mix? That should reduce the surface tension. I haven’t tried that yet, but it seems like it should work. Actually the liquitex you mention might work as well.

    1. Martha Bechtel Avatar

      I hadn’t thought of using the dish soap, definitely worth a try! I’m in the middle of rebuilding my workbench right now, but I was planning on revamping my casting and molding methods via science experiments once that was done. I’ll add this to the list of things to try out. ๐Ÿ™‚

  5. Osarobo Avatar

    Hi Martha!how did you make you silicone mold? you can email me pls send me details on how to mix the silicone.

    1. Martha Bechtel Avatar

      I use silicone caulk to make the molds and the instructions on how to do that can be found in the blog posts Making a 100% Silicone Caulk Mold, 100% Silicone Caulk Molds โ€” Round 2, Fight! and Silicone Caulk and Cornstarch Molds โ€“ Take 2!

  6. JacquelineLe Avatar

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    Just search for – Miftolo’s tools rewriter

    1. Martha Bechtel Avatar

      I know this is spam, but I couldn’t resist… so I Googled!

      And good grief that software is horrible. It just takes an existing article and rewrites it enough that search engines won’t flag it as duplicate content. What on earth would be the point of filling up my blog with the same article over and over (we’ll ignore my annual ‘This year I’ll be an Adult!’ posts. ๐Ÿ˜‰ ).

      Spam me with things I NEED people! Cool art supplies! Nifty fiction writing tools! Pictures of cute puppies!! :p

  7. william casino Avatar

    could you us find chicken wire to put plaster op pairs to from a mountain for train set .

    1. Martha Bechtel Avatar

      Chicken wire by itself, even the fine stuff is not going to hold the plaster well, it’s just too liquid. You are better off using a paper mache method (dipping strips of newspaper in plaster or thinned white glue and then using them to cover the chicken wire to build up a base). Once you have something solid, you can plaster up from there! ๐Ÿ™‚

      I actually had a good bit of luck paint-sculpting with the plaster mix once it had settled to the consistency of thick mud. You -might- be able to wait until that point and then try putting it on the chicken wire, but using a bit of newspaper first is much easier.

      Take a look at this video and see if this might work for you! ๐Ÿ™‚

      https://www.youtube.com/watch?v=l1aMjRFe5gk

  8. Stephen Tashiro Avatar

    Has anyone had luck using “odorless” silicone caulk, “low odpr” silicone caulk or “Silicone II” caulk to make molds? These fancier types of silicone caulk are crowding out the plain vinegar smelling silicone caulk on hardware store shelves. I think the fancier types of caulk don’t employ moisture as a catalyst, so soap and water doesn’t help them cure.

    Fortunately, the local Walmart still sells the ordinary kind of silicone caulk.

    1. Martha Bechtel Avatar

      I did actually try the Silicone II and it did have the problems where it never cured! I tried it with both the cornstarch and the water/soap mixtures and it didn’t set up with either.

      Someday I expect Lowe’s and Home Depot will stop carrying the Silicone I, but I’ve got my fingers crossed that Amazon will still have a source. ๐Ÿ™‚

  9. Frank Novak Avatar

    Knox wood putty, which is a powder that you mix with water, cast well and is much stronger than plaster. Also a note, silicone will not cure in the presents of sulpher. Most clays contain sulpher.

    1. Frank Novak Avatar

      “CORRECTION” Got my projects mixed. That should be DURHAM’S wood putty. It dries hard. Can even be sanded, drilled or tapped. I use it for wood working also.

      1. Martha Bechtel Avatar

        You mean this stuff? https://www.homedepot.com/p/Durham-s-Rock-Hard-DU-1-1-lb-Water-Putty-1-CAN/100184116 Looks like it would be fun to experiment with! I’ll have to see if I can pick some up next time I’m in Home Depot.

        I haven’t used anything beyond Sculpey to try and make silicone casts from so I hadn’t realised the oil-based clays could cause issue. I’ll keep that in mind in the future. Thanks! ๐Ÿ™‚