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 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!
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.)
- DAP Plaster of Paris
- Acrylic paint (any cheap craft paint will do)
- Elmer’s Carpenter’s Wood Glue
- 30ml plastic measuring cups
Water + Plaster + Wood Glue = Fun?
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?
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!
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
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! 😉