I’ve been having a lot of fun playing around with casting using a mixture of plaster of paris and wood glue. It makes for a much stronger cast which lets me do a lot more with it.
But since I’m starting to try and figure out how much things really cost to make it’s time to finesse the mix so it’s a bit more consistent… Bring on the grade school science experimentation!
For those of you looking for the answer without the adventure, here you go! 🙂
The best mix ratio of plaster of paris and wood glue was 2 parts water (and plaster) to 1 part wood glue. The best paint color out of what I tried was Apple Barrel 21471 Spiced Carrot and 21484 Admiral Blue. Making 20ml of this mixture costs roughly $0.28.
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- Cold water: free-ish.
- DAP Plaster of Paris: I use the 4lb red box for $6.78
- Acrylic paint (any cheap craft paint will do): 2oz for $0.99
- Elmer’s Carpenter’s Wood Glue: 8oz for $4.68
- Craft sticks / toothpicks for stirring: reusable, so free-ish
- Paper cups: 100 5 oz cups for $3.99
- 30ml plastic measuring cups: reusable, so free-ish
So I’m spending roughly $12.00 in the name of SCIENCE!
The Experiment Begins!
For this experiment, I am using the kittenloaf mold.
It takes roughly two tablespoons (or 30ml) of plaster mix, which matches up well with the resin casting cups I got from the Alumilite™ Amazing Casting Resin.
Besides, who doesn’t need more plaster cats in their life? 😉
Time to take a look at our ingredients and what role they play in the mix.
Why use these Materials?
The paint is included to make sure the wood glue is mixed well into the plaster and water. Thus I will be focusing on which colors work best.
Adding too much paint can cause issues, so only use a drop!
As you can see in the photo, most colors will only barely tint the end result, but it helps to pick a color that you want to paint it.
Plaster of Paris
I’m trying to keep the amount of plaster relatively consistent in this experiment and focus on the ratio of water to glue.
My current method is to slowly add plaster of
Since the amount of water determines how much plaster to use, I will be holding steady at 1 tablespoon (15ml) and adjusting the amount of wood glue.
You always want to use cold water since hot water will cause the plaster to set faster.
This is the critical element of the experiment!
If you don’t mix the glue well enough, or you add too much… it will float to the surface as the cast hardens. This leaves you with a
Too little glue and it really doesn’t do much of anything.
Silicone is a flexible and waterproof material that makes for easy cleanup after casting. The plaster or
The mold I am using are all made out of 100% Silicone Caulk! Want to learn how to make your own? Check out my tutorials on making them.
I’m still on a bit of a learning curve with these, so you’ll notice the casts come out with bumps and ridges and weird blobs… those are actually from the mold, not the mixture.
Methodology (aka Plaster Casting Tips!)
- Wait for at least 24 hours before using a newly made 100% silicone caulk mold. Otherwise the out-gassing will make bubbles in the casts. (It’s better to wait a week, or soak the mold in a bucket of soapy water for the 24 hours to make sure it’s fully cured.)
- Use cold water only. Warm water will cause the plaster to set faster.
- Always add plaster to the water, never water to the plaster.
- Let the plaster set for 1-2 minutes before mixing so it has a chance to settle and
unclump. (I mix the glue and paint while I’m waiting.)
- Mix the plaster carefully so you aren’t adding air to the mix. Also the more you mix, the faster the plaster will set up so be careful. We are just looking for a nice smooth mix to add the glue to.
- Mix the glue and the paint together so it’s a nice solid color. Then it’s easier to combine into the water-plaster and make sure it’s mixed well.
- Never add the glue to the water before the plaster, otherwise it the plaster will clump up and not mix properly. Also, you tend to end up with the glue floating to the top of the mold.
- Slowly mix the glue-paint into the plaster-water, but keep mixing until the color is uniform. The glue will want to float on top of the water, but will slowly combine and the overall color will turn noticeably paler.
- You can use some sand on a plate to make sure the mold is level before filling. I tried to make them level with the plaster cast bottoms, but it didn’t quite work (my art table also isn’t level).
- Wait several hours before demolding. I’ve gotten to the point where I’ll normally wait overnight, but if you want the plaster still a little wet for easier
cleanupyou can take it out as soon as it doesn’t dent when you scrape it with a toothpick.
- If there are a lot of bubbles, you can smooth things out with a skim coat of a slightly more watery mixture (after it dries completely!) Since I’m testing the hardness, I’m giving it several days to dry out…
The Recipes and Results!
I started out using the measurements on the ‘spoon’ side, but swapped halfway through to using the measurements on the ML side. Sorry for the ensuing confusion!
- A teaspoon (TSP) is 1/3 of a tablespoon (TBS) and 5ml-ish.
- A dessert spoon (DSSP) is 2/3 of a tablespoon and 10ml-ish.
- A tablespoon (TBS) is 15ml-ish.
- 1 fluid ounce (OZ) = 2 tablespoons (TBS) = 30ml-ish
The ratio noted on each mix is water:glue.
The amount of plaster is a rough guess, I add plaster until it mounds just slightly over the waterline. It’s normally roughly equivalent to the 1 part water/2 parts plaster so I’ve used those measurements here.
1 DDSP (10ml) water, 2 DDSP (20ml) plaster, 1/2 tsp (2.5ml) wood glue, 1 drop Craftsmart Ocean Breeze (greenish). (4:1)
Nice color tint when mixed with the glue, but it fades a lot when mixed in with the plaster. However when it dries out it’s very close to white, so it would be great for a white or light colored paint job. Some of the other colors I tried below… not so much.
This is about the feel of the mixture when I do it by eye instead of measuring, so I’m going to use this as my default. It’s solid, but a little too easy to sand. Still it’s not bad if you’re looking to make things a little more solid, but skimp on the wood glue.
1 DDSP (10ml) water, 2 DDSP (20ml) plaster, 1 tsp (5ml) wood glue, 1 drop Craftsmart Sailing Sky (blueish) (2:1)
This was a very light color and didn’t tint well, even when just mixed with the glue. Once the plaster was introduced into the mix it was almost impossible to see. So ‘no’ for the paint color– I ended up trying the ratio again since it did so poorly.
The resulting cast was pretty good, not too soft and there was only a slight glue layer on the back (probably because of the mixing fail). It sands well and seems relatively solid, however there were a few more bubbles in this one than there were with Mixture 1.
2 TBS water, 3ish TBS plaster, 1 TBS wood glue, 1 drop Apple Barrel 21471 Spiced Carrot (2:1)
I forgot to take a pic before I did the skim coat, but it looked almost identical to Mixture 2, 4 and 7 only more orange.
Now this is a color that has some punch! Very easy to tell how well everything was mixing, makes me rather inclined to test more ‘warm’ colors going forward. I do, however, need to use a lot less of it or only use it on things where an orange tint to the final color is a bonus.
There was a very minor glue layer to the back and it seems quite solid. It does still sand, but it takes some effort. This one had some more air bubbles that the first one, but not too bad. Still, it required and extra layer (skim coat) of filler plaster painted over it, so it was a little annoying.
I did a larger batch with this one to see if doubling the recipe would get me two full cats… but that filled two cats, three small horse heads
10ml Water, 20ml Plaster, 5ml wood glue, 1 drop Apple Barrel 20591 Bright Magenta (2:1)
This is a repeat of the recipe I tried before just to see what the color does. Also I needed a good small batch test since the Sailing Sky didn’t mix as well as was hoped.
Nice solid cast, not too many air bubbles this time. Pretty good mix!
The pink looked like it was going to hold up well when I mixed it with the glue, but as soon as it hit the plaster it paled a LOT. But overall it had a nice tint to it and I was able to make sure the glue mixed well, which is the overall point.
15ml Water, 30ml Plaster, 10ml wood glue, 1 drop Apple Barrell 21484 Admiral Blue (3:2)
This one I haven’t tried before, but I was pretty sure would be too heavy on the glue end of things… and I was right. So right.
All of the back of the casts collapsed in on themselves in these odd ripple pattern. I’m assuming it’s because the glue shrank when it dried and there wasn’t enough resistance from the casting mixture to keep it from ripping apart. I had to repour the backs on some of the casts just to make them solid enough to come out of the molds.
The cast did not want to demold, it was tacky and very soft even after several hours extra dry time. Which helped when it came to sticking the parts back together, but overall was not a pleasant experience.
The mixture was wet enough that the cat cracked coming out and I lost a leg on the horse cast, which easily stuck back together. I let this one sit much longer than the others and I don’t think it would have dried out past this point. It left this odd film in the mold that will need scrubbing out before it can be reused.
There were a bazillion bubbles on the cast’s surface, even though I’d been extra careful mixing things. Which means these will need a lot of prepwork to finish– assuming they ever fully harden (it’s been 48 hours!)
All in all, a complete failure.
The blue is actually a great color and I think I’ll keep it for the darker color paint jobs. It’s a little too strong for anything I plan on painting white, but it’s great for black cats or blue-tinted critters.
10ml Water, 20ml Plaster, 10ml wood glue, 1 drop FolkArt 2900 Cardinal Red (1:1)
I was pretty much positive from the start that this is going to be too much glue, but I had to try… FOR SCIENCE!
I was right. So very very right.
And it was just as bubbly as the blue but slightly less sticky, I think because I gave it longer before demolding. Interestingly the red paint broke up into tiny particles, so even though the overall cast came out with a nice tint, I’m not sure how those specks will affect the overall sturdiness.
15ml Water, 30ml Plaster, 5ml wood glue, 1 drop Craftsmart Violet (3:1)
This was an in-between try just too see if I really need the 2:1 for the strength over the 4:1 or if I can skimp a little on the glue and still come out with something nice.
The purple was a dismal failure, but I’ve come to realize the Craftsmart paints just pretty much suck in general when it comes to pigmentation. They are nice for laying basecolors under other paint jobs, but they are junk for being a solo color (or a plaster tint).
Random side experiment: This mix doesn’t quite make two cats, so I ended up experimenting by adding a layer of sand to the back of the mold that was about 3/4 full. The sand soaked up the water (and some of the plaster) and the whole cast promptly fell apart when I demolded it. So if you want to add sand you need to mix it into the plaster– more on that in the aluminum lost wax casting post!
Otherwise, not a bad mixture, but not as strong as the 2:1 and not that much of a price difference to make it worth the sacrifice.
Overall the best mix ratio was 2:1 and the best paint colors (out of what I tried) are Apple Barrel 21471 Spiced Carrot and 21484 Admiral Blue.
The cost of this 2:1 mixture for a kittenloaf (20ml) is $0.28
- $0.00 old water, Toothpicks, Acrylic Paint, Paper cups / Plastic measuring cups. These are all either reusable or only a few pennies.
- $0.17 for 20ml plaster DAP Plaster of Paris: Lowes 4lb red box $6.78, 20ml of plaster weights 0.4oz per the postage scale (which sort of sucks for sub-ounce measurements), 16oz in a pound, 0.4/16 = 0.025 *6.78 = $0.1695
- $0.11 for 5ml Elmer’s Carpenter’s Wood
Glue :8oz is 236.588ml, rounded because there will be some loss to 220 and 5ml of that is 5/220 = 0.02272727272 * 4.68 = $0.10636363636