How to Cast Plaster of Paris in Easy Cardboard Molds

Have you wondered if you could cast plaster of Paris in easy cardboard molds? Yes you can!!! Way back in the day, I used to love experimenting with plaster of Paris. I still do. It’s versatile, easy to find and work with, and its inexpensive. Just like cardboard. A match made in budget-friendly crafting heaven. If you’ve never tried it before here’s how I made these PoP geometric trees using cardboard as a mold.

The same process can be used to create a mold for any simple shape.

My trees are basic triangles and teardrops. Squares, circles, arches, and stars will work too. You can also combine shapes to create something a little more complicated like this Flintstone-inspired shelf.

Or embed other objects while the plaster of Paris cures in its cardboard mold. I added skewers to my mold to create minimalistic Christmas trees. 

In this tutorial, I’ll show you how easy it is to make a cardboard mold for PoP and share a whole bunch of tips too. But before we get there, be sure to follow us on Facebook, Twitter, Pinterest, or Instagram. And don’t forget to subscribe so you’ll never miss a post!

What you need

To make a simple cardboard mold for plaster of Paris, you will need the following:

• Sturdy cardboard box
• Hot glue gun or tape
• Scissors or craft knife
• Cooking oil
• Paintbrush

How to make a easy cardboard mold for plaster of Paris

Whether you’re molding simple geometric shapes or something a little more complicated, I would suggest first sketching out a blueprint or template of some sort. That way you’ll have something to guide you as you create the cardboard mold. I’m using my mold to make teardrop and triangular Christmas trees.

Psst, did you know you can make a plaster of Paris mold using a polystyrene takeout dish?

Making the cardboard mold

Depending on the shape and size of your project, you may need to glue multiple layers of cardboard together for added strength. My Christmas trees are only a few inches tall so I used a regular cardboard box. Cut out the cardboard shapes and the strips that will form the “retaining walls” for the mold with a sharp craft knife or scissors.

My “walls” are about 1” (2cm) high. The height of the cardboard “walls” will determine how much plaster of Paris you can pour inside.

Use the base cardboard shape as a guide and trim the length of the walls.

If your shape has curves, flatten the wiggly waggly in-between cardboard bits first.

It makes it easier to form the walls around the shape and minimizes the gaps. .

Tape or hot glue the pieces together to create the mold. Add extra glue or tape to the bottom of the mold to seal the joins.

You can also reinforce the corners with more tape or glue if needed.

Pour a little oil into a small bowl and paint the inside of the cardboard mold. I used expired peanut oil, but any cooking oil will work. It acts as a release agent and protects the cardboard mold from moisture.

I mentioned earlier that I’ll be using the Plaster of Paris cardboard molds to make Christmas trees so I stuck a skewer into one side of the mold.

Your mold may look different depending on what you’ll be casting. Make any final adjustments as necessary before moving on to mixing the plaster of Paris.

Tips for mixing plaster of Paris

There’s no big secret to mixing plaster of Paris. You basically just add water, but I do have a few tricks that will guarantee a smooth lump free mix.

• Use a disposable container and wooden craft stick or plastic spoon to mix the plaster. It makes cleaning up easier.

• The mixing ratio is normally 2 parts plaster to 1 part water. For a lightweight PoP mixture use a 1:1 ratio. It’s not going to be as structurally strong and will take longer to dry. It’s not a good idea to use a ratio that has more water than plaster.
• If you want the plaster or Paris to set quickly use warm water. The colder the water, the longer it takes to mix and set. 
• Always add the PoP to the water, not the other way around. It minimizes lumps. I like using the island method to mix my plaster of Paris. Add the PoP to the center of the water so it forms an island. The plaster will slowly swell and soften up as it absorbs the water making it easier to get a smooth consistency, without all the hard work.

• Wait for a minute and then slowly and gently stir the plaster until its lump free. 
• If the mixture looks too watery, add more dry plaster and stir well.

• To add color use any water-based paint and stir it in after the PoP is mixed.
• Use the plaster of Paris immediately. It starts to set quickly once mixed.

BTW, did you know; Plaster of Paris is named after the gypsum-rich Montmartre area in Paris, where it was first used in the 17th century. When mixed with water, this gypsum-based material is known worldwide for its quick-setting and fine-detail-retaining properties.

Pouring and demolding the cardboard mold

Pour the plaster into the centre of your mold and gently shake or tap the mold to get rid of any air bubbles.

When the plaster of Paris sets it gets quite hot. It usually takes about 30 minutes to an hour depending on the temperature. So leave it be. You’ll know it’s ready to demold when it’s cold to the touch and makes a tat-tat sound when you tap it gently.  

Carefully remove the glue or tape on the joins and peel away the cardboard pieces.

Discard the cardboard.

Lightly sand or scrape the plaster of Paris to get rid of any imperfections or refine the shapes.

And that’s all there is to it. Let the plaster of Paris shapes cure for at least 12 hours if you want to paint them or add embellishments. It sets quickly but takes a day or so to cure fully depending on the temperature.

Using and displaying the PoP Trees

If you’re making little PoP trees, there are a few ways you can display your creations:

• Glue a ribbon to the top and hang them from the mantle
• Thread twine through a small hole and hang them in your Christmas tree
• Dress them up with paper napkins, paint, glitter, and glue and use them as paperweights
• Add them to miniature landscapes or dioramas
• Insert skewers like I did and stick them in a flowerpot. Plaster of Paris isn’t waterproof so mind you don’t put them outside where they can get wet.

I went for something simple and used a beautiful branch to show them off. Just in case you want to do something similar, here‘s how I did it. Give the branch a good clean to get rid of any bugs that may have taken up residence.

Cut or break the skewers to get trees of varying heights.

Drill a few holes in the branch and glue the skewers inside.

My branch was quite rotten in places so I couldn’t drill too deeply for fear of it splitting apart.

Hide the glue with moss……..

…..Or bark chips.

Add more moss if that’s the way you roll 😀

I never need an excuse to go overboard with moss.

Don’t you just love how the greenery plays off against the jagged wooden edges?

Benefits of using Plaster of Paris

I always enjoy working with Plaster of Paris, aka PoP. It’s versatile and dries pretty quickly, so you don’t have to wait days to make sure it’s cured properly. No one’s got time for that.

Compared to its distant heavyweight cousin, concrete, Plaster of Paris is lighter and less dense making it easier to shape and carve.

And since it’s not so heavy you can get away with using something like a skewer to prop it up.

If you like the idea of using simple cardboard molds to cast plaster of Paris, don’t forget to pin it for later.

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Oh, and if you’re looking for some of the things we used, we’ve got you covered. Disclosure: Clicking on the links below, means we may receive a commission from Amazon. But don’t worry, it won’t come out of your pocket, and it helps us make more amazing crafts to share with you 😉

And if you prefer to buy rather than DIY, then maybe these beauties will appeal.

And as always, wishing you a wonderful, crafty week filled with lots of love. Thank you for popping in for a visit.