Introduction: Plaster Molds for Slipcasting

About: Hello! My name is Ali, and I am passionate about design, making, and thinking about how people interact. I'm currently an industrial design student at University of Cincinnati's design school, DAAP. I am mino…

This project shows the process of carving a prototype out of a block of plaster and then making a plaster mold from that prototype to be used in slip casting.

Step 1: Step 1: Design Your Prototype!

To start this off, it's important to figure out what form you're trying to achieve.

In this case, I was making a vase inspired by the architecture of my city and campus in Cincinnati, Ohio.

I sketched concepts and ideas by hand and then jumped into CAD and used Fusion360 to model up my idea more accurately. From here, I could come up with the general scale I was working within. (5" x 5" x 9")*

*With ceramics, it is always important to remember that you will experience shrinkage in the scale of your piece throughout the entire process.

Step 2: Step 2: Make Your Plaster Block

Once you know the scale of your design, pour a block of plaster with an inch or two of wiggle room in each dimension.

To make a plaster block you will need the following materials:

- dust mask and safety goggles

- cottle boards (4)

- clamps (4)

- 2 clean buckets

- something to mix with

- clay

- sponges/towels

- pottery plaster

- water

- murphy's oil soap cut with a little water and a brush

- a clean/flat/non-porous surface to work on

- scale

First you will soap your cottle board with the oil soap by brushing a light coat on all sides of the boards. This will help them stay clean and seal them from sticking to the plaster.

Next, set up your cottle boards on your clean work surface to acheive your desired dimensions: (6" x 6" x 10" in my case)

Clamp the boards in place and seal all edges and cracks with clay, just in case

Now it is time to make your plaster. In one clean bucket, measure out the appropriate amount of water for your plaster-to-water ratio. I would suggest looking online for this ratio and for formulas that will help you determine how much plaster you will need to make for the size block you are trying to create.

^ The link above will fill in some more details about measurements and timing. I will just go over the general process!

Once you have your water measured, measure out your dry plaster in a separate bucket. Pour the plaster into the water (always dry into wet) and allow it to sink and absorb into the water. First, mix your plaster with either an electric mixer or some sort of mixing device. It is best to aggressively mix your plaster at first for about 30 seconds and then mix it more slowly. Allow the plaster to soak and remove air bubbles by hitting the sides of the bucket. Once the plaster has set enough to create a soft visible peak in the top of the bucket when you run a finger across the surface, pour it into your cottle boards.

Let the plaster set for about 30 minutes, and now you have your material!

Step 3: Step 3: Carving and Chiseling

Depending on your desired design, this step can look very different. In my case, I was trying to achieve a very specific and measured geometric form. I decided to print out orthographic views of my design and to create custom acrylic tools to help measure the specific planes and surfaces that I wanted to create.

Using wood chisels and hammers, files, clay carving tools, water, and sand papers, carve out your form by measuring and drawing the desired form on each surface and carefully and strategically removing material.

Make sure to keep your part moist while carving. When the plaster starts to dry out it will become harder to cut into and more likely to crack and fracture in undesired ways.

Once you achieve the desired form, wet sand with a high grit sand paper to create a smooth surface.

*Added bonus: I decided to 3D print a full scale version of my design to compare the digital version of my concept with the hand carved result.

Step 4: Step 4: Determine Your Parting Lines

Once you have carved and sanded your part to your desire, it is time to determine parting lines.

In this step, you must pay attention to areas where there are undercuts. I determined my parting lines by looking at my part from each angle and determining where I could see the most surface area at one time. From there I drew lines and planned the best way to approach creating each piece of my mold. I used a sharpie to draw these lines.

Step 5: Step 5: Making Pieces of Your Mold

Once you have determined parting lines, you will use your prototype to create each part of your mold.

First, determine which piece of the mold you should make first. After brushing your prototype all over with Murphy's Oil Soap, orient the prototype so that the piece you are creating is accessible by plaster. Build up a wall of clay around your plaster prototype, making sure the clay wall is perpendicular to the prototype surface and aligned with your sharpie parting lines. Set up your cottle boards around the bed of clay and your prototype, making sure the clay is flush to the boards. Clamp the boards in place and use clay to seal the edges again.

Mix the desired amount of plaster, and pour it into your boards on top of the prototype. After 30 minutes or so, remove the boards and separate the new plaster mold part from the prototype and the clay. You might need a rubber mallet to do so!

Step 6: Step 6: Clean Up Your Mold Piece, Add Keys, and Repeat!

Once you remove your first mold piece from the cottle boards and clay, clean up the surface that was casted off of the clay, and add keys to register with your the next mold piece you will create.

Keep in mind that sometimes parts of molds should slide off of each other instead of locking in. Design your keys to work with your design.

Once your first mold piece is clean and has keys, soap it up, and repeat the process for the rest of your mold pieces. Always remember to clean and soap each part so that the plaster doesn't stick to itself.

Step 7: Step 7: Creating the Gate and Cleaning Things Up

Depending on your design, you might have to create a gate. In my case, I wanted to create a gate where the slip would be poured into the mold. That way, the top of my finished product will have extra clay material that can easily be trimmed away, leaving a clean edge. I made the gate by building up a drafted extrusion from the top face of my prototype.

Once you complete all of the parts of your mold, clean up the edges with a file and sandpaper to avoid cracking and to make it easier to handle.

Clean the mold with an alcohol solution, and allow it to dry out completely before slip casting.

You've made a mold!


This is only the very surface level of what it takes to make a mold. I didn't go into great detail about terms or materials used, so if you're looking for more information about anything ceramics related, Ceramic Arts Daily is a treasure trove of knowledge.

Good Luck! :)