
Ah, the great plaster controversy, gritty and sordid, stealthily sifting into the studios of ceramic hobbyists and artists alike. Ask any potter or dabbler in clay and she will most likely hold an avid opinion about the powder: material of miracles, or dust of demons. Often, I have heard a ceramicist say with animosity, “Oh, I do not allow plaster into my studio!”
And perhaps the anti-plasters have a point. Plaster is messy. Clouds of fine white particles waft from open plaster bags, flying free to settle on every surface, creating dusty skims on tables and walls, shelves and chairs. It is impossible to mix a batch of plaster without finding the milky droplets have splattered on work benches and puddled on floors. And if care is not taken, plaster in the studio is plaster everywhere in the studio.
But the real reason for the hostility towards plaster lies in its insidious ability to spoil good clay and ruin ceramic pieces when they are fired.
Project:
Making a Plaster Press Mold of a Tile

Step 1: Prep the Mold Form
The mold form is basically just a container that holds the liquid plaster while the tile is cast. Mold forms can be made from pieces of wood, strips of thin sheet metal, foam insulation sheets, plastic containers and other found objects. Keep in mind that the bottom of the mold form should be flat and smooth.
Step 2: Prep the Tile
Since I am using adjustable cottle boards, I begin with my mold form unassembled. Depending on your preference and the type of mold form you are using (perhaps a piece of sheet metal bent into a circle), it may be easier to assemble it before hand.
Step 3: Assemble the Mold Form
Plaster has a tendency to stick to wood, and to some extent, the sheet metal with which I have experimented. Therefore, it is a good idea to coat the inside surfaces of your mold form (those that will be exposed to the plaster) with a releasing agent. Mold soap is commercially available and works quite well. I have also experimented with diluted dish soap, which has worked for me in a pinch. But, I have to admit, I have fallen into the bad habit of forgoing the releasing agent. Since the four sides of my form detach from each other, I can pull the wood from the plaster with little problem. However, the sides of my molds come out looking very rough (I really don’t care about the aesthetics of my molds), and every once in a while I have to clean off the plaster stuck to the wooden form.
If your mold form is already assembled per the previous step, feel free to ignore the next paragraph.
Since the cottle boards are adjustable, I can easily alter the mold form to the size of my tile. I place the cottle boards around the tile, abutting the end of one cottle board to the side of its neighbor. I then use 1.5-inch clamps to secure the boards to each other, ensuring the sides of the form are strong.
Step 4: Seal the Sides of the Mold Form
If you are working with a mold form that separates from its bottom, the edges of the walls should be sealed where they meet the bottom of the form. Any other gaps between the mold form walls will also have to be sealed to ensure that no plaster leaks out of the form. I have forgotten to seal the edges once or twice and have inadvertently created plaster lakes on my floor. Not fun to clean up!
Step 5: Volume Calculations
Now for the math!
The size of your mold form will determine how much plaster to mix. In order to determine how much plaster you will need, calculate the volume of your mold by multiplying the finished length, width and height (for rectangular molds) and then subtracting the volume of the model tile. (Note: If your mold form or model is non-rectangular, use the appropriate formula to calculate volume. The formula for a cylindrical volume is provided below.)
Water Volume = 0.56 gallons - 0.14 gallons = 0.42 gallons water
Now, since the plaster to water ratio must be calculated by weight, convert the gallons to pounds by multiplying the water volume by 8 (there are about 8 pounds in a gallon of water).
Water Weight = 0.42 gallons * 8 pounds (per gallon) = 3.4 pounds water.
After finding the weight of the water, find the weight of the plaster using the 100 to 70 plaster to water ratio.
Step 6: Weigh Components
Based on the calculations, we will need:
- Plaster: 4.9 pounds
- Water: 3.4 pounds
Add your water to the mixing bucket, and keep your plaster in a separate container for the time being. Mixing buckets are easy to come by, and there is really no need to buy anything special. Currently, I am using an old spackle bucket and an old paint bucket for my containers.
Step 7: Dissolve the Plaster
Step 7b: The Island Method (Optional)
In the beginning of my dabbling, I was taught to mix plaster using the quick and dirty Island Method. The volume of water is estimated, but the amount of plaster needed is not calculated beforehand.
Begin by filling a bucket with water to about ¾ of the total volume needed. As above, slowly sift plaster into the water. Enough plaster had been added once the water is saturated and small, slow-to-dissolve “islands” of plaster form in the center of the bucket.
Cons: The Island Method is not consistent, and the ratio of water to plaster may not be exact. This leads to molds that may not have the optimal durability or porosity.
Step 8: Rest the Plaster
Letting the mixture stand unagitated for a couple minutes allows the plaster to absorb the maximum amount of water, leading to a stronger mold. And I am convinced that the rest period is the key to a smooth plaster slurry. Forego this step at your own peril (I have, and then found lumps in my plaster).
Step 9: Mix the Plaster Slurry
Since I mix relatively small batches of plaster at a time (~ 1 gallon), I mix by hand (wearing rubber gloves). For hand mixing, a plastic spoon or paint stirrer will also work. For larger batches, use a drill with a paint mixer attachment at slow rpms. USG prefers mechanical mixing, stating it leads to a more uniform and thus durable plaster mix.
Step 10: Pour the Plaster
Try not pour the plaster directly over the tile. I have found that with intricate designs some of those pesky air bubbles from the plaster get trapped in small crevices, creating voids in the finished mold. It seems that pouring the plaster slowly and evenly reduces the amount of air bubbles. After the plaster is poured, I also like to tap the sides of the mold form to release any remaining air bubbles.
Step 11: Finishing the Mold
Plaster sets up in about 45 minutes. The reaction of plaster and water is exothermic (heat producing), so I usually wait until the plaster is cool before taking it out of the mold form. My cottle form makes removing the plaster mold quite easy by merely removing the clamps and pulling the individual cottle boards away from the mold.
Once the plaster has been taken from the mold form, I remove the clay model tile from the mold cavity and the sealing clay from around the edges. Since this clay has been contaminated with plaster, I keep it separate from the rest of my clay in a clearly marked plastic container. I can use it again for creating other molds, but I will not use this clay for my other projects since any plaster in the clay can ruin a finished ceramic piece.
I then finish the edges of the mold by rounding them with a knife. This helps to keep the corners from chipping as I use the finished molds. At his point I also smooth any burrs or other irregularities in the tile cavity that may have been produced from the plaster pour.
Finally, with a damp sponge I clean off any clay or plaster dust that is still stuck to the mold. This final cleaning is good practice for keeping your studio dust free, since any residual clay will dry with the mold and eventually flake off.
I set my molds on two furring strips to allow the air to circulate under the molds while they dry. I also try to speed up the drying by placing a box fan by the molds.
1.) Always wear a dust mask while mixing plaster or generating plaster dust. Plaster contains silica, and breathing the dust can lead to respiratory problems over time. See USG’s Pottery Plaster #1 MSDS.
2.) When mixing plaster by hand, wear rubber gloves. Plaster can dry out the skin and cause irritation.
3.) Do not pour any plaster down the drain or clean out plaster equipment in the sink. The plaster will harden in the drain and ruin the pipes.
4.) Keep plaster away from the good clay in your studio. Even the tiniest piece of plaster can ruin a finished ceramic piece.