Published Dec 4, 2023
Plaster Hump Molds
While shopping at estate sales and thrift stores, in addition to searching for clothes and treasures, I always scan the kitchen section for studio inspiration. Pie pans, mixing bowls, and serving dishes are often stacked high and offer a variety of sizes, shapes, and special features at very reasonable prices. While the non-porous forms themselves can be used as slump molds (with the help of plastic sheeting as a barrier), I prefer to make multiple solid plaster hump molds from each form.
Commercially made vessels are often mold made, making for a catch-free release, but I always double check the rims to ensure nothing will trap the plaster positive. If using a rigid or porous material, such as glass or wood, you may want to prep your surface by brushing on a water-diluted coat of Murphy’s Oil Soap as a release agent for removing your plaster positive later.
Prepare your found vessels on a flat, level surface (1). I usually mix enough plaster to fill as many dishes as possible in one go and often prep a couple more than I measure for. The more molds I have on hand, the more work I can make in a single assembly-line batch. Apply a coat of diluted Murphy’s Oil Soap as needed.
Determine how much plaster you’ll need to mix. I use a baby scale for weighing plaster and a clear bucket marked with volume measurements for the water. I typically calculate the cube-based cubic volume for each vessel regardless of their shape (using the cube formula often simplifies the math for me and slightly overestimates the amount needed when calculating for a variety of shapes and forms).
- The volume of a cube (measured in inches): length × width × height = volume
- The volume of a cylinder (measured in inches): π (3.14) × radius² × height = volume
Once the cubic volume has been determined, divide your total by 80 to calculate the quarts of water needed, and then use the chart above to determine pounds of plaster required.
Using room temperature water, add your weighed plaster to your water. Once all the plaster has been added, allow it to soak for at least two minutes before mixing with your hand and breaking up any bound clumps as you go.
Pour plaster into your prepared vessels, pouring slowly against the inner wall to minimize trapped air bubbles, until the vessels are filled to your liking (2). I often leave space below the rim when filling in order to support the plaster positive with my hand when de-molding. Next, gently agitate each mold by tapping the exterior of each vessel or gingerly shaking the entire table surface to encourage trapped air bubbles to rise to the surface.
Once the plaster heats up and cools down you can remove the hardened plaster positives from their molds and allow them to dry on a wire rack overnight. Once all the water has evaporated, and they no longer feel cool to the touch, they are nearly ready to use. Lastly, use a rasp to bevel the edge of the mold, preventing chips and creating an edge for easier handling (3). If you used Murphy’s Oil Soap as a mold release, you can remove its residue by brushing on a couple of layers of white vinegar. After a few mold-making rounds, I will end up with 3–4 of each plaster form and am ready to begin clay production.
the author Margaret Kinkeade is assistant editor for Ceramics Monthly.