Using plaster press molds is a great way to make consistently sized pottery without slip casting or throwing on the wheel. If you’re not proficient in slipcasting or don’t like to throw on the wheel, but want to make the same shape over and over, this technique is perfect for you.
In this post, an excerpt from the Ceramics Monthly archives, Ben Jordan shares his process for making a cup using plaster press molds. To learn how he press molds his cup handles and adds beautiful slip floral decoration, see the October 2017 issue of Ceramics Monthly! – Jennifer Poellot Harnetty, editor.
Why Use Plaster Press Molds for Clay?
The most satisfying aspect about using plaster press molds is the ability to capture and preserve every detail that I put into a clay form.
Making the Mug
I start out by rolling a slab of clay about 1⁄8 inch thick. Normally I use a slab roller but two yardsticks used as thickness guides and a rolling pin work just fine. I use a metal rib to compress the clay as well as to smooth it out, removing the majority of the canvas texture, then trace around paper templates made to fit the particular plaster mold being used onto the clay slab (1). Next, I cut out the pieces from the slab and begin pressing them into their corresponding mold pieces (2).
If the mold is large or especially concave, I use a homemade tool that helps to softly and evenly press the slab into the form without causing cracking or creating thin spots in the slab (3). This tool is made using a soft fabric pouch filled with some fine-mesh sand.
I remove extra clay from pressing the slabs into the mold using a fettling knife, being careful to leave a little extra clay so that when attaching the separate mold pieces together, there is some additional clay at the connection points to ensure a solid and strong seam where the clay comes together (4).
Once all of the pieces are cut and pressed into their corresponding mold pieces, I score and slip the edges, then begin to assemble the mold (5, 6), pushing the three side pieces of the mold together then securing them using a bike inner tube cut to the necessary length. Large rubber bands or mold straps also work well for this.
I then place the fourth and final piece of the mold onto the other three, pressing the pieces firmly together to ensure a snug fit (7, 8). Using my fingers I press the inside of the mold, pushing all of the seams together as well as ensuring that all of the surfaces are compressed and in place. This creates a number of finger marks inside of the mug. I choose to keep these very much part of the composition as important visual evidence of the making process. The contrast of the less refined interior with the smooth and decorated exterior of the mug is also an intriguing relationship.
The porous plaster mold draws moisture out of the clay, allowing the mug to set up and making it easier to remove from the mold. If the mug is left in the mold too long, it becomes overly dry and may also crack. I wait about five minutes before removing the piece, just long enough to let the clay firm up enough to handle (9), and I often use a heat gun to speed up this process.
After the mug is out of the mold, it’s ready to be cleaned up and to have the foot ring attached and thrown on the wheel. I attach the foot by throwing it on the wheel for a few reasons. The first is for practicality: I would have to make a much more complicated mold with more pieces to incorporate the foot. Second, I enjoy the freedom of being able to vary the foot size and shape with each mug by throwing it on the wheel.