Published Mar 31, 2023
While it was once thought of as an industrial process, using ceramic molds is a technique that is increasing in popularity among potters these days. From simple one-piece plaster molds to more complex multi-part molds, plaster mold making allows the potter the freedom to experiment with ideas for form and surface because it takes away a bit of the preciousness of a one-of-a-kind wheel thrown pot.
Potter Linda Gates makes her work using multi-part ceramic molds, not only because it allows her to be creative with the forms, but also because it results in a smooth surface perfect for her digital decal decoration. Today, Gates walks us through the slip-casting process for one of her jug forms using a four-piece plaster mold for the body and a two-piece plaster mold for the handle. – Jennifer Poellot Harnetty, editor
Ceramic decals can be applied to any glazed object, but it makes life easier if the ceramic form has smooth surfaces to avoid the problem of trapped air creating bubbles and holes in the image. This is why I use ceramic molds to make my forms. The ceramic form shown here is a slip-cast jug that I designed as part of a college tableware design project.
With a little modification to the original jug design, I made new plaster molds, one for the handle (figure 1) and the other for the body of the form. The mold for the body of the jug (figure 2) is made in four parts – the two sides, the base, and the reservoir.
To prevent leaks when pouring the casting slip, secure the parts firmly together with strong bands cut from rubber inner tubes (figure 3). If you design your mold to include a reservoir, which makes it easier to maintain an even rim thickness, fill the mold to halfway up the reservoir wall using commercial earthenware casting slip. As the porous mold absorbs the water from the clay, the excess is drawn from the reservoir.
The handle mold is also filled with casting slip. Once the slip is the desired thickness (check by blowing on the edge of the mold where the slip and plaster meet), pour the extra casting slip back into the container and leave the mold inverted at an angle to drain into a bucket. Placing it at an angle avoids stalactites of clay forming on the bottom of the piece.
Tip: To achieve even thickness between multiple casts, it is a good idea to time the first casting and use this as a guideline.
When the mold is well drained and the sheen has gone from the wet casting slip (typically about 20 minutes), remove only the reservoir portion from the jug mold, trim the excess clay from the top and clean it up with a damp sponge. I leave the rest of the mold intact for a further hour or so for the form to firm up for easier handling.
Both ceramic molds are then disassembled and the jug form and handle carefully removed. Both component parts are cleaned up with a fettling knife and damp sponge. The handle is attached, and the form is covered in plastic for 24 hours to ensure a secure join.
To learn more about Linda Gates or see more images of her work, please visit www.lindagates.net/.