Published Nov 22, 2021
Slip casting is probably the most popular way to make ceramic wall pieces, especially if the pieces are made up of repeating forms. If you’ve been wondering how to make a plaster mold for slip casting, today’s post is for you.
In this excerpt from the Ceramics Monthly archive, Jackie Head takes you through the plaster mold making process from making a prototype to pouring the plaster mold. So have a look and cross “learn how to make a plaster mold for slip casting” off your to-do list! –Jennifer Poellot Harnetty, editor.
How to Make a Plaster Mold for Slip Casting - Part 1: Prototypes
The tile outlines are enlarged by 10% to accommodate the shrinkage that occurs when the porcelain clay body is fired to cone 6. I print out the outline of the prototype shapes, cut them out, and trace them onto laminate boards. Separate prototypes of the front and back halves of each of the shapes are sculpted out of solid clay using a clay body with no grog to achieve a smooth surface. Wet clay is roughly packed onto the boards in the approximate shape and desired depth of the tile prototypes. After drying for several hours, the clay begins to stiffen, then I carve the prototypes into a more defined shape (1). Once the prototypes reach the beginning stages of leather hard, I refine the shapes using serrated loop tools and various ribs to achieve their final forms while ensuring there are no undercuts on the prototype (2).
In preparation for creating the molds of the front half of each shape, the prototypes are refined a final time. To accomplish this, a soft paintbrush dipped in water is used to remove any imperfections, and a flexible rib is used to create a smooth surface and reinforce any sharp angles.
How to Make a Plaster Mold for Slip Casting - Part 2: Creating the Molds
Wrap aluminum flashing around the prototype to serve as cottle boards, leaving a 1½–2-inch gap between the flashing and the edge of the clay form. The flashing creates a barrier to contain the plaster once it is poured onto the prototype (3). Once the base of the flashing is sealed with a coil of wet clay, plaster is poured in until the depth of plaster surrounding the prototype is 1½–2 inches.
Once the plaster sets up, the next step is to create the back half. Creating a back to the tile lifts the form from the wall visually, and makes it more dimensional and interactive for the viewer. To begin this process, create registration keys in the first half of the mold. Use a wire tool to carefully release the back of the prototype shape from the laminate board. Place it on top of the plaster mold so that it lines up with the solid clay form that is still in the mold. This step requires patience and attention to detail. It is important to make sure that the front of the tile and the back of the tile line up perfectly. To do this, use a metal rib and shave the sides of the back half of the tile down little by little, just until the edge of the clay from the front tile section becomes visible. Add clay as needed to create a seamless connection, and then refine the back half of the tile (4).
Before wrapping the aluminum flashing around the first plaster mold section to pour the second part of the mold, apply several coats of Murphy Oil Soap to the surface of the plaster. Ensure that all excess soap is removed before pouring the plaster, so that no unwanted marks are captured in the mold. Next, secure a piece of PVC piping to the center of the shape to create a pour hole. Using PVC piping allows me to have consistency with the size and placement of the pour hole when producing multiple molds of the same shape (5).
Want more information about hanging ceramic art? Check out this post for info on how to map out ceramic installation art for the wall.