Mixing plaster for a slip-casting mold is different than for a press mold. To ensure even wall thickness throughout the cast, each part of the mold must be made with the same plaster-to-water ratio.
First, prepare your model. Mine is carved from insulating foam that’s first cut out using a template and a hot wire foam cutting tool. A separate form is cut out for the slip reservoir (1–2) and held in place on top of the main form with T-pins pushed at an angle through the side of the reservoir form into the main form. Bricks secure the model in place when pouring the plaster. I place my model on a ¼-inch piece of Plexiglas and use a corner of the Plexiglas to square up the cottles. The Plexiglas protects the work surface and allows me to move the entire mold if needed.
Brush a 50/50 Murphy’s Oil Soap/water solution liberally on the Plexiglas and the model, arrange the cottle boards, and clamp them together. Seal any seams between the boards with clay coils to prevent plaster leaks (3), then coat them with the soap.
Determining Volume: Method 1
Next, determine the volume of water needed for the pour. I put an estimated volume of water into a 2½-gallon bucket, weigh the bucket, and add more water until I get to the next closest weight on the chart below. Room temperature water works best. Next, weigh out the plaster into a separate dry container using the amount called for in the chart.
Determining Volume: Method 2
If you would like to measure the volume of the water needed more precisely, figure out the volume of the space inside the cottle boards. For a rectangular shape, multiply length × width × height of the space to get the volume. For a cylindrical shape, multiply π × radius2 × height.
To find the volume of water needed to make enough plaster, use the following equation. It divides the required volume by the volume of a 3:2 plaster to water mix by weight, (80 cubic inches). The 3:2 ratio means 3 pounds (1.36 kg) of plaster to 2 pounds (.907 kg) of water. Two pounds of water equal a volume of 1 quart (.95 L). The equation uses a plaster to water ratio of 10:6.6 rather than 10:7, which works well and is easier to calculate:
To determine the weight of the plaster needed, multiply the resulting number of quarts or liters of water by 3.
Pouring the First Section
Slowly pour the plaster into the water so as not to cause splashes. Stir with your hand. It should take less than 2 minutes to get the plaster mixed and get any clumps broken up. I pour as soon as all the clumps are out (4). Pour the plaster through your fingers to avoid splashes. When you have the plaster up to the level you want, put your fingers into the plaster and run them gently over the surface of your model, the Plexiglas, and the cottles to release any air bubbles. Also, remove the T-pins if you used them, but the brick weights should remain. Let the mold set up for a few hours until the plaster has heated then cooled before removing the boards and preparing for the next section.
Cleanup and Prep Work for Multiple-Section Molds
I clean up each mold section and carve registration keys into its surface before I pour the adjacent section. First scrape all the edges of the mold with a fettling knife to remove the sharp slivers of plaster (5). To clean the face of the mold I use water, drywall sandpaper, and wet/dry sandpaper (6). I sand around it and lightly on the top edges with wet/dry sandpaper to get the plaster level to the model’s surface. If using a clay model, use a needle tool to remove any clay that adheres to the plaster close to the model and to remove any plaster on the clay model. If you are using a bisque-fired piece, remove and soak it in water before pouring each adjacent mold section. To cut keys into the plaster, use a quarter or a dime to create a half sphere indentation at two or three locations on the surface.
Before pouring the next section, coat all the surfaces of the mold (even the bottom) and all the surfaces of the model liberally with the soap solution (7). Place the mold on the Plexiglas, arrange and clamp the cottles, seal any seams between the boards and plaster with clay coils (8), and coat the cottles with the soap solution. Find the volume of water needed, mix another batch of plaster, and pour it over the first section to the desired height (9). Put your fingers into the plaster and slide them over the first plaster section, the model, and the sides of the cottles to release any air bubbles on their surfaces.
Finishing, Drying, and Using the Mold
Take the mold apart 2–3 hours after pouring the last section. Use a fettling knife to pry apart the sections (10–11). If it won’t come apart easily, let it set overnight. Remove the model (12–13), sand the outside of the mold to eliminate any rough edges, sand the inside where needed (14–15), check that the interior seams line up while the mold is assembled, and re-sand if necessary (16). Wash the mold with water and place the separated sections into a dry box or place the assembled mold in front of a fan on a rotating wheel head so it dries uniformly.
When the mold is completely dry and before I use it for the first time, I submerse each piece into a tub of water for a few seconds. This starts the capillary action within the plaster so that the casting process works more quickly. Next, remove any remaining soap that is on the casting area with vinegar.
The first cast will probably not release easily from the mold so gently use compressed air (at 35 to 50 psi) aimed at the casting-slip/mold interface or just wait another 10–20 minutes.
After two casts, if the form won’t release, check for undercuts (areas where plaster overhangs prevent the clay from releasing). View the mold sections from above, place a fingernail against the mold at the top edge of the casting area and move it down toward the bottom. If at any point you can’t see the tip of your fingernail, you have an undercut that needs to be sanded away.