Published Oct 17, 2022
With mold making and slip casting, it is possible to create pots or sculpture of any shape imaginable. Most often, when making a piece like the one shown here, one would cast a multi-part mold, one section at a time. But Wade MacDonald takes a different
approach. Wade creates the mold in one piece around the object, then cuts it into pieces using a bandsaw. This technique is sometimes referred to as a "break-away mold."
In today's post, an excerpt from the September/October 2022 issue of Pottery Making Illustrated, Wade shares his break-away mold process. – Jennifer Poellot Harnetty, editor.
Making the Break-Away Mold
First, attach the plastic positive to a piece of melamine using spray adhesive (To see how Wade makes his models using a 3-D printer, see the September/October issue of Pottery Making Illustrated). Because the model is filled with air pockets, it could become buoyant in the plaster, so it is important to ensure that the form is adhered. Brush a generous amount of Murphy’s Oil Soap on the model and on the melamine surrounding the model.
Next, use cottle boards as a barrier for the plaster. I find these give a finished look to the sides of the mold and are very easy to use. Add a pencil mark about 2 inches above the form as a reminder to not overfill the mold (3).
I do not use a formula when mixing plaster. I start by estimating the amount of cold water needed and add pottery plaster until the dry plaster begins to accumulate on the surface of the water. When enough plaster has been added, I wait for 5 minutes before mixing. I use a drill with a mixing blade attached to mix the plaster and then immediately pour the plaster into the mold. Pour the plaster slowly as to not introduce any unwanted air bubbles or loosen the mold (4).
By placing the mold on a table that is low and movable, you can shake the table gently to force the release of any air bubbles trapped in the plaster. Once the plaster hardens, heats up, and then begins to cool down, you can detach the cottle boards. Clean the mold with a large Surform and a scouring pad and flip it onto wood blocks to dry.
When the mold is dry, draw lines where you want to cut the mold using a bandsaw (5). The cutting lines are determined by the shape of the mold, and are intended to eliminate undercuts that lock the model in the mold. Caution: Follow all safety procedures when using a bandsaw. Use a vent hood to gather dust. And, always wear a respirator, eye protection, and ear protection.
Now, I make the first cut (6). If the plastic model can be removed at this point, I do not need to make a second cut. Because of the complexity of this form, the mold must be cut into four parts. After the mold is cut, remove the pieces of the plastic model and spray the mold with compressed air to remove plaster dust and plastic shavings. Gently clean the mold using a scouring pad and water. Soften the sharp edges and make any repairs to the mold, like filling holes left from air bubbles (7). Once the mold is totally clean and semi-dry, it is ready for casting.
Casting the Negative Mold
I use a casting slip that is 6-Tile kaolin based. Always mix your slip thoroughly and sieve it prior to filling the mold (8). Watch the level of the slip while it is drying and top it off if the level gets too low. Agitate the surface of the slip or mist it with water to reduce scumming.
Look for the slip walls to be almost ¼ inch thick before dumping the slip out. With the mold upside down and placed on wooden stilts, let the excess slip drip out. Using a soft rubber rib, scrape the dried slip from the mold. Then, use the rib to remove the drips on the lip of the cup (9).
To see how Wade makes his models using a 3-D printer, adds handles, and decorates his work, see the September/October issue of Pottery Making Illustrated!