Mastering Mold Design: Choosing the Right Approach for Your Slip-Cast Forms Jonathan Kaplan

Picture a vase that is 10 inches in height, with a diameter of 3½ inches at the top and 4½ inches at the bottom, and is cinched in the center. The vase has a flat top and a flat bottom. It can be made a few ways. The simplest is a mold of four parts: two side pieces with vertical seams, a separate, full-size, mold part that contains the slip reservoir, and a separate, full-size mold part for the bottom. 

In another variation, this object can be made in a three-part mold: two side pieces with vertical seams and each side has half of the slip reservoir built into it. The bottom is a full-size separate mold part.

The third configuration has two side pieces with vertical seams each containing half of a slip reservoir. A drop-out spare completes the mold.

Which of these mold designs makes the most sense? The simplest mold configuration is shown in A. It only requires one spare at the top, and this can be wheel-thrown clay, a plaster turned part, or a round, found object.

Example two requires a turned spare to make a slip reservoir in each side part. It's just a bit more work. Finally, the third design requires turning a drop-out spare, as well as a spare for the top slip reservoir in each side. The third mold configuration makes most sense to me as I can make the necessary spares, which will result in a less bulky mold, unless the weight of the mold is reduced by cutting the sides off at a 45° angle on a band saw or putting 45° spacers in the mold box for the set-up. These adjustments require additional work and power tools.

A is an example of poor mold design. While this configuration will certainly work, the seam bisects the foot and will require extra trimming. This type of simple solution is often made as you begin your journey into mold and model making. Nonetheless, this two-part configuration is the easiest way to make a mold. 

A much more appropriate solution is using a drop-out spare for the bottom mold part (F, G). Another very workable solution is C, which has two side parts, a separate mold part for the slip reservoir, and a separate part for the bottom.

Experiencing poor mold design propels us to learn about the better or best way(s) of mold design. While the design in A does have the sensibility of including the slip reservoir in each half of the side pieces, I would consider this mold to have bad form because this configuration has the parting line bisect the bottom surface of the slip-cast object. This does not mean it doesn't work. This configuration works, it is simple, and is just another way. While I would deter you from making a mold with such a parting line configuration, I would totally encourage you to develop good mold-making habits from the beginning. I think what is quite pertinent is that, as someone starting out on this plaster journey, it is also important to both see and understand what the possibilities are, even if they have bad form, or could also be referred to as incorrect design. And maybe better put, questionable mold design.