Mold Design: Decisions, Decisions…..

Three decisions to make when designing a mold for slipcast pottery

When I am asked to make a slipcasting mold, I want to make sure there is agreement on some options. 

Here are three questions I am likely to ask, illustrated by some mug molds I have made over the years:

1) The Spare:

        Slipcasting molds almost always have a reservoir called a "spare" or a "pour hole".  This is an extra space added to the top of the casting cavity. It has two main functions:

- The spare acts as a reservoir to feed the mold as it is casting.  This is desirable because, as the filled mold absorbs water, the volume of the slip is reduced, so level of slip drops. Without a spare, this would expose the top edge of the casting, neccessitating repeated topping up by the potter. The spare keeps the entire piece "underwater" throughout the casting process.

- The surface of the spare acts as a guide for a knife, as the potter trims the opening of the pot.

 Spares can be made in two different orientations, by extending the cavity either vertically or horizontally. Often there is only one orientation possible, but with many vessel molds there is a choice between the two, as illustrated by the mugs molds below. 

  The first mold has a vertical pour hole. In trimming, the knife is held vertically, and the trimming cut is made as the blade rides on the inside surface of the spare. 

   The molds in the second two pictures have spares that extend out horizontally (variously named a "trim ring", a "step-spare", etc.), Here, the cut is made with a bent knife, with the blade riding on the horizontal surface of the spare.

   The vertical spare gives a consistent edge thickness, determined by the distance that the spare is stepped back from the edge.  But if the is wall is cast too thick or thin, you can get a messy situation- so wall thickness must be accurately controlled (by controlling casting time, mold saturation, etc.) On the other hand, a horizontal spare is less accurate but more forgiving. Since you are cutting across the wall, you get a clean, square cut regardless of wall thickness.   There are preferred applications for both types of spares. 

    The last three pictures illustrate the cutting of various spares.

2) The Handle:

     If a piece of pottery has a handle (or similar attachment) there is a decision as to whether it is cast-in-place or added on. 

      The molds in the first two illustrations show mugs where the body and handle are cast in one piece. This is the simpler mold that eliminates the need to join on (or, as the British say, "lute on")  the handle and clean up the joint. On the other hand, the inside of the piece will often show a dimple or even a hollow where the handle attaches. 

        The third illustration shows mold for a mug with an added-on handle. Obviously, this requires you to make two separate pieces in two separate molds, and then join them together. This is more laborious, but gives a cleaner result on the inside surface of the piece. 

3) The Foot:

    Any of these mugs could be made in a two-part mold, with the seam running across a flat bottom. However, all these mugs have a recessed underside, which potters call a foot. This requires a third mold section, which can be configured in two different ways:

 In the first illustration, the additional mold section is an insert that is captured between the two main sections. In the other illustrations, the third section is a full bottom plate, upon which the two side sections rest. Either way can work well.

   This is a quick overview of these issues. The mold configuration will be determined by the shape of the piece and the preference of the potter/manufacturer.


Click the images below to enlarge


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