Slipcasting Plates

How to design molds for slipcast plates, platters, shallow bowls and similar forms


   It would be great if there were a perfect way to do slipcast plates. Unfortunately, although there are several different ways to do this, each have their own problems and  limitations. 

   In industry, flat or open ware( like plates, platters and low bowls) are usually made by some form of either pressing or jiggering…techniques that require an investment in machinery, installation, and molds or dies. These kinds of forms are rarely slipcast in mass production. In smaller-scale studio production, plates sometimes must be slipcast because that is the available technology. 

   Plates are slipcast either by Drain-casting or Solid-casting. Here I will describe these techniques, and point out the benefits and difficulties of each

 

Drain-casting

 One option is to drain-cast the plate like any other typical slipcast product. An open mold is filled, a period of time elapses as the slip casts up against the working surface of the plaster mold, then the mold is inverted to drain, leaving behind the cast. 

Pro:

- The mold is an open, one-part mold. The modelmaking (simple, solid model) and moldmaking (one-part mold))  are relatively easy.

- Slipcasting plates in a one piece mold is a simple operation

- Sometimes it is a benefit that the freshly cast piece is available to decorate while soft and still supported in the mold. Also, the piece can partially dry in the mold for stability.

Con:

- The most important surface of the plate- the face- is created by the draining of the slip, not by a mold surface. This requires good technique and well-prepared slip to get a clean surface, and/or some finishing work down the road. 

-  The wall thickness is controlled only by the casting time and the mold/slip conditions, not by the mold itself, thus it's harder to keep the plates of consistent weight.

-  No molded details or crisp transitions are possible on the face of the plate. 

- The foot ring on the underside must be subtle (or nonexistent) because it will be reflected in the face of the plate. 

Drain-cast mold for a platter  

Drain-cast platter mold with recess in back, fits wheel head

Filling the mold

Draining the mold

Trimming the mold

Finishing the platter on the wheel

Drain-cast molds for large platter. Molds are filled, then drained, then trim ring cut off. In this case, I made the molds to fit on my wheel head, and finished the face of the platter on the wheel. 


Solid-casting

  A plate can also be solid-cast with a two-piece mold, creating both the face and the back of the plate. 

Pro:

- The face and the back of the plate are both formed by the mold, allowing for clean surfaces, crisp transitions and molded detail or decoration.

- The thickness (weight) of the plate is consistently determined by the mold.

- Features such as a foot ring may not "telegraph" through the plate to the other side.

Con:

- The creation of the thin-section model and the two-part mold are more complex and demanding of precision plaster craft. 

- Solid-cast plates can be difficult to produce due to the initial clay shrinkage. The mold section that creates the face of the plate is a positive core (like what a studio potter calls a "hump mold".)  As the clay sets up in the mold, it begins to shrink and compresses on to the core. Often this causes the cast to be difficult (or impossible) to demold without distortion, or alternatively, the piece cracks in the mold. Of course, the greater the depth of a bowl or plate, the greater the problem of the piece "grabbing" the core side of the mold. 

-  The model must be carefully made, such that slip can feed from the source (at the pour hole) to the furthest extremity of the mold cavity. For example, in a solid-cast mold that is poured through the foot, if you had a thin rim with a big fat edge, the thin section will cast up solid and prematurely choke off supply of slip to the edge, preventing a proper cast. 

  Solid-cast plate molds are most commonly cast through an open pourhole in the back. Typically, it is an opening that starts just inside of the foot ring. In this kind of mold, the rim of the plate is solid cast and the mirror (center) of the plate is drain cast. In this kind of  mold, the pour hole can slant out (for trimming the foot while in the mold) or it can slant in (so you can remove the back of the mold first and then trim the foot.)  Either way, the foot ring is a problem and you have to develop a protocol for finishing this area. 

    Ideally, the center (drained) area of the plate should be slipcast to a thickness that matches the solid-cast areas. This kind of hybrid drain/solid cast mold can made to work. In fact, the bodies of toilets are usually cast in this way. 

    Another potential problem occurs if the plate has a thick cross section just outboard of the foot, or the piece is cast thin. In this case,  you can get an empty space between the front and back surfaces.  Thus the design of the plate's cross-section is critical to success. 


Small dish, drain cast with open back  Open-back solid-cast mold for small sculptural dish, DM









Models and molds for solid-casting two sculptural trays (for Rookwood)

Models  and molds of two trays that I made for Rookwood/Mottahedeh. You can see that these molds were designed to be cast through tubes in the undersides of the pieces. It is telling that Rookwood commissioned these molds for the purpose of making initial samples. In production, the same models were used to create dies for RAM press. 

 

   I've also made plate molds in which both the face and the back of the plate are completely formed by the mold. Here, the mold is filled  through small tubes or funnels that feed slip via small pads on the back, sometimes incorporated into the foot. This has the benefit of delivering a fully formed piece. On the other hand, the model and mold are a little harder to make, and there are potential air-trap issues.

  One other alternative is to solid-cast the plate with the mold up on end. The slip is fed through one edge of the plate. Generally more suited to square and other off-round shapes.

Tip:: Sometimes in solid casting, when you open the mold, you will want to force the cast to stick to one side of the mold or the other. In a plate mold, because of the shrinkage issue, you'd want to get the plate to release from the core (face) side ASAP, and stick to the back side. A trick I learned at the Kohler sanitaryware plant is this: On the side you wish the cast to release from, dust the core. before casting (At Kohler, at least 30 years ago, they patted the mold with a  loosely woven bag of silica. Perhaps there are safer dusts to used- I've used baby powder.) Conversely, to make the cast stick longer (such as on the back mold section) prepare the mold with a very thin wash of the casting slip. 

   I conclude by reiterating that, unfortunately, there are no problem-free ways to slipcast plates, platters, shallow bowls, etc.- any form where the primary surface is on the inside. But, in the absence of alternative manufacturing techniques, it can be made to work. I'd be interested to hear of your experiences with slipcasting plates!



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