Sign up for your FREE subscription to the Ceramic Arts Daily Newsletter and we will give you Ceramic Mold Making Techniques: Tips for Making Plaster, Bisque, and Styrofoam Molds, Making and Using Casting Slip, and Decorating Ceramic Surfaces Free! click here to subscribe for free! already registered? log in here to download freebie!

Molds make it possible to repeat patterns and forms for a variety of reasons. For thousands of years, potters have used molds both for forming and decorating and often both have been accomplished at the same time.  Whether you choose to try press molds with slabs of clay or slip-casting molds for slip-cast pieces, you’ll discover that making ceramic molds provide a way to create uniform pieces that can save you time and provide you with the means to concentrate on surface decorations. 

If you’re interested in exploring the world of making plaster molds for slip casting or handbuilding, Ceramic Mold Making Techniques: Tips for Making Plaster, Bisque, and Styrofoam Molds, Making and Using Casting Slip, and Decorating Ceramic Surfaces is a great place to start! Check out this excerpt:

Making Prototypes for Mold Making and Slip Casting

click to enlarge!

Each new piece begins with a prototype, generally made of wood or MDF, from which I make a plaster mold. The prototypes can be made from clay, but I prefer using wood for its durability. I’m not the savviest mold maker, so if at some point I have an accident during the mold-making process, the prototype is safe and intact. I’ve also found that making prototypes from wood is great for achieving sharp, transitional lines and edges (figure 1).

Once I’ve settled on a design, I produce two scale drawings—one illustrating the side view or profile, which includes the number of stacked pieces of MDF I will need to make the model, and one illustrating the top view. Using the first drawing as a blueprint, disks of MDF are cut, glued together, stacked, and turned on a lathe to make a solid round form whose shape is close to the side profile of the finished piece (figure 2). Tip: You can use a Surform tool to shape the MDF if you do not have a lathe. The second drawing works as a cutting template that is glued to the top of the form (see figure 2).

Using a band saw, I cut into the shape of the form, carefully following the outside edges of the glued-on template. The sides of the form are then sanded smooth to erase any irregularities from sawing. Finally, the prototype is sealed with one coat of Minwax Sanding Sealer and two coats of polyurethane.

The casting slip I use has a 16% shrinkage rate so the prototype must be made appropriately larger to accommodate the final size of the pot. Always test the shrinkage rate of your casting slip before making the prototype.

Making the Plaster Mold

When making plaster molds, it’s important to remember that slip casting, like any other building method, is strictly a means to a desired end. It doesn’t have to be an overly technical venture and, depending on the form, can be quite easy. I’ve learned plaster mold making simply by reading books on the subject, and by asking for help from others.

The biggest trick to making plaster molds is figuring out the number of parts to cast. Most of my molds are made with four parts—a bottom, two sides, and a top piece used as a pouring gate or slip reservoir. Before I make a mold, I take my prototype and draw seam lines on it with a black marker so that I know how many parts I will need for the mold (figure 3). Then I add a clay slab to the top of the prototype for a pouring gate (see figure 4). By making my pouring gate just a little taller than need be, I can control the quality of the rim after the piece has been cast.

Next, I embed the form into a block of clay up to the seam lines marking off the first section of the mold, set up cottle boards, seal the seams between the blocking clay and the cottles, and pour the plaster. Parts of the blocking clay are removed as I’m ready to cast successive sections. The image shows the mold halfway through the casting process, with the bottom and first side cast, and the second side and slip reservoir or pouring gate still to be cast (figure 4). Note that the location of the seams has been planned so that they correspond to edges or places where planes and curves shift, rather than flat faces of the form. This makes them easier to clean up, and makes them less noticeable in the finished form. 

This process and more are covered in Andrew Gilliatt’s DVD Layers of Color: Exploring Form and Surface Pattern in Slip-Cast Pottery, which is available in the Ceramic Arts Daily Bookstore.