In pottery, press molding refers to the process in which clay is forced into a mold in order to take on a certain shape. Once the mold is removed, the piece produced is a uniform replication of the inside of the mold (similar to casting with metalwork). Press molds work great when reproducing multiple pieces of ceramics—from bowls to plates to tiles. Using this method, you can also create pottery shapes that would be difficult to throw on the wheel. Repeating the shape again and again by using a mold also saves you time that you can otherwise spend on the finishing touches.
Press Mold Uses
Press molds (also known as spring molds) can be used to reproduce large quantities of a specific type of ceramics. They can also be used to make the raised decorations commonly seen in the famous Wedgwood pottery, founded by Josiah Wedgwood, the "Father of English Potters." You can use press molds to make ceramic jewelry pieces, like intricate beads and pendants.
Hump and slump molds are available for purchase through most art supply stores and can be used for nesting bowls or uniform plates. (A hump mold forms a convex shape and a slump mold yields a concave form.) You can also buy decorative molds in detailed shapes and patterns that can then be added to an existing piece of pottery.
Types of Press Molds
There are two types of press molds. The first and most common type is a plaster cast press mold made by combining Plaster of Paris with talcum powder (talc shortens the setting time of the plaster and helps lessen the risk of cracking). The liquid mixture is then poured into a rubber mold and set. Plaster cast molds are fired, like ceramics, to remove the water content, but at a much lower temperature (a maximum of 500 F).
The second type of mold is a bisque mold, which is super easy to make yourself. To do so, evenly roll out your clay to the thickness you desire using rolling clay slab sticks (3/8-inch is a good starting point). Then, cast your mold to form the shape of anything from cake tins to plates. You can use the open end of a bucket to make a plate or bowl mold. Just lie your rolled clay on top and let it dip down as deep as you want your finished piece to be. Make sure to line your mold form with a mold release (such as talcum powder, cornstarch, plain water, or pantyhose) so that your clay doesn’t stick to it. Then, let it dry to leather hard. After that, it’s ready for firing.
You can also slip cast molds using liquid clay or form your mold over Styrofoam, which works great if you’re carving your own design into it. Lastly, you can use objects like shells, fossils, or rocks pressed into the clay to create your own forms.
Press Mold Techniques
Using a press mold to make pottery is actually similar to making a bisque-fired mold on your own. First, roll out your clay evenly and place it over your mold. (Try not to roll your clay too thin to reduce the risk of breakage when moving it.) Next, line your mold (skip this step for plaster molds) so that the clay doesn't stick to it. Leave your clay to dry to leather hard and then carefully remove it from the mold. The clay is very fragile at this stage. Trim any excess clay from around the edge of the mold to finish it before firing. Your work is now ready for bisque firing and glazing.