What is a press mold?
Press molding has been defined as ‘a technique in which clay is forced into a mold in order to take the shape of the mold, and then removed to form a ‘positive’ of the mold’. Therefore, press molds are great for many reasons, namely as an easy way of reproducing multiple pieces of ceramics from bowls to plates to tiles. With this method you can also create pottery shapes that would be difficult to produce on the wheel.
You can repeat the shape again and again and save yourself a lot of time in the process. Press molds can also be known as sprig molds, which Pottery Magic concisely describe as being ‘one-piece molds from which flat-backed, shallow castings are produced when a piece of clay is forced into the impression’. Sprig molds are also used for creating raised decorations, commonly seen in the infamous Wedgewood pottery. Wedgewood was founded by Josiah Wedgewood, who is known as ‘the Father of English Potters’ and has been manufactured in Stoke on Trent in the UK since 1759.
What are they usually made from?
There are two main types of press molds. The first and most common is the plaster cast press mold, which is made with plaster of Paris with a material like talc mixed in (this shortens the setting time for plaster of Paris and also helps lessen the risk of cracking). The liquid mixture is then poured into a rubber mold and set.
Plaster cast molds are still fired like ceramics to remove the water content, but at a much lower temperature (usually a maximum of 500 degrees Fahrenheit). Another type of mold is the bisque mold, which is super easy to make yourself. All you need to do is roll out your clay, to whatever thickness you desire (around three eighths of an inch is a good thickness to start with).
Make sure your clay is rolled completely evenly, using rolling clay slab sticks. Then you can use whatever you like to create your mold, anything from cake tins to plates. A great idea is to use the open end of a bucket to place your rolled clay on top of, letting the clay dip as deep as you want your plate or bowl mold to be. Make sure you line whatever you are using as a mold, so your clay doesn’t stick and let it dry to leather hard. Then it’s ready for bisque fire to the required temperature. You can also slip cast molds with liquid clay or form your mold over Styrofoam, which is especially great (not to mention easy) if you’re carving your own design into it. You can also also use objects like shells to press into clay molds to create your form.
How do I use a press mold?
Using a press mold is actually fairly similar to making your own (bisque fired) mold. You'll need to roll out your clay evenly and place over your mold. A hump mold is where you form your clay over a convex form and slump molds are when you form your rolled clay over a concave form. For any materials other than plaster (these are porous and the clay will shrink in it) you will need to line your mold, so the clay doesn't get stuck in it.
A nifty tip is sprinkling your mold with cornflour to line it (cornflour will burn off in the kiln) or to line the mold with a pair of tights, which you can peel off the mold before firing. Leave your clay to dry to leather hard before taking it off the mold. You’ll need to be very careful when taking your work off, as the clay is fragile when moved at this stage. Try not to roll your clay out too thin when using the mold to reduce the risk of breakage when moving it. You’ll also need to trim any excess clay off the edge of the mold and make sure all of your edges are neat before firing. Your work is then ready for bisque firing and glazing.