Press molds are the easiest and most effective ways to recreate your work multiple times. One of the oldest of the mold techniques is a bisque fired press mold. Bisque firing is basically firing greenware (unfired pottery) at temperatures between cone 6 and 8. The bisque firing will evaporate any trapped water (and chemically bonded water) in the clay. This will lighten the piece and increase how porous the clay body is, meaning it will be much easier to glaze.
Making molds using this method is incredibly easy, you line your original piece with well-rolled clay (make sure to line the mold with newspaper first if the mold you are using is not porous) and wait for it to dry before firing. Once dry you can use it as a mold as many times as you like. There are two types of molds when bisque firing, hump molds which are convex pottery shapes and slump molds which are concave. Hump molds are better if you are adding handles to your work afterward. If you make your slump or hump mold out of clay then you will be able to dry them in the kiln. Be aware of the fact that the clay will shrink, so the piece you are recreating will be smaller than the mold.
What is slip casting and how to do your own slip casting?
Slip casting is defined as ‘the process of forming ceramic ware by pouring slip into usually plaster molds’. Slip casting and the use of plaster molds became popular around the 17th century when potters realized that you actually recreate much more intricate pieces than just by using bisque fired press molds. The slip is made from clay body with a process that's sometimes known as slaking, which is when clay breaks down when it is mixed with water. Slaking sees the clay swell and trapped air burst.
The process of making slip is done by mixing warm water with bone-dry clay and leaving it for around 24 hours, for the slaking process to work fully, and finally then stirred. From then the slip clay's specific gravity is measured, this is basically to work out the ratio of water to clay within your slip mixture. A deflocculant is then added to the slip, this is so that the particles in the clay do not stick together. The final consistency of the slip should be thick and creamy. If you are making your own slip, you should always wear a respirator mask when making it.
The slip is then transferred into a plaster cast mold, which you should fully fill to the brim. You’ll need to wait for enough time for the plaster mold to absorb as much liquid from the slip as possible (timings will depend on the size of the mold) before gently pouring out the excess slip from the mold and neatening off the edges before firing. This method is perfect if you want to make several identical pieces.
Tips and tricks for making press molds:
If you’re making your own hump mold then you can take any bowls and plates from your household that you love the shape of and pour plaster into them to create your own versions. If you are using a plaster mold then a good tip is to use cheesecloth between the plaster and the clay, it will help you to be able to easily remove the clay. Also, a good way to speed up the drying process is to put the mold on your kiln.
Decorative ideas for press molds:
There are a thousand different ideas that you can use with press molds and potters love it for its complete creative freedom. For example, you needn’t just make traditional pots and bowls, you can make imprints of flowers, animal shapes and even casts within your molds. London based ceramicist Rachel Dein makes the most exquisite plaster cast tiles that depict plants and flowers that wouldn’t look out of place in a Victorian botanical illustration book.