Making Ceramics Products
Ceramics begins with a plaster gypsum mold and clay casting slip. With the mold secured by straps or strong elastic bands, fill the mold slowly through the gate (opening) in the mold with slip (Refer to picture on top left). Normally a casting time of 20 to 25 minutes produces a clay wall with a thickness of 3/16 of an inch. During the casting of the mold, check the level of the slip in the mold and periodically add or top-off with more slip.
Larger pieces, such as floor vases, table lamp bases, etc. will require more time to form a thicker clay body.
When the slip reaches the desired thickness (use a clock or timer), empty the unused slip into a pail. This slip may be used again to fill another mold. Turn the mold upside down and allow the mold to continue drain. This procedure is known as the 'drain-casting' method of casting. Other methods of forming clay might be the pinch method, the coil method, wheel turning, jiggering, and pressing.
The same methods are used for casting earthenware, stoneware, porcelain or red, buff clay slips.
The plaster in the production mold continues to draw the moisture naturally from the clay. After some time, usually 45 to 60 minutes later, the clay body begins to pull away from the plaster wall of the mold. This releasing action occurs because the clay body now has less moisture in it than the wall of the plaster mold.
At this point, the new casting known as 'greenware' must be removed from the mold. Release the straps or remove the bands holding the mold pieces together. Gently pull on the wet clay body or tap using your open hand on the mold.
After the greenware has dried, the mold parting lines known as 'fettles' are removed using a knife or sandpaper. After the fettles have been removed the piece is further wet sponged to smooth the fettles and the surface.
The 'cleaned' greenware is now ready to be fired in a kiln to around 2000 F. degrees. The newly fired piece of clay is known as 'bisque'...