Mixing the clay
- 1 Clay arrives by truck or rail in powder form. The powder is moistened with water and mixed in a huge tank with a paddle called a blunger. Multiple spindles mix and re-mix the clay, in order to evenly distribute water. A typical batch mixed at a large production potter is 100,000 lb (45,400 kg) and they often mix up two batches in a single day. At this point, the slurry is about 30% water.
- 2 Next, the slurry is filter pressed. A device presses the slurry between bags or filters (like a cider press) to force out excess water. The resulting clay is thick and rather dry and is called cake now and is about 20% water.
- 3 The cake is then put into a plug mill in which the clay is chopped into fine pieces. This chopping de-airs the clay as pumps suck out air pockets that are exposed by this process. The cake is then formed into cylinders that are now ready to be molded or formed.
Jiggering
- 4 The fastest way to produce a regular, hollow pot is by using a jiggering machine. Thus, hollowware such as vases is largely made on jiggering machines. The clay cylinders made in the plug mill are sent to the jiggering machine. In order to make a vase, a wet clay cylinder is dropped onto the jiggering machine by a suction arm which positions the clay inside a plaster mold. A metal arm then comes down into the wet clay cylinder forcing it against the interior wall of the plaster mold thus forming the new vessel. The plaster mold, with wet clay .
- inside, is then lifted off the machine and set in dryer. As the clay heats up and dries slightly the new, wet clay pulls away from the plaster mold and can thus be easily removed. Thus, the factory must have thousands of plaster molds in order to make these vases or other hollowware as a plaster mold is used to make each new vessel. The factory may be able to make as many as 9 pieces of pottery in a single minute.
- 5 A machine takes the rough edges off the molded piece. The cleaned pieces are placed on a continuously-moving belt which leads to tunnel dryers, which heat the pieces and reduce the water content to under 1% moisture before glazing and firing.
Slip casting
- 6 Pottery with delicate or intricate silhouette is often formed by slip casting. A pourable slip or slurry is poured into a two-part plaster mold, the excess is poured out, and the slip is permitted to stiffen and dry. The plaster mold sucks up some of the excess water and helps hasten the drying process. The plaster mold is opened when the greenware (undecorated clay piece still a bit wet) is stiff enough, the piece is cleaned of rough edges and seams from the mold, and the slip-cast greenware is ready for drying in the heated dryers.
Glazing
- 7 After the pieces have been dried, they are ready for glazing. The pieces may be entirely covered in one color of glaze by being run under a waterfall of glaze that completely coats each piece, or the pieces may be sprayed with glaze. Deep hollowware such as vases have to be flushed with glaze by hand to ensure that they are completely coated on the inside. Glazes are generally applied to a thickness of 0.006-0.007 in (0.015-0.017 cm). Other pieces may be more decoratively glazed. Some pieces are printed with screen-printing, others have a decorative decal applied by hand, others may have lines or concentric rings applied by machines, and still others may be painted by hand.
Firing
- 8 Kilns may be heated by gas, coal, or electricity. One large production potter uses tunnel kilns fired with natural gas. Large cars or wagons (about 5 ft or 1.5 m square and nearly 5 ft or 1.5 m tall) are loaded with unfired pottery and sent to the kilns, firing approximately 20,000 dozen pieces of pottery in a single week. Newer furnaces run at higher temperatures than older kilns and require a shorter firing time, running at about 2,300° F (1,260° C) the pots remain in the kilns about 5 hours, thus allowing the factories to move pieces more quickly through production.
- The kiln changes the glaze into a glass-like coating, which helps make the pot virtually impervious to liquid. Single-color production pottery requires only one firing with the new kilns and glazes. (Many glazes require that the greenware be fired once and made into a bisque or dull white, hard body, then glazed and fired again; however, this is not necessary with some new production glazes.)
- 9 The unglazed foot (or bottom) of the pottery is polished on a machine with a cleaning pad. The piece is then placed in a bin and is sent to packaging, ready to be shipped out for sale
8 Kilns may be heated by gas, coal, or electricity. One large production potter uses tunnel kilns fired with natural gas. Large cars or wagons (about 5 ft or 1.5 m square and nearly 5 ft or 1.5 m tall) are loaded with unfired pottery and sent to the kilns, firing approximately 20,000 dozen pieces of pottery in a single week. Newer furnaces run at higher temperatures than older kilns and require a shorter firing time, running at about 2,300° F (1,260° C) the pots remain in the kilns about 5 hours, thus allowing the factories to move pieces more quickly through production.
The kiln changes the glaze into a glass-like coating, which helps make the pot virtually impervious to liquid. Single-color production pottery requires only one firing with the new kilns and glazes. (Many glazes require that the greenware be fired once and made into a bisque or dull white, hard body, then glazed and fired again; however, this is not necessary with some new production glazes.)
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