Wikijunior:How Things Are Made/Ceramic/Ceramic Wares
Ceramic wares are typically eating utensils that are made from ceramics. Do note that ceramics are highly resistant to heat; meaning the ware won't get too hot to touch although we put hot food inside the ware. Also the ceramics are also scratch resistant and waterproof making it suitable for making it a coic of eating utensils. In addition, due to environmental concern, nowadays ceramic ware are preferred choices for eating utensils.
Kaolinite clay (40% aluminum oxide, 46% silicon oxide, and 14% water) is the main ingredient in making ceramic ware.
Glazes are made up of materials that fuse during the firing process making the ware vitreous or impervious to liquids. Glazes must have three elements: Silica, the vitrifying element (Provide coating of glasslike form);flux, which fuses the glaze to the clay; and refractory material, which hardens and stabilizes the glaze. Colours are optional and usually are metallic oxide such as antimony (yellows), copper (green,turquoise,or red), cobalt (black), chrome (greens), iron, nickel, vanadium, etc.
The glazes are weighed and put into a ball mill with water. The glaze is mixed within the ball mill and grinds the glaze to reduce the size of the natural particles within the glaze.
Step 1 - Clay arrives in powder form. The powder is moistened with water and mixed in a huge underground tank with a paddle called blunger. Multiple paddles mixes the clay in order to distribute the water evenly. A typical batch mixed in a large production potter weighs 50,000kgs. At this point, the slurry is about 30% water.
Step 2 - The slurry is filter pressed. A device presses the slurry between bags or filters to force out excess water. The resulting clay is thick and rather dry and is called cake (~20% water).
Step 3 - The cake is then put into a pug mill in which the clay is chopped into fine pieces. This chopping deairs/vacuum the clay as it sucks 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. Depending on type of the wares, the process will differs in type of forming process used.
Bowl / Plate
Step 4a - The fastest way to produce a regular, holloware is by using a jiggering machine. Thus, holloware such as vases is largely made on jiggering machines. The clay cylinders made in the pugmill are sent to the jiggering machine. In order to make a vase, a wet clay cylinder is dropped on to 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.
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 bisque firing the pieces and reduce the water content to under 1% moisture before glazing and firing.
Vases / Jug
Step 4b - Ceramic ware with delicate or intricate silhoutte 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.
Step 5 - After the pieces have been bisque firing, 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 maybe 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 (1.5-2mm). 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 linesor concentric rings applied by machines, and still others maybe painted by hand.
Step 6 - Kilns may be heated by gas, coal, or electricity. Newer type of furnaces run at higher temperatures and require a shorter firing time-running at about (1,260°C). The wares remain in the kilns about 9 hours, thus, allowing the factories to move pieces more quickly through production. Before entering the kiln, the patterns cannot be seen due to the glaze powder coating the designs. The kiln firing changes the glaze into a glass-like coating which produces glossy surfaces and make the ware virtually impervious to liquid.
Step 7 - The pin mark due to firing pins is polished on a ginetting (ginnetting) machine with a grinding stone to remove the stilt marks. The stilt marks are left over of the pin stuck to glaze during firing and may cause injury if not removed.