A talented artist can create intricate and beautiful pieces with porcelain. However if the materials are not selected and mixed just right, and if the firing of the work is not exactly correct, then the artist's efforts will be wasted.
The craft of making porcelain originated in China where the early makers used a type of clay called kaolin, and a type of granite called pegmatite. Nowadays it is made all over the world, and the main components are clay, specifically kaolinite, as well as feldspar and silica because in all of these the particles are extremely small. By varying the proportions of these ingredients, the properties of the porcelain can be changed.
Having been carefully selected, the raw materials need to be thoroughly crushed to make sure that all of the particles are extremely fine. The crushing is brought about using three types of equipment. Firstly swinging metal jaws crush the material. Then hammer mills, which are rapidly moving hammers, grind it further. Finally a ball mill, consisting of large rotating cylinders filled with steel or ceramic balls, further reduces the size of the particles.
The ingredients are then passed through a series of screens to make sure all of the particles are exactly the required size. Water is then added, forming a slurry, which is filtered with a magnet to remove any iron that may be in the mix. Iron is commonly present, and is undesirable because if it oxidizes (rusts) it will turn the mix an unattractive reddish color.
Now the material is ready to be shaped into a beautiful porcelain body in one of the following four ways.
- The first method is called soft plastic forming, which is when the porcelain material is manually molded using one of three methods. Sometimes it is formed by wheel throwing, where the potter places a piece on a wheel and shapes it as the wheel turns. Alternatively, the clay can be shaped more mechanically by jiggering. This is when the clay is placed horizontally on a plaster mold of the desired shape, and then a heated die is brought down from above to shape the other side of the piece. The third possibility is called ram pressing, where the clay is placed between two plaster molds, which shape it while forcing the water out of it. Using a vacuum on the upper half and pressure on the lower half, the body is then removed from the mold.
- The second method, used to shape less plastic bodies, is known as stiff plastic forming. Here the material is forced through a steel die, which results in a long, even sausage shape, which can then be cut into the desired length, or used as a blank for other forming actions.
- Thirdly, dry bodies can be shaped simply by pressing into either a rigid die, or a flexible mold. Sometimes the pressure is from one direction only, and in other cases it is pressed equally from all sides.
- The fourth method involves pouring the wet slurry directly into a mold which is porous. The liquid drains out through the mold, leaving a rigid layer of porcelain which can be removed.
Once the porcelain has been shaped, it goes through its first firing, or bisque-firing. The temperature is relatively low, and it allows for contaminants to be vaporized, and minimizes later shrinkage.
The beauty of most clay products, including porcelain, lies not so much in its form as in the outer glaze. The glaze materials need to be ground and mixed with water, forming a slurry. The slurry also needs to be passed through a screen and a magnetic filter to remove contaminants. There are many ways to apply the glaze to the bisque-fired porcelain. It can be delicately painted on with a brush, or poured or sprayed over the piece, or the piece can be dipped into it. Varying the proportions of the individual ingredients in the glaze will produce different effects.
Probably the most well-known porcelain pieces are characteristically blue and white. To create this stunning style, firstly pieces are created as clean, shaped white clay, and bisque fired. The intricate blue designs are then painted on, usually with cobalt oxide. Then the piece is coated with a layer of transparent glaze before the final kiln firing.
Finally the porcelain ware must be fired in a kiln. Some kilns have a single sealed chamber and can fire only one batch of ware at a time. However the large commercial kilns are constructed inside a tunnel several hundred feet long. This way the ware can be moved from one zone to another, with each one continuously set to a particular temperature such as a preheating zone, a central firing zone, and a cooling zone.
The scientific basis for the firing process is complex, as firstly the carbon-based impurities are burnt out, particular chemicals decompose, and gases are produced which must be released from the ware. But eventually the feldspar and flint react with those decomposing minerals to form liquid glasses which shrink and bond the grains. Finally, as the ware cools, the liquid glasses solidify.
The porcelain product emerges from the kiln strong, white and translucent and ready to be used in so many ways. The artistic pieces emerge in all their delicacy and beauty, with the colors embedded in the strong, shiny glaze.