HOW BONE CHINA IS MADE
In the past of the work involved in making bone china was done by hand. Today, the potter is helped by modern machinary. Clay can be shaped automatically and modern kilns fire the china in minutes rather than hours or days. Potters used to work by trial and error but now scientists and techniciansmake sure that each piece is as perfect as possible.
Today, the designer, the scientist, the figure painter and the computer operator are just a few people needed to make fine bone china.
Bone China Mix: The Ingredients
In many ways, making bone china is like baking a cake. The ingredients are mixed together, a shape is formed, it is baked in an oven and, finally, decorated. As with a cake, a mistake at any stage will spoil the finished piece and every process involved in making bone china calls for experience and skill.
1. All the ingredients are individually mixed and then blended together with a precise amount of water in a tank called a blunger until a creamy liquid clay is produced. This is called ‘slip’.
2. The slip is sieved and then pumped over powerful electromagnes to remove particles of iron which would cause dark spots to appear on the finished china.
3. It is pumped into nylon filter bags which are hung in a long row on a machine called a filter press. This squeezes out the excess water to form slabs of clay which feel rather like plasticine and now contain about 23% water.
4. Finally, the clay goes through a pug mill which is rather like a giant mincing machine. It thoroughly kneads the clay and a vacuum pump removes air bubbles that would cause the clay to crack in the kiln. The clay leaves the mill as long sausages which are now ready for the potter to shape.
Before anything can be made someone has to decide what shape the pieces are to be. This is the job of the designer who must make sure that they are practical to use and, of course, beautiful to look at.
Using the designer’s drawings, a plaster or clay model is madefor each piece. From these, identical palster-of-paris moulds are taken which are used to make the china. These moulds are always larger than the finished piece will be because the clay will shrink when being fired in the kiln.
SHAPING THE CLAY
The potter calls the things he makes ware. Plates and saucers he calls flatware and cups, teapots and jugs holloware.
1. Flatware is made by forming a pancake of clay, a ‘bat’, on a revolving disc which is thrown onto a plaster mould. The mould shapes the front of the plate and a metal profile-tool is lowered to form the back. This process is known as jiggering and can be done by hand or by automatic machines.
2. Holloware is made by throwing a ball of clay into a revovling mould and lowering a profile-tool to form the inside shape of the cup or dish. As the clay is pressed against the mould the outside shape is made. This is called jolleying. Handles are stuck on later.
Many things like teapots, jugs, bowls and sometimes, cups are too complicated to be made by the jolleying machine. These are made by casting in plaster-of-paris moulds. Specially prepared slip is poured into the mould and the porous plaster absorbs water from it to form a layer of set clay which gets thicker and longer it is left. When it is thick enough the slip that is left is poured away leaving a clay shape on th einside of the mould. Often things are cast in a number of pieces and later stuck together with slip.
When the ware is dry enough it is carefully trimmed to remove any excess clay and the surfaceis made perfectly smooth with a damp sponge. This is called fettling and sponging. The ware is now ready to be fired for the first time.
Bone china is fired at a temperature of around 1,200 degreed C. which is hot enough to bond all the ingredients together and make the clay hard, strong and translucent. During firing, the high temperatures make the clay soft and great care must be taken to make sure that the pieces do not lose their shape. The ware is stacked onto truncks in a special way to avoid this.
All the water in the clay is driven out during firing and so the pieces shrink by about 13%. They leave the kiln feeling dry and biscuity-hence the name of the process. After being brushed and inspected for any faults they are ready to be glazed.
The glaze on a peice of china is really a very thin coating of glass used to make the pieces perfectly smooth and attractive. Glaze can be colourless or coloured, opaque or transparent and each is made to a very precise formula. It is applied by either dipping by hand into a tub of glaze or by automatic spraying. After drying the ware must be fired again.
Making sure that the peices do not touch each other, the ware is again loaded onto trucks for a glost firing at a temperature of around 1080 degrees C. This turns the glaze into a brilliant glass coating and fixes it firmly to the piece. After another inspection the china goes to be decorated.
There are many ways to decorate bone china but the most popular are with enamel colors and precious metals. These and other kinds of decoration are described in the separate section.
After being decorated by hand, with lithographs or by printing, the colours need to be fixed permanently into place by being fired. This is the enamel firing and, at a temperature of between 800 and 860 degrees C., the colors sink into the glaze which protects them from wear. Some kinds of decoration need two or more enamel firings because different colors are obtained at different temperatures.