Porcelain - the white gold
A brief insight into the history of porcelain
Porcelain, like clay, is a non-biological material and is the result of the decomposition of rock. Entire mountain ranges are eroded by the weather over millions of years and some produce gravel, others clay and others feldspar. These minerals are found in different purity and composition all over the globe. In China, the secret of making porcelain was discovered about 2000 years ago. The Chinese porcelain came to Europe by ship or via the Silk Road and long transport routes cost money then as now. Because of this, alchemists, pharmacists and hobby-Venicians in Europe have been tinkering with the recipe for centuries. In search of the philosopher's stone and the recipe of gold, Mr. Böttger stumbled upon the recipe of white gold around 1700, more or less voluntarily. From this invention the Meissen Porcelain Manufactory emerged, which laid the foundation for porcelain outside of China.
The components of ceramic clay
Clay is a naturally occurring raw material. It consists on average of 50 % feldspar, 25 % quartz and 25 % kaolin. Feldspar is a silicate mineral, a diffuse group of elements such as calcium, sodium, potassium and silicon. These layered silicates are the shaping factor in clay - they give it plasticity and make the clay literally fat. Quartz is known to many, it is the basic component of window glass. Quartz is responsible for glazing in clay and is not plastic and difficult to process. It only melts at temperatures of over 1100 °C and thus separates stoneware from stoneware. The last component is kaolin. Kaolin is a snow-white raw material and again originates from the decay of feldspar. More precisely, it consists of aluminum oxide and silicate. Kaolin seams are rare and even rarer to find in good quality, that is, as free as possible from impurities such as iron or manganese. In the pharmacy it is called Bolbus alba or pipe clay.
The components of porcelain
In contrast to clay, porcelain is a man-made compound. It consists of the same resources as clay, but in an unnatural composition. Feldspar, which gives plasticity, is present in European porcelain in 25%, quartz in 25% and white kaolin in 50%. Kaolin is not plastically deformable and gives the porcelain its immense hardness. Due to the chemical octahedral and tetrahedral arrangement, the phyllosilicates are firmly bonded together. This layering allows an immense fire resistance and forms a particularly dense structure when fired.
Hard paste porcelain and soft paste porcelain?
These two designations do not stand for the consistency of the mass. Both types are available both as solid mass and as castable mass. The distinction refers to the firing temperature. In principle, the hotter the firing temperature, the more stable the porcelain is. However, there are countless types of porcelain and even low fired porcelain can be very hard. By the way, on the Mohs scale, hard paste porcelain ranks just behind diamond!
Due to its composition, European porcelain can also be called hard paste porcelain. The proportion of kaolin (the substance responsible for strength) is around 50%. It is fired at temperatures of up to 1450 °C. This extreme temperature also ensures that the porcelain shrinks by a good 20%. This contraction leads to a compression of the material. As a result, the shards become hard, impact-resistant and immune to acids and cutlery.
The composition of Chinese porcelain is slightly different. Approximately 25 % is feldspar, 45 % quartz and 30 % kaolin. Chinese porcelain consists largely of glass and is fired at only 1300 °C. This also explains the name "soft paste porcelain". The above mentioned "bone china" is the most expensive and noblest porcelain. Due to the high quartz content, it is very impact resistant and at the same time very translucent. Therefore it looks more like polished milk glass. Why is this so?
Bone China Porcelain
Bone China is also made of feldspar, quartz and kaolin. Bone ash is added to the raw mass. This is obtained by burning animal bones, which are then ground. The exact recipe of the raw mass is of course top secret and was the reason for wars 300 years ago. Bone ash is rich in calcium oxide and calcium phosphate and these compounds, together with the high quartz content in the mixture, give bone china its extremely translucent character. Opaque components such as feldspar are replaced by transparent ones such as calcium. Another special feature is that the biscuit firing (the first firing) of bone china takes place at 1280 °C and the later glaze firing at only 1080 °C. Despite the lower firing temperature, bone china porcelain is just as resistant as European hard paste porcelain. Even cutlery, acids from salads or colorants from coffee cannot harm this material. This is due to the high proportion of quartz, because quartz is also a very material.
Porcelain and the processing
Due to its composition, porcelain is a difficult material to process. The quartz it contains is not plastic, does not absorb water and cannot be compacted. If quartz meets feldspar and kaolin, a white, tough, sticky mass is formed. The consistency is reminiscent of a diagonal cross between chewing gum, toothpaste and a shot of bathroom silicone. Tough like old chewing gum, sticky like toothpaste on your finger and greasy like silicone without a rinse. It is largely poured or pressed/ squeezed during industrial processing. Plaster molds have been used for this purpose for a good 300 years. Some of them are still in use today and are treated accordingly like gold dust.
Porcelain and the pottery wheel
Due to its composition, porcelain is very difficult to process on the turntable. On the one hand, it is tough and can hardly be shaped with the hands. On the other hand it is difficult to move it in one direction. Because as soon as the mass is a 10 cm high cylinder, it sinks back together again. Furthermore, it likes to stick to the fingers. If you remove your fingers from the mass to moisten it, you want to take it to the water bowl. So you have to moisten your fingers before you notice that it gets dry. Once you have managed to bring this unpleasant mass into the desired shape, you are facing the next hurdle: The loosening from the turntable. As already mentioned, porcelain sticks, especially to the turntable! It is therefore advisable to turn on plasterboards or exchangeable plates. When turning, however, porcelain is similar to clay and you will quickly be captivated by this enchanting material. The mineral composition even makes it crunch a little when turning and it glitters like fresh snow!
Porcelain and plaster moulds
Gypsum is a problematic substance in ceramics, including porcelain, because gypsum transforms into another substance at temperatures above 130°C (calcium sulfate dihydrate becomes anhydrite). This transformation is space demanding! At the relatively low temperature, however, the clay/porcelain is still very sensitive. By this blasting, the gypsum accidentally contained in the soft mass damages the unshaped body. One calls a so developed dent also a "Kalkspatz". However, plaster also has impressive advantages. In contact with water, it is water-absorbent (hygroscopic), but does not attract/chew room humidity and does not swell on contact with water. It is therefore ideal for moulds or impressions - as long as they are not damaged. A distinction is made between different plaster moulds and different processing methods. The most common forms in professional production today are the explosive forms. These moulds do not have a classic seam and are quite sensitive. After the plaster has hardened, they are split into halves with wedges during production. The resulting broken edge fits into each other much better than that of smooth forms. This almost invisible seam requires very little rework. In squeeze molds, the tough porcelain is pressed into the mold and the halves are later joined by hand.
Porcelain is being garnished
After the parts are cast, they have to be connected somehow. In the technical language one speaks of garnishing. This is a critical moment, because porcelain is not plastic due to the missing feldspar. The individual parts are difficult to join and must be processed with great care. Because the white gold has a memory.
Porcelain and its memory
Even if it is an unnatural, non-biological material, a soul dwells in it. It is an angry mass with the proverbial elephant memory. It does not forgive if the wrong step was taken at the wrong time. Even slight bends, which quickly happen when removing from the plaster mould, are visible again after the glaze firing. It does not forget. Even garnish spots become visible again after the glost firing if the work has been done incorrectly. The commercially available plaster molds for the hobby sector should also be mentioned here. The seams are sometimes one millimeter thick! When processing porcelain, this is an order of magnitude that cannot be processed. The industrial plaster moulds have a seam of a tenth of a millimeter - that's all it should be when processing porcelain.
The three firings!
The only difference between bisque firing and porcelain firing is that the firing is at a slightly higher temperature. Instead of 800 to 850 °C, as with ceramics, porcelain is bisqued at 900 - 950 °C. Only the glost firing at approx. 1400 °C reveals the quality of the processing. If careless work was done during processing, you will see it again. Large seams or garnish spots appear magically again. This memory is also a point why good porcelain is still relatively expensive today. Now that the pieces have undergone two firings, the last firing follows! This takes place at approx. 800 °C and is used for baking colors. Gold, platinum and other colors are not as temperature-resistant as porcelain and must be fired at the end. The colors are applied to the glazed porcelain. At 800 °C the glaze melts slightly and the colors or metals are absorbed into it because they sink in.
In English, porcelain is also simply called "china" - a product that is named after an entire country? It illustrates the value that was and still is attributed to this white gold. Even though the value of most porcelain has changed, Chinese tableware made of "bone porcellain" still fetches top prices today. This is not surprising, because one must not forget the energy costs, all the work and necessary meticulousness. Likewise, a lot can happen at the three firings. As a rule of thumb, you can remember that in about 15% of the porcelain pieces survive all three firings without damage. About one third of them are grade I - the remaining 85% are waste!