Science in everyday life: white gold from the alchemist's kitchen

White Gold from the Alchemist's Kitchen

Whether it's a coffee cup or a soup bowl - porcelain is in every household. Because it does not react chemically with other substances and therefore cannot take on any taste, it is particularly suitable for cooking and serving food.

Like all ceramics (from the Greek kéramos, "potter's clay"), porcelain consists of fired raw materials containing clay minerals, is heat-resistant, hard and waterproof. It also has an attractive appearance. Thin-walled parts even appear slightly transparent like glass - in fact, this material is a solidified mixture of a crystalline phase and a glassy phase.

The raw materials used are feldspar, quartz and china clay (kaolin) in a ratio of 1:1:2. The main component of kaolin is kaolinite (chemical formula Al₂(OH)₄[Si₂O 5_{]), added alumina (Al}2_O3_{), silica (SiO}2 _{) and water. The exact composition depends on the temperatures at which the body is later to be fired.}

These raw materials are prepared by the supplier and ground into powder, mixed by the porcelain manufacturer and then shaped by pressing, turning or casting.

After pre-drying, the first firing takes place at around 1000 degrees Celsius (this is called glow firing). thereby all the water escapes and a solid but still porous blank is formed. Porcelain is only made from it during the next firing, in which the glaze is also melted. The raw materials for this are essentially those already mentioned; immersed in such a dispersion (called slip), the pores of the blank absorb liquid by capillary action, the powder particles are drawn along and form a thin layer.

Now follows what is known as smooth firing, at 1300 to 1440 degrees, depending on the type of porcelain: So-called soft porcelain such as vitreous china and bone china are produced at a lower temperature, hard porcelain at a higher temperature. The first resists mostly creamy white to yellow and is used in the home, the pure white hard-paste porcelain is better able to withstand the stresses of gastronomy, hospitals and retirement homes. Feldspar melts first in glaze firing, both in the formed body and in the glaze. Most of the quartz then dissolves in its melt. The kaolinite ensures that the body remains dimensionally stable. Tiny crystals of alumina and silica called mullite also form. The result after cooling is a body of seventy percent glass in which tiny crystals of quartz (five percent) and mullite (25 percent) with diameters of 0.5 to ten micrometers are embedded are. The outer transparent glaze layer is about a quarter of a millimeter thick.

The body shrinks by up to 14 percent during glaze firing. This makes the construction of porcelain parts very difficult. Especially when two parts that have to fit into each other are manufactured separately - for example a jug with a lid - the designers have to take shrinkage into account. The color is rarely applied before the glaze is fired. Because for temperatures around 1400 degrees there is only black, Blue and green as colors. Today, the decoration is applied automatically or by hand as a decal from a carrier material and ceramic colors and then fired a third time. If it is heated up to 900 degrees Celsius, the colors melt on the glaze and stick there - this is how gold edges are applied, for example -, at 1280 degrees they penetrate and are thus protected against damage.

Did you know?

Stoneware is not porcelain, because it is heated to 1100 to 1300 degrees Celsius during the first firing, which means that the escaping water causes bubbles to form in the crystal structure. It therefore remains porous and the glaze, which is then melted at 900 to 1200 degrees, can chip off more easily.

• The resilience is indicated by the hardness of the glaze and the edge impact strength. Hardness is usually measured according to the scale of the German mineralogist Friedrich Mohs (1773-1839), who arranged ten minerals in such a way that those with higher values could scratch those with lower hardness. Talc has a hardness of 1, diamond has a hardness of 10. Hard-paste porcelain with a 6-7 outperforms steel (5-6), so knives and forks don't scratch the plate. The edge impact resistance is of particular importance for the heavily used commercial porcelain.
• Porcelain can basically be colored, at least if the temperatures of the glaze firing do not exceed 1300 degrees Celsius. Then the dyes decompose, oxidize or are reduced. And if it does succeed, the colors cannot be reproduced. A German manufacturer of hotel porcelain, BHS-tabletop, was able to produce cream-colored hard porcelain for the first time last year - making it similar to Chinese porcelain, which is perceived as noble. A special ceramic colorant and the control of the process parameters during glaze firing were decisive.
• Shards is the pottery term for any fired pottery. So the old saying "shards bring luck" means "full clay storage jars bring luck".

• The first porcelain was probably made in China almost 1500 years ago. In the 13th century, Chinese porcelain found its way to European royal courts and was weighed there with gold. Between 1600 and 1730, the Middle Kingdom developed a lucrative export branch from this, but in 1709 the alchemist Johann Friedrich Böttger (1682-1719) invented porcelain for the second time (as a random product when trying to make gold). His client, King August the Strong, then founded Europe's first porcelain manufactory in Meissen in 1710.

"Science in Everyday Life" is a regular rubric in Spectrum of Science. A collection of particularly beautiful articles in this category has just been published as a dossier. © Spectrum of Science

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