HARD-PASTE PORCELAIN

Definition and origin

The term porcelain refers to ceramics made front similar materials and baked at high temperatures which are light, durable, and vitreous.* Porcelain combines the positive qualities of glass and clay - glass is smooth and translucent while clay retains its shape when moulded. However, due to the addition of a few more minerals, porcelain is stronger than either glass or clay. It is also extremely beautiful and valuable: Chinese Ming Dynasty (1368-1644 AD) bowls can fetch a million dollars on the international art market.

For around fifteen hundred years, porcelain has been employed as tableware and decoration, but its more recent applications include: dental crowns and electrical insulators.

Porcelain was lir5t made in China. During the Tang Dynasty (618-907 AD), small amounts were used by the court and the very rich. High-quality porcelain, like that manufactured today, was not widely available until the Yuan Dynasty (1279-1368 AD).

Chinese porcelain was traded with kingdoms in Central, Southeast Asia, and the Middle East from the seventh century. By the Middle Ages, it had reached Europe.

European obsession

Porcelain was consumed in enormous quantities by European royal families, nobles, and the church, all of whom tried desperately to discover its chemical composition. The English word, porcelain‘, derives from the Portuguese name for a sea creature, the nautilus, which has a spiral orange vitreous shell from which it was believed at one time that porcelain was made. Other more astute Europeans contended the ceramic contained crushed glass or bone.

Early experiments in the production of porcelain included adding ground glass to clay. The result is called 'soft-paste’ as it is weaker than true porcelain.

So great was the frenzy for possessing Chinese porcelain, or attempting to recreate their own hard-paste, that a number of European principalities endangered themselves financially, spending as much of their budgets on pursuing porcelain as on their armies. Frederick II of Prussia (now in Germany) was one such fanatic. Fortunately, for Prussia, two scientists – Johann Bottger and Ehrenfried von Tschimhaus - in the monarch's service, solved the porcelain puzzle. Their discovery, made in 1707, combined clay with ground feldspar - a mineral containing aluminium silicate.

Meanwhile, in England, the recipe was a little different: ash, from cattle bones, was mixed with clay, feldspar, and quartz. This became known as ‘bone china‘, and is still manufactured. Although not true porcelain, it remains popular in the US and the UK because it is harder than porcelain.

Constituents

The raw materials from which porcelain is made are abundant. They are: white clay (china clay or ball clay), feldspar, or perhaps flint, and silica - all of which are noted for their small particles. Feldspar and flint are used as fluxes, which reduce the temperature needed for firing, and bind the glass, silica, and clay granules. Porcelain may also contain other ingredients like alumina or steatite.

Manufacture

To produce porcelain, the raw materials are selected and weighed. Then, they are crushed in a two-stage process. Jaw Crushers work first; mullers or hammer mills subsequently reduce particles to 0.25cms (0.1 inch) or less in diameter. A third crushing, using ball mills, takes place for the finest porcelain. During purification, which follows, granules that are not of uniform size are screened out. Magnetic filtration then removes iron, commonly found in clay, because this prevents porcelain from forming correctly. The fifth Stage, preparatory to firing, is formation. There are several types of formation by hand or machine. After formation, the ware undergoes its initial firing in a kiln - a special oven.

A glaze is a glassy liquid similar in composition to porcelain. If a porcelain object is painted, a glaze covers the paint, or its decoration may just be the glaze. Glaze is applied by painting or dipping and takes place after the first firing. Not only are porcelain wares gorgeous, but their decoration and glazing are also of great interest.

In making porcelain, the temperature in the kiln is critical - high enough to reconstitute the elements, yet low enough to vaporise contaminants and minimise shrinkage. A typical temperature is 1454° Celsius (2650° Fahrenheit).

During the firing process, a number of chemical reactions occur. Carbon-based impurities burn out at 100-200°C (215-395°F). As the kiln is heated, carbonates and sulfates decompose. When heated to 700-1100°C (1295-2015°F), the fluxes react with the decomposing minerals to form liquid glass. After a certain density is reached, at around 1200°C (2195°F), the ware is cooled, causing the liquid glass to solidify.

Pause for thought

So, next time you dine from fine porcelain, take a moment to reflect on the complicated history and sophisticated manufacture of this exquisite product.