Porcelain & ceramics

1. Definition

Ceramics comes from the word "keramos" which means clay. The generic term of ceramics includes current products such as baked clay or terracotta, earthenware, sandstone, porcelain, but also the ceramic products of high technology (carbon, alumina, silicon...) for high tech industries.

Ceramics owe their distinctive qualities as much to the composition of the body (clay mix) as to the process of firing. The nuances in colours, the varieties of appearance depend on this combination.

We can distinguish two main categories of ceramics: porous (earthenware, terracotta and some sandstones) and non porous (porcelains and sandstone).

2. Properties of ceramics

The objects made of ceramics are usually hard and resistant. Ceramics can keep their solidity even at very high temperatures. They generally have a low thermal conductivity and are used as thermal insulators or refractory materials.

Dense and hard, porcelain is nonporous and white. Some sandstones have similar characteristics but earthenware and terra cotta are very porous and not very resistant to chipping and shock.

3. Comparison

Baked clay or terracotta
Glazed terracotta is a mixture of calcareous clay and clay fired at 900°C -1000°C (1650°F – 1830°F) which create in fact a porous material. It is mainly intended for the production of horticultural, culinary or decorative potteries.

Earthenware / Faïence
Earthenware refers to pottery, which, after firing, is still porous. Earthenware is a ceramic made from potash, sand, feldspar and clay. It will normally be porous (10-15%) unless the surface is sealed with a glaze. Firing of earthenware is normally done at low temperatures 980 to 1.080°C (1.795-1.980 °F).

There are several types of earthenware :
- the calcareous earthenware, used for the manufacture of decoration pieces;
- the feldspathic earthenware, used in the manufacture of tableware;
- the "fine earthenware", made of a white and fine clay. It is covered with a transparent glaze;
- the earthenware fired at "high temperatures" : decoration and glaze are cooked at the same time. The manufacture is then delicate.
- the earthenware fired at "low temperatures" : the decoration is painted on the glaze cooked.

Sandstone
The sandstone consists of a clay-like base ground, which contents silica. Cooked at 1.250-1.310 C° (2.280°F-2.390°F) the clay is vitrified and various nuances are obtained (grey, ivories, brown, beige). The sandstone thus remains opaque but intense heat gives it a very tight texture, which makes it impermeable. It is used mostly for the decorative potteries.

Porcelain
The porcelain is a hard ceramic. It is obtained from a mixture of kaolin (50 %), clay, feldspar (25 %) and quartz (25 %), first fired at about 950°C (1 240°F) and then between 1 300°C and 1 400°C (2 370°F to 2 550°F).

Thanks to the kaolin the porcelain remains white after cooking (fire in reduced atmosphere). At 1 400°C (2 550°F), the porcelain is vitrified.

4. Technical characteristics of the porcelain

The porcelain is nonporous (-0% of porosity), translucent under low thickness, resistant to thermal and mechanical shocks.

Porcelain offers great mechanical and thermal resistance due to a firing at high temperature 1400°C (2550°F). This brings a higher food safety because of the scratching resistance.

Porcelain is easy to clean and oven and freezer proof (-25°C to 300°C / -10°F to 500°F).