Processes
There are various manufacturing processes used to obtain a white and translucent ceramic, the majority of which produce soft-paste porcelain or hard-paste porcelain.
Hard-paste porcelain is composed of the natural mineral materials kaolin, quartz and feldspar; it is translucent and vitrified throughout its mass. Kaolin is a white, very refractory clay mineral with a melting point of 1,800° C; by introducing a flux (calcium, feldspar or phosphate of lime), this melting point is lowered, enabling the porcelain to be vitrified between 1,300 and 1,450° C.
When it is coated for decorative purposes, the chemical nature of the coating is similar to that of the paste; it simply has more flux to give it a perfect affinity.
Soft-paste porcelain is made without kaolin from a very complex mixture composed of white marl with limestone, sand and flux to which a "frit" is added, made of silica and flux. This is all fired at around 1,200° C and the paste produced is white and translucent. It is described as "soft" because it may be scratched easily with a steel instrument.
These three ingredients - kaolin, quartz and feldspar - are mixed with water to obtain a paste in a more or less liquid state adapted to the forming procedure by casting in a porous mould (gravity or pressure casting) or by sizing if the plastic paste is pressed against the sides using a tool.
The parts obtained are then dried before being fired below 1,000 °C leading to fragile and porous porcelain. Glazing or varnishing involves covering, by soaking or gun spraying, the bisque with a coating (glaze or varnish) made up of a watery metallic pigment dispersion. A second firing at temperatures between 1,260 °C and 1,400 °C causes vitrification in the bisque and transforms the glaze into a vitrified film, giving the porcelain its translucency. Porcelain that has been fired at high temperature (from 1,200 to 1,400 °C) without a glaze is called biscuit porcelain.
The decorative elements can be applied by hand, or in the form of stickers generally produced by screen printing. After application, a final firing fixes this decoration on the part.
Properties
Porcelain is characterised by the whiteness of paste and is often used as a transparent cover. Porcelain is recognised by its translucency.
Applications
It is mainly used for tableware and, more broadly, bottling (perfume, spirits) and hardware (door handles, backplates). More recently, it has started to be used in lights and urban furniture. Its electrical insulation property has led to its use in the manufacture of switches, sockets and isolators. Its hardness is harnessed for thread guides in the textile industry.