Quartz

Quartz is a mineral with the chemical composition SiO2 and trigonal symmetry.

It is the most stable form (modification) of silica on the earth's surface and the second most abundant mineral in the earth's crust after feldspars. At a temperature of more than 573 °C (under a pressure of 1 bar) deep quartz changes to high quartz by changing its crystal structure.

With a Mohs hardness of 7, quartz belongs to the hard minerals and serves as a reference value on the scale up to 10 (diamond) according to Friedrich Mohs. Quartz gives real hard porcelain its hardness! It often forms well-developed crystals of great variety of forms and species (see Modifications and Varieties), whose crystal surfaces are glassy. Quartz has no cleavability, breaks like glass and shows a greasy shine on the fracture surfaces.

Quartz in porcelain production

For porcelain production, quartz is the third, essential stock raw material. It is added to the porcelain mass as finely ground piece quartz in sand-like form.

In the hard firing of porcelain, the feldspar first melts into a glass-like mass, the feldspar glass. During this sintering process, the feldspar releases further microscopically small amounts of quartz. Since the melting point of quartz is much higher (high quartz 1,470 °C) than the firing temperature of porcelain (approx. 1,350 °C), minimal undissolved quartz crystals (mullite) remain in the feldspar glass mass.

These quartz crystals ultimately give the hard-fired body its hardness and resistance to heat, cold and chemical substances.

The raw material "quartz" has a high influence on mullite formation. A common porcelain mass consists of kaolin 50%, feldspar 25% and quartz 25%. The quartz determines the reaction behavior, the transparency and the relative rate constants of the quartz dissolution (grain size of the quartz between 60-88 micrometers). The reaction behavior of porcelain masses at higher temperatures is thus essentially determined by the dissolution of the quartz. This also explains the great influence of different types of quartz on the porcelain firing. The more disturbed the structure of the quartz grains is, the easier they react and the easier they dissolve.

One can certainly define that "quartz" gives the porcelain its hardness.