The raw materials of a glaze fall into three main categories according to their chemical properties:
– Former
Silica is the glaze former. It creates a network and melts at 1707 °C. Silica, or SiO?, has the form RO?, meaning one radical and two oxygen atoms. Any molecule with this structure belongs to this category.
– Neutral, stabilizing, structuring
The other essential element of a glaze is alumina, which is provided by kaolin, a mineral that naturally occurs in the soil. It settles into the silica network and strengthens its structure.
Alumina, or Al?O?, has the form R?O?, meaning two radicals and three oxygen atoms. Any molecule with this structure belongs to this category. It melts at 2020 °C.
The silica–alumina mixture melts at 1590 °C.
– Flux
The silica–alumina mixture still melts at too high a temperature. A flux must therefore be added to lower the melting point. It also integrates into the silica structure.
Fluxes have the form RO or R?O, meaning one or two radicals and one oxygen atom. Any molecule with this structure falls into this category. Examples include chalk (CaO), soda (Na?O), potash (K?O) and lead (PbO).
Metal oxides (iron, copper, cobalt, etc.), which provide color, are used in small quantities and do not belong to these categories. Iron is the exception, as it can be introduced in larger amounts. Since it is Fe?O? (R?O?), it belongs to the structuring category. Zinc oxide ZnO (RO) can, on the other hand, belong to the flux category.
Natural raw materials
Each element is supplied by natural raw materials.
- Fluxes such as soda (Na?O) or potash (K?O), often called KNaO by ceramists, are provided by sodium or potassium feldspars.
- Alumina is supplied by kaolin (Al?O? · 2SiO?).
- Soda is provided by sodium feldspars.
- Calcium carbonate is supplied by calcium feldspars or chalk.
- Magnesium oxide is provided by talc.
- Phosphorus comes from bone ash.
- Lithium comes from lithium feldspar or lithium carbonate.
- K?O and Na?O is provided by nepheline syenite.
Frits
For earthenware glazes, frits are used. A frit is essentially an artificially created feldspar. These frits are produced through an initial firing, then ground and reintroduced into a glaze. The fluxes are trapped within a network. This technique makes it easier to use toxic elements such as lead (the most powerful flux) and also lowers the melting temperature of other fluxes such as CaO and KNaO, making them usable in earthenware firing.
Color
Metal oxides provide the colour. Some are extremely powerful, such as cobalt, where sometimes only 0.5 % is enough. Iron, on the other hand, can reach high percentages—up to 20 %—when deep browns are desired.
Iron: red to brown in oxidation, blue in reduction
Copper: blue-green in oxidation, red in reduction
Cobalt: blue
Chromium: green or red
