The glaze is used for surface treatment of ceramic products such as earthenware, stoneware and porcelain and is in contrast to the crystalline porous base material has a glass-like, closed surface layer.
It was used since the beginnings of pottery is mainly to provide the porous body with a waterproof coating. So could be produced even at a low firing temperature of articles which are suitable for storing foods and beverages.
At a firing temperature of 1250 ° C stone tools are seared by the shards. Porcelain is fired at temperatures close to 1400 ° C. This results in intergranular glassy phase which provide closed porosity, and optionally a self glaze. However, the surface is often rough and has the color of the corresponding basic substance. The glaze is made by additional substances with which a hard enclosed surface coating and different colors can be produced. The constituents of the glaze form together and with the base material, a glass layer composed of a mixture of various oxides.
Glazes are applied to improve the aesthetic effect ( color and effect glazes ) or used to improve mechanical and electrical properties.
At harnesses the glaze reduces the surface roughness, so they are easier to clean, and the hardness is increased, so that the performance characteristics are improved, as it is less scratched.
High voltage electrical porcelain insulators are glazed to increase by an intrinsic compressive stress, the strength of the insulator. During this process, a suitable chemical composition of the surface is reached, the ( no water uptake ) is reduced by reducing the conductivity of the leakage current. The reduced roughness prevents the same time before a faster contamination.
Chemically, glazes ( like other glasses as well) from a mixture of mineral powders.
The minerals are on the one hand the network formers, such as silica (as quartz flour), fluxes or melting point, such as alkali and alkaline earth oxides, usually sodium and calcium, which are often added in the form of felspar or boron and lead compounds, which are often be used as a frit, and aluminum oxide as consistency and viscosity enhancers.
Lead glazes are particularly resistant to corrosion, while the low-melting components of sodium and potassium are easily removed.
In the salt-glazed rock salt (sodium chloride) is added to the fire whose smoke gases flow around the kiln. The released at high temperature sodium oxide combines with the shards and lowers the melting temperature of the surface layer, so as to form a glass layer.
The higher the firing temperature and the achievable resistance, the more limited the color palette. While the color white by dispersion occurs (addition of tin oxide or zirconium oxide), other colors can only be achieved by adding a coloring metal oxides. It is known that cobalt blue glaze. Green is produced by chromium oxide, brown tones by manganese or iron often already present. Under a reducing combustion atmosphere leads to a proportion of iron gray blue shades.
Low Burnt colorful ceramic glazes often contain soluble components that give off during use so much substance that they are still toxic. This often applies to ornaments with applied engobe, which are not entirely " glazed " and are compared with glazes rather crystalline and in the surface less closed.
Porcelain objects that are smooth fired at 1450 ° C, on the other hand are considered safe - even if they contain in themselves toxic coloring substances. Here, the heavy metals are permanently glazed in the silicates and tied it.
The painting of porcelain and pottery can be done as underglaze painting with high-fired colors at high temperature, or temperature-sensitive on-glaze colors, reduce heat to the glazed goods.
The green ( unfired ) ceramic is first subjected to a biscuit firing, inter alia, in the preparation of porcelain. The firing temperature is lower, not as high as the final firing after applying the glaze ingredients. After Schrühen the ceramic suspensions with the glaze ingredients is poured, dipped or brushed in water ( frits, dissolved in water powder). This bearing surfaces remain free to prevent fusion with the furnace internals.
When closing fire melts the glaze and their constituents unite with each other and with the shards. They form glassy mixed oxides.
Is the expansion coefficient of the glaze layer is greater than that of the base material, cracks may form. These cracks are sometimes recognized as a design element and used ( crackle ). In the opposite case, that the tension of the glaze layer is higher, so the glaze layer is under constant compressive stress, the strength is increased, which can also be intentional, depending on application.