Vitrification
Vitrification means "to vitrify" or "to fuse into glass". It comes from the Latin term "Vitrum", which stands for glass. In general physics, vitrification describes the solidfication of a liquid by increasing its viscosity during the cooling process, whereby crystallization fails to occur and thus an amorphous material (glass) is formed.
Vitrification of ceramic materials
In ceramic knowledge vitrification refers to the ccoling process that occurs after the fusion process of various clays, kaolin and other earthen raw materials and renders the body sintered by firing impermeable to water. In porcelain production, vitrification is the last part of the "glass phase".
The influence of different glass phases plays an important role in the production of ceramic materials - as it does in porcelain - in many respects. This is not only the case with the eponymous transparent glaze. Above all, a melting phase is formed in the body during high-firing, which in many ceramic combinations solidifies to different degrees of vitreousness during cooling. For example, feldspar porcelain fired between 1,320 °C and 1,400 °C has a proportion of about 60% glass phase and 30% mullite (temperature dependence of the equilibrium phases of a porcelain mass consisting of 50% kaolin, 25% feldspar and 25% quartz).
Ultimately, all ceramic tableware consists more or less of silicate minerals sintered by fusion, but they differ in the degree of their vitrification. Thus, the term "vitrified" does not represent a particular unique manufacturing method or a unique quality characteristic. Much more decisive is the question: WHAT was vitrified at WHAT temperature?
Vitrification of porcelain (glass phase - cooling process)
Genuine feldspar porcelain (50% kaolin, 25% feldspar and 25% quartz) is fired between 1,300 °C and 1,400 °C. Due to the raw materials and the level of the firing temperature, the mixture of minerals produces the mullite.
After the highest firing point in the furnace, the cooling phase (glass phase) begins. The mass is slowly cooled down at high heat. The body and glaze combine to form a homogeneous combination of two layers with almost the same coefficient of expansion. The porosity of the body is 0.01 up to 0.03% at max and its density 2.1 to 2.6 g/m3.
In porcelain production, vitrification is a completely natural process.
Cooling phase of ceramic tableware
We explain in this section the behavior of ordinary utility ceramics, which is very different from technical and medical ceramics. In the tableware sector, such ceramics are known as stoneware and earthenware.
Due to the lower firing temperature - between 1,000 °C and 1,260 °C and the raw materials that are inferior to porcelain (clay instead of kaolin), no needle mullite formation takes place in ceramic tableware. In the cooling phase, glass phase formation is minimal compared to porcelain, reaching a porosity of 0.3 to 13% and a density of 0.19 to 2.0 gcm3 depending on material composition and firing temperature. The porcelain expert speaks of the absence of the glass phase in such ceramics.
Thus, these ceramic tableware are clearly inferior to porcelain in terms of their physical stability and surface resistance. They break more quickly. Even under low chemical or mechanical load, the glazes scratch and corrode. Due to the different firing temperatures of smooth firing and glaze firing, the body and glaze of ceramic tableware have different coefficients of expansion, which also makes the ceramics sensitive to thermal stresses. (see crackle cracks).
Vitrification of tableware (Vitreous China)
From the roots of the English Staffordshire Potteries (potteries from the town of Staffordshire / Central England) grew in post-war England manufacturers such as Churchill, Dudson, Steelite, etc. The older generations certainly still know the English service "hunting motif" from the 1970s in the colors blue and pink. These companies specialized in the production of earthenware, a low-fired ceramic that allowed for an underglaze process. They adapted the process invented in the 1940s in the USA for hardening sanitary ceramics (sinks and toilet bowls) called vitrification and implemented it for the production of ceramic dishes.
Vitrification, through the addition of a very finely ground quartz mixture, nepheline syenites and other additives, results in a flux similar to feldspar for the fusing process, which achieves a more homogeneous structure and reduced porosity in the hard firing. In short, the ceramic body is densified by artificial glass phase reinforcement. This achieves a water impermeability similar to that of porcelain, and the body is in many cases even superior to ordinary feldspar porcelain in terms of stability and mechanical resistance..
The low firing temperature makes it possible to display a more vibrant play of colors in the decoration on ceramic tableware such as Vitrified Tableware, but unfortunately this does not make it more durable.
Like its base material, Vitrified China is subject to different firing temperatures of glost firing and glaze firing in the lower temperature range (1,100 °C to 1,240 °C). Thus, the body and glaze of such vitrified ceramic ware also have different coefficients of expansion. In contrast to stoneware and earthenware, this results in the - mostly not to crackle cracks, but to a higher sensitivity to corrosion and glaze abrasion.
Vitrified China, portrayed in advertising as particularly strong, elevates this finishing technique above ordinary ceramics and earthenware, but on the other hand documents a mullite-replacing glass phase inferior to porcelain.
Glass phase of our High-Alumina-Porcelain
Since the glass phase in the production of genuine porcelain is a natural process, we avoid calling the heading of this section "vitrification of porcelain". If we were to do the same as the English companies - from an advertising point of view - we too would have to emphasize this advertising slogan "Vitrified" - which, by the way, is unknown to most consumers.
The High Alumina porcelain is fired between 1,350 °C and 1,400 °C, which is higher than ordinary feldspar porcelain. Due to the addition of enriched silicate mineral Alumina (Al2O3), an extremely dense and hard corundum, the mullite content in the glass phase increases considerably. We do not want to disclose details about this, please. In any case, "considerable" means superior breaking strength to ordinary feldspar porcelain up to 400% and more. Die porosity, density and stability of High-Alumina-Porcelain is also superior to Vitreous China.
However, the decisive factor for the quality and durability of High Alumina porcelain is the homogeneous bonding of body and glaze with almost the same coefficient of expansion due to the joint hard firing, in which body and glaze melt together simultaneously. This homogeneity and the high proportion of needle mullite make the High Alumina porcelain resistant to thermal shock (+-180 °C).
In all disciplines of use, High-Alumina-Porzellan is superior to Vitreous China and also to ordinary feldspathic porcelain - this is of high utility and sustainability for the consumer.