vitrification (Gas-phase)
The English term vitrification (vitrification) means in the German translation “vitrify” or “melt into glass”. It comes from the Latin term “Vitrum”, which stands for glass. In general physics, vitrification describes the solidification of a liquid by increasing its viscosity during the cooling process, with no crystallization and thus an amorphous material (glass) is formed.
Vitrification of ceramic materials
In ceramic theory, vitrification refers to the cooling process that occurs after the fusion process of various clays (clay), kaolin and other earthen raw materials and makes the sintered body impermeable to water. In porcelain manufacture, 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 with porcelain – in many respects. Not just with the transparent glaze that gives it its name. Especially in the shard, a melting phase occurs during high firing, which in many ceramic combinations solidifies in different glassy forms during cooling. The feldspar porcelain fired between 1,320 °C and 1,400 °C has a proportion of around 60% glass phase and 30% mullite (temperature dependence of the equilibrium phases of a porcelain mass made of 50% kaolin, 25% feldspar and 25% quartz).
Ultimately, all ceramic tableware consists more or less of fused silicate minerals, but differs in the degree of their vitrification. Thus, the term “Vitrified” does not represent a particularly unique type of production or a unique quality feature. Much more important is the question: WHAT was vitrified at WHICH temperature ?
Vitrification of porcelain (glass phase – cooling)
Real 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 firing temperature, the needle mullite, which is decisive for porcelain, is formed from the mixture of minerals.
After the highest firing point in the furnace, the cooling phase (glass phase) begins. The mass is slowly cooled down over high heat. Shards 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 to max. 0.03% and its density is 2.1 to 2.6 g/cm 3 .
Vitrification is a completely natural process in porcelain production.
Cooling phase of ceramic tableware
In this section, we explain the behavior of ordinary everyday ceramics, which is very different from technical and medical ceramics. In the tableware sector, such ceramics are known as pottery, earthenware and stoneware.
Due to the lower firing temperature – between 1,000 °C and 1,260 °C and the raw materials that are subordinate to porcelain (clay instead of kaolin), there is no needle mullite formation in ceramic tableware. In the cooling phase, the formation of glass phases is minimal compared to porcelain and the body reaches a porosity of 0.5 to 13% and a density of 0.19 to 2.0 g/cm 3 depending on the composition of the material and the firing temperature. In the case of such ceramics, the porcelain liner speaks of the absence of the glass phase.
This ceramic tableware is clearly inferior to porcelain in terms of physical stability and surface resistance. They break faster. The glazes scratch and corrode even under low chemical or mechanical stress . Due to the different firing temperatures of smooth firing and glaze firing, the shards and glaze of ceramic tableware have different coefficients of expansion, which also makes the ceramic sensitive to thermal stress ( see craquelure cracks ).
Vitrification of Tableware (Vitreous China)
From the roots of the English Staffordshire Potteries (pottery from the city of Staffordshire/Middle England) manufacturers such as Churchill, Dudson, Steelite etc. grew up in post-war England. The older generations probably still know the English “hunting motif” service from the 1970s in the colors blue and pink. These factories specialized in the production of earthenware, a low-fired pottery that allowed an underglaze process. They adapted the method invented in the USA in the 1940s for hardening sanitary ceramics (sinks and toilet bowls) called vitrification and implemented it for the production of ceramic tableware.
By adding a very finely ground quartz mixture, nepheline syenite and other additives, the vitrification leads to a feldspar-like flux for the fusion process, which achieves a more homogeneous structure and reduced porosity in hard firing. In short: The ceramic shard is compressed by an artificial glass phase reinforcement. This achieves a level of impermeability to water that is similar to that of porcelain, and in terms of stability and mechanical resistance, the shard is often even superior to ordinary feldspar porcelain.
Due to the low firing temperature, it is possible to display a stronger play of colors in the decoration on ceramic dishes such as the Vitrified Tableware, but unfortunately this does not make them more durable.
Like its basic material, vitrified china is also fired at different firing temperatures for smooth firing and glaze firing in the lower temperature range (1,100 to 1,240 °C). This means that the shards and glaze of such vitrified ceramic tableware also have different coefficients of expansion. As a result – in contrast to stoneware and earthenware – there are usually no craquelure cracks , but a higher sensitivity to corrosion and glaze abrasion .
Vitrified China , presented 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 that is inferior to porcelain.
Glass phase of our high-alumina porcelain
Because the vitreous phase is a natural process in the manufacture of real porcelain, we avoid titled this section as “Vitrification of Porcelain” . If we were to do it the same way as English companies – from the point of view of advertising – we would also have to emphasize this advertising slogan “Vitrified” – which is by the way 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. The addition of enriched silicate mineral aluminum oxide (Al2O3) , an extremely dense and hard corundum, significantly increases the proportion of mullite in the glass phase. We do not want to reveal any details about this. In any case, “considerably” means a breaking strength superior to ordinary feldspar porcelain by up to 400% and more . The porosity , density and stability of the High Alumina porcelain is also superior to Vitreous China .
However, the decisive factor for the quality and durability of the high-alumina porcelain is the homogeneous combination of shards and glaze with almost the same coefficient of expansion through joint hard firing, in which the shards and glaze fuse together at the same time. 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, the high-alumina porcelain is superior to vitreous china and also to ordinary feldspar porcelain – this is of great benefit to the consumer and high sustainability.