Alumina

Alumina in porcelain production

It is actually not entirely correct to describe alumina - alongside feldspar, kaolin and quartz - as a separate raw material in porcelain production and to categorise it here! In technical ceramics, however, alumina - more correctly referred to as aluminium oxide - takes first place among the frequently used high-performance ceramic materials. We mention this mineral oxide here in the main group of raw materials for our porcelain, as it is used for the tableware.

• Alumina
• High Alumina
• Ultra Alumina

from Holst Porzellan plays a very special role.

The mineral "Alumina" with the chemical form "Al2o3" is already present in the basic materials of porcelain, kaolin and feldspar, depending on the design and quality. However, in a purely concentrated (processed) form, it can significantly change the properties of the porcelain as an additional mineral (aluminium oxide), both in the body and in the glaze. Aluminium oxide has the following properties:

• High electrical insulation
• High mechanical strength
• Extremely high compressive strength
• High hardness
• High temperature resistance

Alumina (aluminium oxide) is a mineral form and belongs to the corundum family. Alumina should not be confused with aluminium, which is a chemical element (metal) of the large group bauxide.

Aluminium oxide is the oxygen compound of the element aluminium. The starting point for this substance is the aluminium ore bauxite. Caustic soda splits this ore into aluminium hydroxide. High-quality, high-purity AI2O3 is produced through calcination and complex grinding processes.

Alumina has its own deposits and - depending on the country and origin - is also contained in small quantities in some types of kaolin. According to Dr Khalil M. Ibrahim (Department of Earth and Environmental Sciences, Hashemite University, Jordan), aluminium oxide can also be extracted from low-grade kaolin, which is abundant in his country. Unfortunately, we do not have any results on this.

Unlike dolomite or magnesium, it cannot be used as a substitute for kaolin or feldspar, but is used exclusively as an additive to increase hardness. With a hardness of 9 Mohs, this corundum ranks directly behind diamond, which naturally explains why it can increase the hardness of crockery and porcelain to such an extreme degree.

With the formula AD85, the technical porcelain sector knows a porcelain mass that is mixed with 85% aluminium oxide. The density of the material is 3.4 to 3.45 g/cm3. This is due to the formation of a stable aluminium oxide phase when the Al2O3 content exceeds 80%. Crystallographic examination reveals the aluminium oxide phase in a glassy matrix. The significant properties of AD 85 are due to the stability of the aluminium oxide phase present.

Unfortunately, in most cases the addition of alumina requires a higher melting point (> 1,400 °C), which most porcelain manufacturers are unable to achieve due to various factors. These are, for example

• The high-firing kilns are not designed for this output.
• Conventional burning aids cannot be used.
• Conventional porcelain recipes burn in the oven.
• The glazes cannot follow the high firing temperature.
• The mixing ratios have not been determined.
• Only low-grade aluminium is used.

Definition of aluminium porcelain

The exact composition of the raw materials and the proportion of Al2O3 in the porcelain body is not standardised for the designation "Alumina porcelain" and all other designations that use the term "Alumina" to indicate a particular stability of the body. There are no minimum requirements or standard values for "Alumina porcelain". Laboratory testing for Al2O3 in the finished fired porcelain is also very costly. It can therefore be concluded that "Alumina porcelain" is a quality designation and definition that is to be found in a legal vacuum.

Country & industry specific definitions of Alumina porcelain

Although Germany in particular plays an important international role in the composition of raw materials and the production of Al2O2 for the porcelain industry, the addition of Al2O3 to the porcelain paste for greater stabilisation of the body in the tableware sector was virtually unheard of until the 2000s. In Europe, the first time Al2O3 was added to the paste was in 1985 in Poland. The first series with this "more stable" porcelain were not ready for the market until almost 10 years later. The proportion of Al2O3 at that time was around 2-4%.

Every porcelain-producing country sources its raw materials from different countries and primarily utilises existing deposits in its vicinity. However, raw materials that are decisive for the quality of the finished porcelain cannot be substituted at will. This is mainly because machinery and kilns are becoming increasingly similar internationally and the requirements for raw materials in industrialised porcelain production are subject to very uniform criteria.

Through our decades of experience in porcelain production, we have learnt that there are not only differences in the definition of alumina porcelain, but also that the manufacturers in the countries themselves often work with very different and even nebulous proportions of Al2O3 added to the body. Here are a few examples

• China Guangdong Province: from 30% Al2O3 in the mass
• China Guangxi Province: from 33% Al2O3 in the mass
• Vietnam: from 35% Al2O3 in the mass
• Bangladesh: from 35% Al2O3 in the mass
• India: from 33% Al2O3 in the mass
• Brazil: from 30% Al2O3 in the mass

Proportions & compositions of Al2O3 in porcelain

In the absence of exact norms and standards, we at Holst Porzellan have developed the "greatest common multiple" from the variety of individual definitions of "alumina porcelain" and the proportion of Al2O3 added to distinguish the quality and stability levels of porcelain tableware from one another.

Calcined aluminium

Calcined alumina", which is used in the production of ceramic materials among other things, is "pre-fired" aluminium oxide and corresponds to corundum. In a broader sense, the term "calcinatio" refers to pulverisation into a finished raw material. In contrast to all other additives used in the ceramics industry, this corundum is probably the "hardest" thing that can be used to produce ceramics. Incidentally, calcined aluminium oxide is used in the textile industry to produce particularly light and resistant bulletproof vests.

Synthetic aluminium

Many raw material manufacturers are therefore trying to change the chemical formula of alumina. Corundum can now also be produced synthetically and can therefore be adjusted for different applications. Some of these formulas allow for lower firing temperatures and have now also become established in well-known porcelain brands up to a high firing temperature of 1,260 to a maximum of 1,280 °C. In most cases, however, the use of alumina also leads to discolouration of the body, so that pure white porcelain cannot be achieved. Alumina porcelain from China with a slightly yellow appearance is often used under the name "Durable". According to the laws of firing temperature, these products all belong to the stoneware group. In addition, all products of this type (known to us) suffer from a loss of Mohs hardness, which ultimately results in a maximum score of 5.

Please read more about the "High Alumina Porcelain" from Holst Porzellan/Germany.