Additives

Additives in Porcelain industry

Since the mid-90s, many manufacturers have been experimenting with additives and substitutes. On the one hand to increase the stability of the cullet, on the other hand to reduce raw material costs, others simply to differentiate themselves from the competition. In an exchange of views with the Fraunhofer Institute Würzburg in July 2014, we learned how badly the kaolin deposits on this planet are. Like crude oil, kaolin is a raw material that cannot be reproduced and must be handled more sustainably in the future.

Basically, additions of additives and admixtures change the properties of "real" hard porcelain and limit many a positive property in one way or another. By 2015, no manufacturer will have succeeded in adding additives without lowering the firing temperature. However, it is important to maintain a minimum temperature of 1,300 °C to achieve hard porcelain with a surface hardness of 5 to 6 Mohs.

In this section, we do not want to deal with the large group of pigments, which in its multitude usually only influences the color of the body. Apart from this, the firing method (oxidation or reduction) has a significant influence on the final reaction of the additives. Additives that change the density, hardness and/or stability of the cullet are important for us. Currently known (status 9/2019) are three large groups, which we would like to introduce to you here.

Dolomit

Dolomite is a mineral from the hexagonal crystal system, also known under the names dolomite spar, rhombus spar or pearl spar. Dolomite mineral is a rather frequently occurring mineral from the mineral class of the "carbonates and nitrates" with the chemical composition CaMg[CO3]2 and is thus chemically seen a calcium magnesium carbonate. The addition of dolomites using special formulations can reduce the amount of kaolin and enrich the raw material with a hardness of up to 4 Mohs.

Promising and melodious names are used for various types of alumina masses in which the kaolin portion is substituted by other (more favorable) clays. The probably best known name is "New Bone China" and is supposed to remind of the appearance of the original Bone China by its coloring. In Germany we find designations like ivory porcelain, cream porcelain or silk porcelain. As far as the Mohs hardness is concerned, they all do not correspond to the classic hard porcelain.

Magnesium

Magnesium minerals give the cullet a high resistance to thermal shock.

The magnesium mineral is one of the ten most common elements of the earth's crust. It is found as an additive in numerous other minerals and in the leaf green of plants. The extraction of magnesium is very favorable (cheap) in comparison to kaolin, but as a substitute (substitute for kaolin) it offers a Mohs hardness of only max. 2.5. Magnesium makes porcelain very hard against mechanical impact, but too soft to be kept in the large group of porcelain. Magnesium porcelain can be fired at a maximum of 1,260 °C and thus belongs to the hard stoneware group. In porcelain production, magnesium tableware causes toxic gases and, above all, extreme Co2 emissions, even when modern filter systems are used. As a result, the use of magnesium as a substitute or additive was already banned in China in 2016/2017.

Sodium silicates

Sodium silicates (Na2SiO2) are chemical compounds from the group of sodium compounds and silicates, which occur in several molar ratios (monosilicate, metasilicate, polysilicate and many others). They are more or less hydrated or soluble, depending on the type of production and the degree of purity. Sodium silicates are crystalline - or as a mixture of different silicates - glassy (amorphous) solids or solutions viscous in water.

When casting porcelain in plaster moulds, sodium silicate is added to the slip in small quantities as a release agent during processing. Depending on the type and shape of the casting mould, residues can lead to undesired seam formations.

Chamotte

Chamotte is a leaning agent that increases porosity and reduces drying and firing shrinkage. In ceramics production and pottery, leaning is a process technique used to mix clays that are too fatty with mineral or organic additives. This is especially important for handmade ceramics. The plasticity of the clay is reduced by an appropriate leaning, making it less tough and sticky and more stable for vessel construction. During the drying process, leaning supports the clay mass which shrinks due to water release, and during firing it ensures a better temperature distribution within the fired ware. Depending on the type and quantity of the previously used magnetization, various physical and mechanical properties such as temperature insensitivity, water absorption capacity, density and hardness can be influenced in the fired body.

Xonotlit

Xonotlite - from the group of silicates (Ca₆[(OH)₂|Si₆O₁₇]) - is a common additive which is added to the casting slip as a filler. Xonotlite promotes the processability of the slip by calcination. Calcination is the heating of minerals containing calcium and magnesium carbonate with the aim of dewatering or decomposing them.