Hardness

Hardness of Ceramics and Porcelain

To define "hardness" for ceramic materials (in this case porcelain and other materials), it is necessary to get started in the chemistry and the doctrine of materials science. "Hardness" refers to the mechanical resistance of one materials to the penetration of another and provides information about its quality and resistance. Hardness is also considered as a measure of the wear and fatigue of a material. Hardness and its testing are the focus of materials science and solid state physics and is measured in fracture toughness and is thus clearly one of the material properties.

A hardness test of mineral substances can be determined by different methods. We introduce you to the techniques recognized in the field of lateral science.

Source: Institut für Angewandte Geowissenschaften, Karlsruher Institut für Technologie (KIT-Campus Süd), Adenauerring 20b, D-76131 Karlsruhe

Recognized and widely implemented, especially in the porcelain and ceramics industry, is a materials test according to the process engineering of Friedrich Mohs (mineralogist and geologist 1773-1839). This is based on the knowledge that harder substances can scratch softer substances. The hardness scale of Friedrich Mohs is a "relative" definition and does not give the mineral exact physial values, e.g. Volt, Newton or Lux. The hardness scalerefers to an understandable, representable softness - or just hardness - that has a mineral or mineral-containing material. In the presentation of ceramic hardnesses, the indication of the "Mohs hardness"is applicable as an applicable evaluation parameter and has gained international acceptance. The Mohs hardness thus represents a significant quality feature of porcelain and ceramics.

The counting and presentation of Mohs hardness was carried out until 2001 under the regulation of DIN-Standard norm DIN EN-101. In this case, the detected hardness of damage was shown out. By this kind of counting the real hardness of the mineral equivalent to 8 was only 7. These numbers of the physical result of the hardness were adapted in 2010 by a new version of DIN Standard and converted to the real value of the hardness by DIN-EN-15771. Accordingly the hardness No. 8 dropped down to the new hardness of No. 7. 

The (first) scale of hardness by Mohs

Source: Institut für Angewandte Geowissenschaften, Karlsruher Institut für Technologie (KIT-Campus Süd), Adenauerring 20b, D-76131 Karlsruhe

Die The current scale of hardness by Mohs (State 2011)

(Source: Universität Koblenz)

Crucial is not the temperature of first firing (usually bisquit firing), but glaze firing, since the surface hardness of the ceramic body usually measures the hardness of the glaze and not the hardness of the body. The indication of the respective, comparable ceramic substance may differ slightly by the addition of additives (for example alumina oxide). Furthermore there may be differences as to wether the smooth burn (hard firing) takes place in the oxidation process or reduction firing.

(*) This scale is based on our test and assessment values and reflects the result of a continous survey we have been conduction since 1993, taking into account the hardness yields of more than 400 factores.

However, the above scale of Friedrich Mohs does not represent the considerable differences in material hardness in comprehensible, readable values. The hardness scales according to Stanley P. Rockwell (Rockwell designation / Rockwell hardness) or the Rosival hardness according to A. Rosival (geophysicist and mineralogist 1860) would be more suitable - 1923), which provide more precise, readable information on the hardness of a material since 1920, because they prescribe more concrete methods of determination. While Rockwell considers the penetration depth of a material in fracture toughness, Rosival measures mass loss through stress by determining the "grinding hardness" of a resource and by illustrating the weight loss of a material by processing a fixed amount of abrasive powder in time and pressure. (Source: Dr. med. Vet. H.c. Hugo Freund, University of Wetzlar).

Crockery, whether made of porcelain or other ceramic grades, is subject in everyday use - especially in commercial use - more of a mechanical load (wear) by rinsing and destacking, as the vertical penetration of foreign substances. Therefore, we consider the Rosival hardness to be most suitable for a demonstration and differentiation of ceramic hardnesses, especially to offer the layman an understandable and concise assessment of the ceramic hardness.

The hardness scale by Rosival

(Source: Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Str. 49b, D-50674 Köln)

(*) The conversion of the "comparable ceramic material" was done according to the textbook "Mineral and rock determination for students of geology and mineralogy" of the Institute of Applied Geosciences of the Karlsruhe Institute of Technology (KIT-Campus South) Adenauerring 20b, D-76131 Karlsruhe

According to Rosival, between high-fired porcelain (30,8) and ordinary stoneware (4,2) there are 26,6 Rosival degrees of hardness and 84,4% lower hardness of the stoneware than the high-fired porcelain. Even the hard ceramic fired between 1,240 °C and 1,280 °C (Durable / New Bone China / Viterous) is 82,5% inferior to hard-fired porcelain. Substituting these values in a tabular manner gives the following picture

hardness scale for dish

(*) Representation after DIN-EN-15771 since 2001.

(**) Die Umrechnung des "vergleichbaren keramischen Materials" erfolgte nach dem Lehrbuch " Mineral- und Gesteinsbestimmung für Studierende der Geologie und der Mineralogie" des Instituts für Angewandte Geowissenschaften des Karlsruher Instituts für Technologie (KIT-Campus Süd), Adenauerring 20b, D-76131 Karlsruhe​, in Anlehnung an die Umsetzung nach DIN-EN-15771

Acting factors

The determination of mineral surface hardness - whatever the hardness tests is applied - is always based on a uniform surface structure. But the kind of production of ceramic and porcelain often shows "Pinholes" and "Spots". Please read more about this here: "Defects of Porcelain - Pinholes". A standard or classification of numbers, sizes and depths of pinholes does not exist. But rule of thumb says, that as more and clearly pinholes are visible, the more the hardness of the glaze comes down. Very often laboratory technical verifications in accordance of DIN-EN-15771 lead porcelain plates to a hardness of 5, even though the pinhole-free zones of the glaze have the hardness No. 6.

Derivation and conclusion

It may be true that due to new developments and techniques, the mineral and chemical composition of ceramic materials varies from manufacturer to manufacturer or even from country to country. It is also correct, that the firing method (mono firing vs biscuit and hard firing) has an influence on the amount and thereby on the strength of the glaze and thereby changes the factor of corrosion. Even if a tolerance of 25% is granted for the quality determination of the especially durable dish (eg Durable / Fine China / Dynamite), there still is a quality difference of at least 25% of high-fired porcelain compared to all other ceramic materials burned below 1,300 °C.

That is exactly what makes the difference!

High alumina porcelain by Holst Porzellan is fired at up to nearly 1,400 °C and thus combines the good characteristics of Mohs's hardness and the physical stability of durable porcelain. Read more about High Alumina