Glaze hardness
This section deals with the hardness of the glaze. Porcelain, as we know and appreciate it, is an end product that generally consists of two different components.
- the body
- the glaze
- the decoration, if applicable
You can find our explanations of body hardness and a more detailed explanation of the hardness scales in the section /ware/properties/hardness.
General information
Hardness in the general sense refers to the mechanical resistance that a material offers to the mechanical penetration of a harder test specimen. A distinction is made between different types of hardness depending on the type of impact. Thus, hardness is not only the resistance to harder bodies, but also to softer and equally hard bodies. The definition of hardness differs from that of strength, which represents the resistance of a material to deformation and separation.
Hardness is also a measure of the wear behaviour of materials. Hard spectacle lenses scratch less, hardened gear wheels wear less. Hardness is particularly important when selecting cutting edges for tools such as milling cutters or turning tools. Hard cutting edges remain sharp for longer, but break more quickly. Hardness and its testing are important focal points in the fields of solid state physics, materials science and the analysis of materials, as well as in geosciences for characterising rocks and minerals. Hardness is one of the material properties of a material along with crack toughness, strength, ductility, stiffness, density and melting temperature.
Mohs hardness scale
The old German hardness scale according to Friedrich Mohs assigns hardness 8 to hard porcelain. Following the change in the Mohs hardness assessment method in 2010, the hardnesses were reclassified in the DIN-EN-15771 standard. According to the new assessment method, hard porcelain now achieves a Mohs hardness of between 5 and 6.
The largest proportion of this is due to porcelain's quartz content with a hardness of 6. Quartz alone in its unfired state is capable of cutting simple glass or utility glass. This means that porcelain can damage all materials with a hardness of 1 to 5. As a reminder: a diamond has a hardness of 10, i.e. only 4-5 degrees of hardness higher than hard porcelain! These values clearly show that porcelain is extremely harder than cutlery, even if the latter is made of stainless steel or chrome steel. Incidentally, this is also the proof or the reason for the black stripes of "cutlery abrasion": It is not the porcelain that is damaged by the cutlery, but the softer cutlery is rubbed off on the harder glaze, leaving behind the annoying black stripes and streaks.
Hard water
In everyday use, especially in commercial use (in hotels, hospitals and canteens), porcelain can basically only be damaged by itself or attacked by quartz crystals in the washing-up water. For example, if the bases of porcelain plates are not polished clean, they can cause scratches and damage to the glazes of the items underneath when they are pulled back and forth. The same happens with stainless steel worktops, which are "rubbed off" by rough porcelain bases and leave grey and black stains on the feet.
The rinse water also contains a number of solids that make our water "hard". This problem is usually solved by so-called "descaling systems", which are designed to remove or destroy the solids from the water. Whether osmosis systems, electromagnetic water treatment, softeners or other salt or chemical-based water treatment systems, it is much more difficult to deal with quartz crystals in water than with oxides or surfactants, for example. Regular rinsing with quartz-containing water can also mechanically damage the glaze through the quartz crystals, in extreme cases even removing it down to the shard. Incidentally, this process is often incorrectly referred to as glaze corrosion.
Kaolin
Please bear in mind that the hardness we describe only applies to hard-fired (T=> 1,320 °C) feldspar porcelain with a kaolin base (Al2Si2O5(OH)4). This compound, which contains water, oxygen, aluminium and silicon, has a hardness of approx. 1.5 in its unfired state and is considered a recognised mineral. Incidentally, kaolin owes its name to the Middle Kingdom. The Chinese use the term "Gaoling" to describe a storage room. Incidentally, the cleanest kaolin in the world also comes from China in the province of Jiangxi.
Degrees of hardness
If the composition of the raw material is changed, porcelain loses its hardness and therefore also its resistance. The addition of stabilising additives usually lowers the glost firing temperature and the hardness levels quickly drop to 4. If you compare the hardness 10 of a diamond with the hardness 6 of porcelain, the difference in resistance quickly becomes apparent! Many variations of porcelain in particular (Stoneware, Durable, New Bone, etc.) are a third (33.33%) softer than hard-fired feldspar porcelain with a hardness of 4 Mohs.