For most people it is simple; a plate is to eat from, you can drink from a cup, a vase to put flowers in, a toilet bowl to... etc. etc. A number of these products are also made of glass but, especially cutlery, is often made of ceramic. And ceramic is then an all-encompassing term for products made of earthenware, porcelain, stoneware, terracotta and other clay products. The term "ceramics" is derived from the ancient Greek word keramos which was a designation for clay products.
The origin is therefore always clay, which is brought into the desired shape and then fired at high temperatures (sometimes up to over 1150°C). Clay is a predominantly inorganic material consisting of fine-grained clay minerals such as silicates, quartz or metal oxides and can be plastically deformed if the water content is sufficient.
Depending on the technique used, the object will have different properties in terms of porosity, strength and surface structure. A number of techniques have been developed for shaping the clay, such as hand shaping (gluing together), pinching, casting in (often plaster) moulds and turning on a disc. Texture and decoration can then be added to the shape before it is fired. After firing, it can then be glazed to protect the object, make it waterproof and/or give it colour and shine.
The temperature in the kiln in particular has a decisive influence on the later end product and its qualitative properties. Because the higher the firing temperature, the more resistant the ceramic becomes. And it is also the temperature that divides ceramics into its sub-terms: a distinction is made between earthenware - which is fired below the sintering limit - and sintered material that is exposed to temperatures above this limit.
Terracotta and pottery
The category that is fired below the sintering limit includes earthenware and red-brown terracotta. The latter is easy to recognize, especially because of its color and is known to many when used in flower pots. And this is also one of the advantages of this low-fired ceramic: due to its high porosity, the material absorbs a lot of water and then slowly releases it to the plants. This is a decisive advantage, especially in warmer regions. However, caution is advised in winter in temperate climate zones, as frost can cause the porous material to crack.
Despite its porosity, earthenware also has advantages, which is why it is still often found on the market. Due to the high lime content, the clay is easy to process and the production process is considerably cheaper than stoneware or porcelain due to the lower firing temperatures, which usually has a positive effect on the price.
However, earthenware dishes are less impact-resistant and durable than sintered earthenware. In addition, they must be covered with a glaze in any case, because the material itself is not completely waterproof. Earthenware is often sold as dishwasher safe. However, the unglazed base of the plate, small cracks or damage to the glaze can sometimes be problematic. Soaking in the sink or a long wash cycle in the dishwasher can allow small amounts of water to penetrate and soak the material. If the ceramic is not carefully air-dried after washing, mold can develop.
Sintered pottery - stoneware and porcelain
The category of sintered earthenware includes both stoneware and porcelain. Both types of ceramics are fired at temperatures above the sintering limit of 1150°C and therefore absorb almost no water, even when unglazed. They are easy to maintain, particularly impact-resistant and therefore very durable.
Unfortunately, stoneware and some types of earthenware are visually indistinguishable or barely distinguishable from each other. Both subtypes of earthenware are made of a light gray or soft beige clay and are often covered with colorful glazes. It is therefore important to inquire about the material before making a purchase. Stoneware is much more robust and can usually be cleaned in the dishwasher without any problems.
Porcelain, on the other hand, is easier to distinguish. The composition of the clay contains a high proportion of kaolin, which means that the material is very light in colour and has a thin-walled and often slightly translucent appearance. Classic porcelain is often white or - if the clay is coloured with pigments before casting - pastel-coloured. It is usually additionally covered with a transparent glaze. Because porcelain clay is rare and is fired at particularly high temperatures, it is also called white gold. Its special feature is therefore usually reflected in the price.