What kind of soil can be used to make porcelain?

The clay required for making porcelain should have formability, moderate drying shrinkage, moderate firing shrinkage and refractoriness. When making porcelain, the mineral and chemical composition of the clay must also be taken into account. Ceramics are an important calling card of Chinese civilization. China is the country where pottery originated earliest in the world and also invented and created porcelain.

Porcelain is made mainly from clay, along with other natural mineral materials, through processes of crushing, mixing, shaping and firing. So, what kind of soil can be used to make porcelain?

The clay required for making porcelain should meet three requirements in terms of performance: First, it should have formability, that is, plasticity. When clay is mixed with an appropriate amount of water, it forms a clay ball. Under the action of a certain external force, it deforms but does not crack. When the external force is removed, it can still maintain its shape unchanged. Second, it should have moderate drying shrinkage performance. During the drying process of clay materials, the water surrounding the clay particles evaporates, causing the particles to come closer together and resulting in volume shrinkage. If the shrinkage is excessive, it will cause the clay body to crack. Therefore, it is necessary to maintain an appropriate drying shrinkage rate. Thirdly, it should have moderate firing shrinkage performance and refractoriness. When clay is calcined, a series of physical and chemical changes occur, causing it to shrink again. When it reaches a certain temperature, it will gradually melt and soften. If the shrinkage is excessive, it will cause the porcelain to deform. To ensure that the clay does not soften during firing and reaches a firing temperature suitable for the glaze, it should have an appropriate content of aluminium oxide, which is one of the main chemical components constituting the "skeleton" of the porcelain body.

To determine whether a kind of clay is suitable for making porcelain, the mineral composition and chemical composition of the clay should also be taken into consideration. In terms of mineral composition, clay is composed of main minerals such as kaolinite and montmorillonite, associated minerals such as quartz and feldspar, as well as some organic substances. The main minerals of clay naturally have good plasticity. The more main minerals there are and the finer the particles, the higher the plasticity will be. In terms of chemical composition, the main mineral of clay is hydrated aluminosilicate, whose main chemical components are silicon dioxide, aluminum oxide and water. Due to different geological formation conditions, it may also contain small amounts of oxides of potassium, sodium, calcium, magnesium, iron, titanium, etc. The chemical composition of clay varies greatly due to the influence of its constituent minerals.

Clay comes in various colors such as white, gray and yellow, which is related to the types and contents of coloring oxides and organic substances it contains. However, the color of the fired porcelain body is not directly related to the color of the clay, but is more closely related to the content of coloring oxides and the firing atmosphere. The Ru Kiln of the Song Dynasty, with the saying "After the rain, where the blue clouds break, this color represents the future", was mainly made of clay with high contents of iron oxide and titanium dioxide. The body was mainly light grayish-white, which was more harmonious and unified with the Ru porcelain glaze that was as blue as the sky.

Some clays are soft and can spread out in water. They can be used directly after being washed to remove impurities, such as kaolin and ball clay. At the beginning of porcelain-making, this kind of soft and fine clay was usually used to make porcelain, such as Yue kiln celadon. Kaolin, which contains about 40% of aluminium oxide by mass, requires a temperature of around 1400 degrees Celsius to be sintered. For instance, some white porcelains from Xing and Ding kilns are not directly made from kaolin as a single raw material for the convenience of firing. Some clams are dense and blocky, and need to be crushed and washed before use, such as porcelain stones. Jingdezhen has been using porcelain stones to make porcelain since at least the Song Dynasty.

The celadon and white porcelain of the Song Dynasty in Jingdezhen is as smooth and lustrous as jade and is made from porcelain stone. Porcelain stone is rich in sericite, has a low content of alumina, a high content of potassium and sodium, and a low refractoriness. The porcelain fired from it has a high light transmittance and is particularly suitable for firing small items. Blue and white porcelain of the Yuan Dynasty was large and heavy, with exquisite paintings. It was made from a variety of sedimentary clays with high alumina content. During the Ming and Qing dynasties, the imperial kilns in Jingdezhen gradually adopted a "dual formula" of porcelain stone and kaolin to make porcelain, whether it was blue and white porcelain or overglaze colored porcelain, which gave Jingdezhen porcelain the reputation of being "as white as jade, as bright as a mirror, as thin as paper and as resonant as a chime".

A single piece of porcelain spans a thousand years, and civilization transcends mountains and seas. The combination of clay bestowed by nature and the craftsmanship of skilled artisans ensures the continuous inheritance and promotion of the charm of Chinese porcelain.